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Identification of genes associated with shell color in the black-lipped pearl oyster, Pinctada margaritifera

Identification of genes associated with shell color in the black-lipped pearl oyster, Pinctada... Lemer et al. BMC Genomics (2015) 16:568 DOI 10.1186/s12864-015-1776-x RESEARCH ARTICLE Open Access Identification of genes associated with shell color in the black-lipped pearl oyster, Pinctada margaritifera 1,2* 3 3,4 1 Sarah Lemer , Denis Saulnier , Yannick Gueguen and Serge Planes Background: Color polymorphism in the nacre of pteriomorphian bivalves is of great interest for the pearl culture industry. The nacreous layer of the Polynesian black-lipped pearl oyster Pinctada margaritifera exhibits a large array of color variation among individuals including reflections of blue, green, yellow and pink in all possible gradients. Although the heritability of nacre color variation patterns has been demonstrated by experimental crossing, little is known about the genes involved in these patterns. In this study, we identify a set of genes differentially expressed among extreme color phenotypes of P. margaritifera using a suppressive and subtractive hybridization (SSH) method comparing black phenotypes with full and half albino individuals. Results: Out of the 358 and 346 expressed sequence tags (ESTs) obtained by conducting two SSH libraries respectively, the expression patterns of 37 genes were tested with a real-time quantitative PCR (RT-qPCR) approach by pooling five individuals of each phenotype. The expression of 11 genes was subsequently estimated for each individual in order to detect inter-individual variation. Our results suggest that the color of the nacre is partially under the influence of genes involved in the biomineralization of the calcitic layer. A few genes involved in the formation of the aragonite tablets of the nacre layer and in the biosynthesis chain of melanin also showed differential expression patterns. Finally, high variability in gene expression levels were observed within the black phenotypes. Conclusions: Our results revealed that three main genetic processes were involved in color polymorphisms: the biomineralization of the nacreous and calcitic layers and the synthesis of pigments such as melanin, suggesting that color polymorphism takes place at different levels in the shell structure. The high variability of gene expression found within black phenotypes suggests that the present work should serve as a basis for future studies exploring more thoroughly the expression patterns of candidate genes within black phenotypes with different dominant iridescent colors. Keywords: Differential expression, Biomineralization, Nacre, Pearl, Pigmentation, Albino Background basis [3–5]. Most studies have reported a relatively sim- Color polymorphisms of mollusk shells have fascinated ple genetic basis for color polymorphisms involving one humans throughout history. Color polymorphism results or two loci with dominance [6–8], although more elabor- from a combination of multiple factors starting from ate polymorphisms are likely to arise from complex mul- inner genetic to mitigating environmental factors related tigenic systems [9, 10]. Recent studies have shown that to biochemistry, substrate or nutrition [1, 2]. Experimen- marine shellfish tend to display more complex patterns tal crosses have shown that shell color has a genetic of pigmentation than freshwater shellfish [11]. Examples of complexity are mostly found on shells displaying a nacreous layer (i.e. mother of pearl) such as the gastro- * Correspondence: [email protected] Laboratoire d’Excellence “CORAIL”, USR 3278 CNRS-CRIOBE- EPHE, Perpignan, pods in the family Haliotidae (abalone), the cephalopod France, Papetoai, Moorea, French Polynesia 2 family Nautlidae (chambered Nautilus), or the bivalves Present address: Department of Organismic and Evolutionary Biology, of the families Pteriidae, Mytilidae and Pinnidae. Because Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA of its high industrial value, the composition and Full list of author information is available at the end of the article © 2015 Lemer et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Lemer et al. BMC Genomics (2015) 16:568 Page 2 of 14 formation of the nacreous layer have been studied in multiple taxa at different levels of expertise: proteomics [12–21], genetics [22–26, 21], optics [27, 28], mineralogy [29] and chemistry [30–32]. The nacre is made of regu- lar layers of aragonite tiles and is unique from its irides- cence. Iridescence is usually attributed to a diffraction effect caused by the evenly grooved surface microstruc- ture, similar to that of a diffraction grating [33, 34, 27]. The rainbow-like diffraction colors arise from surface repetition of the nacreous layers (growth lines) and are named by gemologists “orient” [29]. However, not all iri- descences arise from surface regularities and the pig- ments present in the binding regions of the aragonite tiles and integrated in the biomineralization process also play a role in the color of the nacreous layer [29]. There- fore the process of biomineralization of the nacreous layer is of great economic interest to the pearl aquacul- ture industry. As a result, extensive studies have been conducted to identify proteins responsible for the nacre formation by screening proteins contained in the shell and genes specifically expressed in the calcifying tissues i.e. the mantle [35, 36]. A wide variety of proteins and genes have been identified and their functions in nacre formation have been partially characterized (reviewed in [35, 37–41]). Our knowledge of the “biomineralization toolkit” in- volved in the formation of the calcitic and aragonitic layers is expanding and we now know more about how the different shell layers are synthesized and how their production is genetically regulated [37]. Never- theless the precise molecular mechanism controlling color polymorphism in nacre remains unexplored and Fig. 1 Picture of dissected specimens of Pinctada margaritifera of the three studied phenotypes. a. Full albino (FA; white shell and is far from being understood. To address this key white mantle), b. half albino (HA; white shell and black mantle), c. question we searched for specific genes displaying dif- black phenotype (C; black shell and black mantle) ferential expression patterns among different shell color phenotypes of the Polynesian black-lipped pearl oyster, Pinctada margaritifera. The nacreous layer of albino specimens and black specimens is an ideal P. margaritifera exhibits a large array of color vari- method to identify the genetic processes directly ation among individuals, although the shell is usually linked to shell color polymorphisms. simply referred to as black. The heritability of nacre Albinism is induced by the absence of melanin or by color variation patterns in P. margaritifera have been non-functional melanin molecules as a result of one or demonstrated by experimental crossing and by main- more mutations in genes involved in the melanin bio- synthesis chain [43–49]. Melanin has been identified in taining single colored dominated lineages (CL. Ky personal communication). An interesting feature of P. many mollusk species, including cephalopods, gastro- margaritifera is the rare occurrence of albinos, 1 out pods, and bivalves [50]. In P. margaritifera, melanin is secreted in the epithelium of the median and internal of 10,000, displaying total or partial absence of color- ation. Albinos are characterized by a white shell (peri- fold of the mantle edge and in the pallial area [51]. The ostracum and calcitic layer) and a white mantle mantle edge is directly responsible for the biomineraliza- tion of the shell and more specifically of the nacre layer (Fig. 1a) in opposition to the common black shells (Fig.1c).Insomerare cases themantle ofalbino [21]. As a consequence, it is the chosen mantle part used specimens remains black (S. Planes and S. Lemer, by professional grafters in pearl culture manufacture to pers. observation; [42]; Fig. 1b); supporting the mul- isolate grafts , that will be transplanted together with a tiple genetic origins of the absence of color in the marble in the gonads of a recipient oyster. Three pro- shell. In the context of our study, the comparison of teins closely related to the tyrosinase, known to play a Lemer et al. BMC Genomics (2015) 16:568 Page 3 of 14 role in melanin biosynthesis, have previously been char- Total RNA was purified using the RNeasy Mini Kit acterized in the pallial or edge region of the mantle of (Qiagen, USA). Pinctada fucata: OT47, Pfty1 and Pfty2 [52, 53]. The The suppressive subtractive hybridization technique localization of these proteins suggests that they could be (SSH) [54] was used to characterize genes involved in the integrated in the shell layers including the nacre layer origin of shell color by comparing expression between the during the biomineralization process. phenotypes C and FA. For the forward library, cDNA from In this study, we aimed at identifying genes in- five phenotype FA samples was used as tester and cDNA volved in the origin and potentially the polymorphism from five phenotype C samples was used as driver, and of color in the nacreous layer of P. margaritifera.We vice versa for the reverse subtractive library. The construc- hypothesized that the variety of colors found in the tion of the libraries, clone sequencing, vector and adaptor nacre is a combined result of variation in expression trimming and differential screening using dot blot of genes involved in the formation of the shell’s hybridization were outsourced to Rx. Bioscience Ldt microstructure and of genes involved in pigmentation. (Maryland, USA). Briefly, both forward and reverse sub- In order to test this hypothesis, we identified and tracted libraries were produced from 2 ng mRNA. First compared coding genes differentially expressed be- and second strand cDNA synthesis, RsaI endonuclease tween normal black specimen of pearl oysters and enzyme digestion, adapter ligation, hybridization, and two types of albinos using a suppressive and subtract- PCR amplification were performed as described in the ive hybridization method, that we then tested and val- PCR-select cDNA subtraction kit manual (Clontech, Palo idated with RT-qPCR runs. We identified genes Alto, CA, USA) with modifications as described in [55] involved in the formation of the calcitic layer, the ara- for the Mirror Orientation Selection protocol. Differen- gonite tabletsof the nacreand in thebiosynthesis tially expressed PCR products were ligated into pJetBlunt chain of melanin suggesting that color polymorphism cloning vector (Fermentas) and transformed in Escheri- takes place at different levels of the shell structure. chia coli competent cells. For the differential screening by dot blotting, 1000 clones per library were randomly trans- ferred on two nylon membranes and hybridized with C Methods and FA cDNA probes, respectively. Nylon membranes Biological material were then auto-radiographed and superimposed for iden- In order to identify genes involved in the origin and po- tification of differentially expressed clones. Among clones tentially in the variation of shell color, mantle tissue of showing the most intense differential signal after Pinctada margaritifera from 3 phenotypes was obtained hybridization to cDNA probes, 960 were randomly se- from adult specimens of similar size and raised in the lected for sequencing in each library. same pearl farm in the Gambier archipelago (Mangareva, French Polynesia) in 2011. The three sampled pheno- types were: the normal black phenotype of P. margariti- Sequence analysis fera (black shell and black mantle) referred hereafter as For each library, the 960 clones were sequenced and vector phenotype C (Fig. 1b); the full albino phenotype (white trimmed by RxBioScience Ltd. (Maryland, USA). High- shell and white mantle) referred hereafter as phenotype quality expressed sequenced tag (ESTs) (>100 bp) were as- FA (Fig. 1a); the half albino phenotype (white shell and sembled into clusters or identified as unique sequences and black mantle) referred hereafter as phenotype HA.We used for database searches with the BlastX and BlastN pro- sampled five individuals of each phenotype. The mantle grams on the NCBI server (http://www.ncbi.nlm.nih.gov/ edge of each individual was dissected by a professional BLAST/) and UniProt. (http://www.uniprot.org/blast/) grafter following the method used for pearl production, Search of homology was also conducted in an EST bank of in order to insure the use of the specific part of the Pinctada margaritifera [21]. In addition, functional annota- mantle producing the calcitic and aragonite layers. Each tion was performed in Blast2Go (http://www.blast2go.com/) sample was immediately preserved in RNAlater RNA where gene information were obtain by blasting sequences Stabilization Reagent (Qiagen, USA) transported to the in Gene Ontology (http://www.geneontology.org/). Enriched laboratory and stored at −80 °C. Molecular function GO terms were then uploaded to REVIGO (Reduce + Visualize GeneOntologyhttp://revi- RNA isolation and subtractive cDNA library construction go.irb.hr/) for visualization. REVIGO summarizes the long Total RNA from mantle tissue of individuals of phenotypes list of GO terms by removing redundant terms and group- C, HA and FA was isolated using TRIzol reagent (Invitro- ing related terms based on semantic similarity [56]. The gen) according to the manufacturer’s instructions. The EST sequences used in this study have been submitted to RNA pellet was washed with 70 % ethanol and air-dried, the online database (Accession numbers: JZ845577- dissolved in DEPC-treated water and stored at −80 °C. JZ845610; JZ845790-JZ845792). Lemer et al. BMC Genomics (2015) 16:568 Page 4 of 14 Differential expression validated by quantitative RT-PCR RT-qPCR data analysis Quantitative real-time PCR (RT-qPCR) was used to analyze The Ct values of the replicates for each housekeeping the expression profiles of selected genes. Total RNA was gene and each candidate gene of each phenotype (C, FA extracted and reverse-transcribed (2 μg) using oligo dT and HA pooled or individual samples) were reported. primers and the Superscript II enzyme (Invitrogen). For each candidate gene, the level of transcription was At first, the cDNA from each of the five individuals normalized using the following calculation: for each phenotype (C, FA and HA) were pooled in ΔCt ¼ Ct −Ct Target M equal amounts (25 ng/μL each). Quantitative RT-PCR analyses were run on the three pooled samples for all se- where Ct is the mean Ct of the target gene in one Target lected genes. The genes showing the most significant dif- of the tested phenotype (C, FA and HA pooled or indi- ferential expression profile among the three phenotypes vidual samples) and Ct is the mean Ct of the house- were then selected to conduct subsequent RT-qPCR ana- keeping genes in that same phenotype. lysis on each individual sample separately in order to de- Relative quantification of gene expressions was estimated tect individual variability (Table 1). for each gene in each phenotype using the ΔΔCt method as Both pooled and individual RT-qPCR analysis were described in [59]. Relative quantification relates the PCR conducted as the following: in each 25 μl reaction, 10 μl signal of the target transcript (here FA or HA phenotypes) cDNA (diluted 1:100 in water) was mixed with 12.5 μl in a treatment group to that of another sample (here C Brillant® II SYBR Green Master Mix (Stratagene) and phenotype). We used the following equation: 0.2 μM of each primer (primer pairs listed in Table 1). ΔΔCt ¼ ΔCt − ΔCt This allowed for the consistent use of standardized ther- Target C mal cycling conditions performed using a Mx3000P where ΔCt is the ΔCt obtained for a target gene in Target Real-Time PCR System (Stratagene): 95 °C for 10 min, one of the tested phenotype (FA and HA pooled or indi- followed by 40 cycles of 95 °C for 30 s, 60 °C for 1 min vidual samples) and ΔCt is the ΔCt obtained for that and 72 °C for 1 min which were found to give efficien- same gene in the phenotype C. In the pooled analysis cies > 90 % (see below). The RT-qPCR reactions were ΔCt is the ΔCt of the pooled C individuals whereas in run in triplicate for the pooled analysis and in duplicate the individual analysis the ΔCt is the mean ΔCt ob- for the individual analyses. Melting curve profiles (95 to tained for each individual from the C phenotype. 55 °C decreasing by 1 °C every 30 s) were assayed to en- Finally the relative expression fold for each gene in sure that we were looking at a single product. Each run each tested condition in relation to the phenotype C was included three positive cDNA controls also used as estimated using the following equation: interplate calibrators, and three non-DNA template con- trols (water) for each primer pair and four housekeeping  ðÞ ΔΔCt E 2 genes: RNA and export factor binding protein 1 or REF 1 [57], 18S [58], Elongation factor 1 or EF1 (EF1S: where E is the reaction efficiency for each target gene. ATGCTGCCATGGTTGATATG/EF1: GTGGCCTCAGC For each target gene in each tested phenotype of the TTTCTCAAC) and Glyceraldehyde-3-Phosphate De- pooled analysis, a relative expression fold superior to 2 hydrogenase or GAPDH (GAPDH1S: AGGCTTGAT- was considered as significantly differentially expressed GACCACTGTCC / GAPDH1R: AGCCATTCCCGTCA [60, 61]. For the individual analysis, significances among ACTTC). EF1 and GADPH were chosen based on mean relative expression folds of each gene were deter- their ubiquitous and constitutive expression pattern in mined by an analysis of variance (ANOVA). P. margaritifera tissue. A 10-fold dilution series was created from a random Results pool of cDNA from our samples (including C, FA and HA Subtractive cDNA library construction and sequencing phenotypes), ranging from × 100 dilutions to × 100,000 di- Two SSH libraries were produced to identify differentially lutions. Triplicate RT-qPCR reactions were carried out as expressed genes between normally black colored and full described above for each gene at each dilution. Mean cycle albinos specimens of Pinctada margaritifera.Nine hun- threshold (Ct) values for each dilution were plotted dred and sixty (960) clones were sequenced from each li- against the log10 of the cDNA input for each gene to gen- brary, yielding 677 and 580 high quality cDNA sequences erate efficiency plots. The reaction efficiency of each gene (>100 bp), after removing low-quality regions and screen- assay was calculated using the following equation: E = ing for vector contamination, from the C and FA libraries 10(−1/slope) where E is the reaction efficiency and respectively. The ESTs from the C library generated 120 ‘slope’ is the slope of the line generated in the efficiency contigs and 238 singletons (358 total analyzed sequences plots. Only the genes yielding an efficiency comprised be- after discarding redundant sequences), indicating that the tween 90 % and 110 % were retained. overall redundancy of the library was 18 %. The ESTs from Lemer et al. BMC Genomics (2015) 16:568 Page 5 of 14 Table 1 Characteristics of the genes selected for the pooled and individual RT-qPCR analyses. The proteins, primers and efficiencies values are indicated. The genes selected for the individual analysis are indicated in bold Gene code Protein Primers Efficiencies % Genbank accession # KRMP Lysine (K)-rich mantle protein Joubert et al. 2014 95 JZ845792 SHEM 9B Shematrin 9 A_shem9b 5’- F CCCCGTATCCTCCATATCC 101 JZ845577 A_shem9b 3’- R GCTATTACCGGAGTACCCTACG SHEM 1B Shematrin 1 C_Shem1b F 5’- CGCTATCGTTGCTCTCATTG 101.00 JZ845578 C_Shem1b R 3’- TCCACCTCCTCCTCCTCTTC TYR 2A Tyrosinase 2 A_tyr2a F 5’- GCGGCTCTACTGTCAAATGG 112.00 JZ845579 A_tyr2b R 3’- CTGGACCTTTCAGGGACTGG TYR 2C Tyrosinase 2 A tyr2c F 5’- CCCGTGGCCTGGATAGTC 101.00 JZ845580 A tyr2c R 3’- TTTTCTTCCATCACTGCTACATTG FLAV Flavonol cinnamoyl CoA reductase-related A_flav aF 5’- AGCAGGGTTATCACGTCAGG 96.00 JZ845581 A_flav bR 3’- ATCCTTTGGTGCCTGTGC METH Methionine-rich nacre protein A_Meth bF 5’- ATGCGGAGGATACTGTGCTT 100.00 JZ845582 A_Meth bR 3’- CGGGGCTGGATAGACTCATA CLP3 Chitinase 3 protein A_clp3b F 5’- TCACAAAATGGATCATAACGTACC 94.00 JZ845583 A_clp3b R 3’- GGACTGCCTTTGAATGTCG PDZ PDZ domain protein A_PDZa F 5’- TGAGCTTCAGAGAGGTGACG 100.00 JZ845584 A_PDZa R 3’- AACATTTGGTGAGGGTTTGG MP10 Mantle protein 10 A_MP10d F 5’- ATTATGGACCGGGCAAGC 100.00 JZ845585 A_MP10c R 3’- GAGGACAGGAACATCAACAGG EGF Epidermal growth factor receptor A_EGF aF 5’- CATTCCCCATCTTCCTCAAA 108.00 JZ845586 A-EGF aR 3’- GGACTTCCTGGGATGTTGTC PHOS Phosphotyrosine protein phosphatase A_Phos bF 5’- AATTATGACACAGAGGGATCTAAGG 98.00 JZ845587 A_Phos aR 3’- CATAGAAACCACCACAACATCG YTH YTH domain family protein 2-like A_YTH aF 5’- AATTGAGACACATTCGGTTGG 93.00 JZ845588 A_YTH aR 3’- TTACCGCTCTCTGTCCTTGC PIF Pif177-like protein A_pif aF 5’- TTTTGAATTACACGACTGCTTTG 93.00 JZ845589 A_pif aR 3’- TTCAGTAGAACTAACGCTAAATCCAG CHIT Chitin synthase A_Chit aF 5’- AATCCAATTTCCCGCAGTC 90.00 JZ845590 A_Chit bR 3’- TTGTTGTAGACATTAGCGACGTATC YBOX Y-box binding protein A_Ybox aF 5’- CGTACATCAAACTGCCATTACC 97.00 JZ845591 A_Ybox aR 3’- CTACATCAAATTCCACCTTCTCC COLL Collagen alpha-1 (XI) chain precursor A_Coll bF 5’- CGTCCAATCAATCCAGGTG 105.00 JZ845592 A_Coll aR 3’- TGGGTCCAAAAGGTGAAATG GIGA Gigasin A_Giga aF 5’- TCTCTTCCGGTCAAAAATGC 91.00 JZ845593 A_Giga aR 3’- TCGCTGTACTATTTCCGTTCG SHEM 4 Shematrin 4 A_Shem4a F 5’- GCTTCCCATCGGTTTATGG 92.00 JZ845594 A_Shem4b 3’- R TGCCAACATTTCCGTATCC CAL Calreticulin A_Cal aF 5’- TCACCATCCATTTCATCATCC 102.00 JZ845595 A_Cal aR 3’- ACCAGAGGACTGGGACAAGC PEROX Peroxidase A_Perox aF 5’- TGCTGGGACTCACTCTATCCA 107.00 JZ845596 A_Perox bR 3’- TCAAGCCATCAAAGAAACATCTT MP8 Mantle protein 8 Joubert et al. 2014 101.00 JZ845791 ASP Aspein shell matrix protein Joubert et al. 2014 99.00 JZ845790 Lemer et al. BMC Genomics (2015) 16:568 Page 6 of 14 Table 1 Characteristics of the genes selected for the pooled and individual RT-qPCR analyses. The proteins, primers and efficiencies values are indicated. The genes selected for the individual analysis are indicated in bold (Continued) MAT Matrilin Cartilage matrix protein A_Mat aF 5’- TTGTTCTGGATGGGTCTTCG 109.00 JZ845597 A_Mat aR 3’- TCAAAGCCCGCACATCAG SHEM 8 Shematrin 8 C_Shem8 bF 5’- CTCCACCACCAATGACGATT 111.00 JZ845598 C_Shem8 aR 3’- TTTCGGGGGTGTTAACGTAG FER Ferritin C_Fer cF 5’- GGGCCTGATTGACAGACTTCT 96.00 JZ845599 C_Fer cR 3’- GAGGGCGCATTGTCCTTC PERL Perlin matrix protein C_Perl bF 5’- TTCCAGATATTACACCCTGTGCT 100.00 JZ845600 C_Perl aR 3’- CGTTACCGTTTCCACCAAAA PRISM Prism uncharacterized shell protein C_Prism aF 5’- TGGTTCCAAGTGTATTTGTCCA 96.00 JZ845601 C_Prism aR 3’- TCCATAGACGCACACCTTTG MP88 molluscan prismatic and nacreous layer 88 C_MP88a F 5’- TCTGTGAAAATTTTGATAAACTGAA 109.00 JZ845602 C_MP88a R 3’- GAATAAAAGTTTAATGTTCCATTCCT CHK1 Checkpoint-like protein C_CHK1a F 5’- CGGGCAGGTACTCATTCC 95.00 JZ845603 C_CHK1b R 3’- GCTACCACATCCAAGGAAGG LAMIN Laminin receptor precursor C_Lamin bF 5’- GAAGCTGAGAAAGAAGAACAGACC 101.00 JZ845604 C_Lamin aR 3’- TGTTGTGGTGGCTGTATTGC SEST Sestrin 1-like protein C_Sest aF 5’- TTTTTGCTACCAGGACTTTGC 91.00 JZ845605 C_Sest aR 3’- GTGTCAACATCTGTCATCTCACC FIBRO Fibronectin 3 C_Fibro aF 5’- AGTAGCTGATCGTGCATTCG 97.00 JZ845606 C_Fibro bR 3’- AACTCACAGGACACCAAAACG SHELL Nacre uncharacterize shell protein 6 C_Shell_cF 5’- CGTTCTATCCCTGGTCATCC 101.00 JZ845607 C_Shell_dR 3’- GGACGTGGATTTTCCTTGG SERP Serine protease inhibitor C_SerP_cF 5’- AGGTGTGTACCATTCTTCTACGG 92.00 JZ845608 C_SerP_cR 3’- GCAAACATCTCCTCCATCTCC ZINC Zinc metalloprotease C_Zinc bF 5’- CAGAGATGGTTTTGTGTTACTTACG 105.00 JZ845609 C_Zinc aR 3’- GCTTTGAGGCATTCATGTCC TRANS Translocon-associated protein subunit C_Trans aF 5’- TGCCCTAAAGAGGGAAAGC 102.00 JZ845610 gamma-like C_Trans aR 3’- CCACAATTAGCAGCACAAGG the FA library generated 82 contigs and 264 singletons the ESTs from the C library were related to the struc- (346 total analyzed sequences after discarding redundant tural constituent of ribosome category (See Additional sequences), indicating 54 % redundancy of this library. file 1). In both libraries, a large number of categories The average length of the ESTs was 521 bp and 781 bp in were attributed to binding (57 % and 66 % of the ESTs the C and FA libraries, respectively (Table 2). from the C and FA libraries respectively, Figs. 2 and 3); Singletons and consensus clusters were subjected to which includes protein and ion binding, organic cyclic BLATX searches in UniProt; 58 % of all the blasted se- and heterocyclic compound binding, transcription factor quences from the C library and 59 % from the FA library and DNA binding (Figs. 2 and 3). Overall, 23 categories obtained a sequence description. Most of the sequences of genes were solely found in the FA library, including of known gene function presented similarities to pre- functions such as transcription factor activity, extracellu- dicted proteins from unknown species (records that have lar matrix binding or metalloenzyme regulator activity no known mapping to the NCBI taxonomy) and P. (Fig. 3 and Additional file 2). margaritifera. The 358 sequences obtained from the C library and Confirmation of differentially expressed genes by the 346 sequences issued from the FA library were sep- RT-qPCR arately annotated and classified according to the terms Quantitative real-time PCR experiments were carried of the main Gene Ontology vocabulary for molecular out to validate gene differential expressions and to function level 3, using the Blast2GO software. 18.9 % of search for differential expression among the black (C), Lemer et al. BMC Genomics (2015) 16:568 Page 7 of 14 Table 2 General characteristics of the black (C) and Full Albino PIF, COLL, SHEM 4, MP8, ASP and PRISM). In addition, (FA) SSH libraries the expression of 2 more genes were down-regulated in CFA the HA phenotype (FLAV, ZINC) relatively to the C phenotype. Within the five up-regulated genes in the FA Sequenced clones 960 960 and HA phenotypes, two are known to play a role in the Retained sequences 677 580 shell calcitic layer biosynthesis (SHEM 1 and KRMP) Analyzed cDNA 358 346 and one is known to be involved in the shell nacreous Mean EST size (bp) 521 781 layer biosynthesis (SERP). Of the 10 down-regulated Contigs 120 82 genes found in the FA and HA phenotypes, four are EST in contigs 439 316 known to play a role in the shell calcitic layer biosyn- thesis (SHEM 9, SHEM 4, PRISM and FLAV) and one is Singletons 238 264 known to be involved in the shell nacreous layer biosyn- Redundancy (%) 18 54 thesis (PIF). The gene ZINC, involved in melanin bio- synthesis, was oppositely expressed in the two albino full albino (FA) and half albino (HA) phenotypes. Thirty condition: up-regulated in the FA condition and down- seven genes (37) associated with aragonite formation, regulated in the HA condition (Fig. 4). periostracum and calcite biosynthesis, shell protein Once the previous results were compiled, 9 genes were matrix formation, melanin biosynthesis, ion binding, oxi- finally selected for the subsequent individual RT-qPCR dase activity and transcription factors were selected to analysis based on their high differential expression level perform the expression analyses on the pooled individ- and their known or predicted role at different levels of uals (Table 1). the shell formation: SHEM 1, SHEM 4, PDZ, PIF, MP8, The RT-qPCR performed on the pooled individual in PRISM, SERP, ZINC and KRMP. In addition and al- each of the three phenotypes revealed that four genes though they did not show significant differential expres- were found to be significantly up-regulated in the FA sion in the pooled analysis, two more genes, CHIT and phenotype (SHEM 1, PEROX, SERP and ZINC) and two TYR 2a, were included in the individual RT-qPCR ana- in the HA phenotype (KRMP and SERP) relatively to the lysis because of their known role in melanin biosyn- C phenotype (one being shared with the FA phenotype; thesis. Some genes showing differential expression Fig. 4). The expression of eight other genes was down- patterns in the pooled analysis were however not in- regulated in the FA and HA phenotypes (SHEM 9, PDZ, cluded in the individual RT-qPCR analysis because they Fig. 2 TreeMap visualization of the summary of GO Molecular function found for the black phenotypes, obtained with a Revigo analysis. The size of each boxe is correlated to the frequencies of the occurence of the GO-term. Boxes with the same color are grouped by semantic similarity (SimRel, similarity = 0.7) and correspond to the same upper-hierarchy GO-term which is found in grey in the middle of each box Lemer et al. BMC Genomics (2015) 16:568 Page 8 of 14 Fig. 3 TreeMap visualization of the summary of GO Molecular function found for the full albinos phenotypes obtained with a Revigo analysis. The size of each boxe is correlated to the frequencies of the occurence of the GO-term. Boxes with the same color are grouped by semantic similarity (SimRel, similarity = 0.7) and correspond to the same upper-hierarchy GO-term which is found in grey in the middle of each box belonged to the same sub-family and showed similar ex- individual gene expression variability in the black speci- pression pattern as another gene selected for this ana- mens (1 being the value corresponding to an absence of lysis, e.g. SHEM 9, COLL or ASP. The genes SHEM 9 variation). and SHEM 4 displayed the same expression pattern and are part of the same shematrin sub-family; as a conse- Discussion quence we choose to analyze only SHEM 4 and SHEM 1 The nacreous layer of the Polynesian black-lipped pearl which both belong to two different shematrin sub- oyster Pinctada margaritifera exhibits a large array of families. The same reasoning was applied to the gene color variation among individuals. In order to study the COLL that is supposed to have a similar role as the gene corresponding molecular mechanisms behind the nacre CHIT. In addition we tried to select a somewhat equal color variation patterns, a selection of genes differentially number of genes involved in the different shell layers expressed between normal black specimens of pearl oyster (calcitic and aragonite) and in pigmentation, thus ex- and two types of albinos were identified using a SSH cluding the gene ASP, involved in the calcitic layer for- method and characterized. A subset of these genes encode mation and for which we already had selected 4 genes. for proteins involved in shell formation and in the melanin The RT-qPCR performed on each individual of each biosynthesis chain. phenotype separately, revealed the same expression pat- terns as the pooled analysis for most genes. The gene Origin of shell color induced by shell matrix protein SHEM 1 displayed a different expression pattern than expressed in the nacre and calcitic layers the one obtained from the pooled analysis: i.e. absence The genes PIF and COLL, directly involved in biominerali- of differential expression in any tested conditions. The zation of the nacreous layer, both showed higher expres- genes SHEM 4, MP8, SERP and KRMP failed to display sion in the black phenotype compared to both albino differentially significant expression patterns among phe- phenotypes in all RT-qPCR analyses for PIF and in the notypes because of high individual variability in the ex- pooled analysis for COLL. PIF codes for the gene Pmarg- pression of those genes in the C phenotype (which we Pif, previously identified by [21]. It is a homolog to Pif-177 used as a reference value, Fig. 5). In fact, individual RT- found in Pinctada fucata [62] and is known to specifically qPCR analyses revealed that the relative expression of bind to aragonite crystals. Pif-177 is an important compo- most targeted genes in the C phenotype significantly dif- nent of the nacreous layer and takes part in the initiation fered from 1 (except for SHEM 1, TYR 2a, PIF and of aragonite crystallization as well as subsequent stacking ZINC). This result highlights the high amount of inter- of aragonite tablets in the nacreous layer. Pif-177 also Lemer et al. BMC Genomics (2015) 16:568 Page 9 of 14 takes part in lamellar sheet formation and therefore it is an essential component of the organic matrix for normal growth of the nacreous layer [62]. Berland [24] showed that the down-regulation of the gene coding for Pif-177 led to a cessation of growth of the nacreous layer. The gene COLL (coding for Collagen alpha-1 XI chain precur- sor) is involved in the growth of collagen II fibrils in humans and seems to be related to chitin in bivalves [63]. Here, we show that both PIF and COLL are most likely in- 0.1 volved in the establishment of color in the shell of P. mar- garitifera. Because the proteins produced by these genes are specifically expressed in the organic matrix and regu- late the orientation of the c axis of the aragonite crystals [62, 64], we hypothesize that the color glints observed in 0.01 the nacre are entailed to the orientation of the aragonite crystals. We however encourage conducting in situ hybridization experiments in the near future to confirm this hypothesis. The weak expression of these genes in the albino phenotypes suggests that the arrangement of albino aragonite crystals differs from those of black shells and contribute to a colorless nacre. Differential expression patterns were however not de- tected for all genes involved in nacre formation. The genes coding for the proteins chitin and pearlin showed 0.1 low expression in the albino phenotypes but this differ- ential expression was not significant (0.6 instead of < 0.5 for under expression). Chitin is a polysaccharide widely distributed in living organisms and is considered to be a 0.01 fundamental template in biomineralization, as an alter- native to collagen [63]. In bivalves, chitin is involved in shell matrix scaffolding. It is the core of the organic framework surrounding the aragonite tablets in the nac- reous layer [64]. Pearlin is also known to play a direct role in the formation of the aragonite crystals in P. mar- agaritifera [12, 65–68]. The fact that these two proteins deeply involved in nacre formation are not showing dif- ferential expression patterns among phenotypes tends to indicate that the nacre is not the predominant shell layer 0.1 responsible for the color of the shell of P. margaritifera. However, this conclusion has to be regarded with cau- tion as there are many more genes involved in the for- mation of the nacreous layer that were not explored in the present work and that could display differential ex- 0.01 pression. In addition, other processes such as post- transcriptional mRNA treatments were not taken into Fig. 4 Relative gene expression levels obtained with the pooled account here. The different expression patterns observed analysis. For each tested gene, the white histogram represents the for COLL and CHIT also suggest that the two amplified expression in the full albino phenotype (FA), the grey histogram in the fragments code for two distinct proteins playing a differ- half albino (HA) and the black histogram in the black phenotype (C). TYR2a and TYR2c are 2 fragments of the same gene coding for ent role in the biomineralization process. Tyrosinase. Significant differential expressions (>2 folds) in phenotypes On the other hand, our analyses detected differential (FA) and (HA) relative to phenotype (C) are indicated by a red star expression of many genes known to be involved in the formation of the calcitic layer of P. margaritifera, sug- gesting that variation in shell color could primarily take place in the calcitic layer. These genes code for organic KRMP SHEM9 SHEM 1 TYR 2a TYR2c FLAV METH CLP3 PDZ MP10 EGF PHOS YTH YBOX COLL GIGA SHEM 4 CAL PEROX MP8 ASP MAT SHEM 8 FER PERL MP88 CHK1 LAMIN SEST FIBRO SHELL SERP ZINC TRANS PRISM PIF CHIT Normalized expression levels (log scale) Lemer et al. BMC Genomics (2015) 16:568 Page 10 of 14 formation of the calcitic layer: KRMP and SHEM 1.The two genes were found to be highly expressed in both al- bino phenotypes compared to the black phenotype, in the pooled analysis. SHEM 1 codes for another homolog 10 of the shematrin family: shematrin-1, which was classi- fied as belonging to a different sub-group of shematrins, 1 along with shematrin-2 and −3in P. fucata [69]. These differences imply that they may play a different role in 0.1 the formation of the calcitic layer and could explain the opposite expression pattern observed for SHEM 1 com- 0.01 pared to SHEM 4 and SHEM 9 in P. margaritifera. KRMP stands for lysine (K)-rich matrix protein and 0.001 codes for a matrix protein family participating in the framework formation of the prismatic layer [72]. Fig. 5 Relative gene expression levels obtained with the individual The high expression of PRISM, MP8, SHEM 4, SHEM analysis. For each tested gene, the white histogram represents the 9,and ASP and the weak expression of SHEM 1and expression in the full albino phenotype (FA), the grey histogram in the KRMP in the black phenotypes suggests that the origin half albino (HA) and the black histogram in the black phenotype (C). of color in the nacre is under the influence of genes dir- Significant differential expressions in phenotypes (FA)and (HA)relative to ectly involved in biomineralization of the calcitic layer. phenotype (C) (determined by an ANOVA) are indicated by a red star The two opposite expression patterns observed between genes highlights the complexity of this genetic process. matrix proteins produced in the mantle and were found to be highly expressed in the black specimens. Proteins Role of pigments in the origin of shell color produced in the mantle are secreted into the extrapallial A gene not directly linked to the calcification process, space, where calcium carbonate (CaCO ) crystallizes to ZINC displayed a significantly higher expression pattern build unusual microstructures [69]. The mechanism of in full albino (FA) phenotypes compared to half albino this process is unknown, but may involve interactions of (HA) and black phenotypes (C). ZINC stands for a gene the matrix proteins and inorganic ions present in the coding for a protein with a metallic zinc ion binding and extrapallial space, leading to crystallization of CaCO catalytic domain: Zinc metalloprotease. This gene is a and morphogenesis of the species-specific appearance of homolog of Tyrosinase related protein 1 [73] which is the shell [69]. Here, we detected high expressions of two known to be involved in the melanin biosynthesis path- genes in the black phenotype, PRISM and MP8, coding way: a tyrosinase-like protein detected in the periostra- for recently discovered proteins: uncharacterized shell cum layer of P. fucata by [52] and named OT47. matrix protein 18 and mantle protein 8, both believed to Tyrosinases are considered to be involved in many bio- be involved in the formation of the calcitic layer (B. logical activities of mollusks, including native immune Marin, pers. comm.). Three more genes, homologs of response [74, 75], formation of egg capsules [76–78], genes found in P. fucata and coding for proteins of the byssus [76, 79, 80], shell matrix proteins [76, 81, 82], and shematrin family, SHEM 4 and SHEM 9, and a gene periostracum [76, 83]. OT47 was found to be expressed coding for Aspein (ASP) also showed high expression in the middle fold of the mantle edge of P. fucata sup- patterns in the black phenotypes in the pooled analysis. porting the assumption that it contributes to the perios- Shematrin proteins form a family of mollusk proteins tracum development. The periostracum is an uncalcified exclusively expressed in the mantle, and particularly in layer covering the outer surface of mollusk shell. It is the edge region of the mantle [69, 20]. Recent findings among the strongest mechanically and the most chem- suggest that shematrin proteins are synthesized in the ically inert structures in the animal kingdom [76]. In mantle edge and secreted into the prismatic layer of the addition to the directly protective contribution to the or- shell, where they are thought to provide a framework for ganism, the periostracum is also thought to play a signifi- calcification and to be responsible for the toughness of cant role in shell biomineralization [84]. The periostracum the shell [69]. Aspein is an acidic protein that has a se- encloses the extrapallial space on the ventral side and iso- quence rich in aspartic acid and is expressed at the outer lates it from the external environment, enabling the forma- edge of the mantle [70]. It is the shell matrix protein tion of supersaturation conditions, which is a requisite for that controls the precipitation of calcite in P. fucata and the formation of calcified layers [85, 86]. Previous studies it is the most acidic protein found in molluscan shell also showed that it could serve as a substrate for the initial matrix protein [71]. The opposite pattern was observed deposition of calcium and even influence the prismatic for two genes coding for proteins participating in the layer formation [84, 85, 87–89]. SHEM 1 TYR 2 PDZ PIF SHEM 4 MP8 PRISM SERP ZINC KRMP CHIT Normalized expression levels (log scale) Lemer et al. BMC Genomics (2015) 16:568 Page 11 of 14 Albino phenotypes are supposed to be mainly the result Inter-individual variability in gene expression within black of the absence of melanin biosynthesis or of synthesis of phenotypes non- or partly functional melanin protein [44, 90]. The high One major outcome of the individual analysis performed expression of ZINC in the full albino phenotype compared on 11 genes is the existence of high gene expression to normal black and half albino phenotypes suggests that variability within the black phenotypes for most of the the full albino phenotype might overcompensate for a non- genes tested and mostly for SHEM 4, MP8, KRMP, CHIT functional melanin protein by over-expressing the gene and SERP, characterized by high standard deviation coding for this protein. Finally, FLAV (coding for Flavonol values and gene expression levels different than 1 (1 be- Cinnamoyl COA reductase related) was found to be highly ing the value corresponding to an absence of variation ; expressed in the black phenotypes, in the pooled analysis. Fig. 5). It is well known that what is commonly named Flavonol Cinnamoyl COA reductase is an enzyme that con- as the black-lipped pearl oyster is in fact a pool of a wide tributes in the phenylpropanoid biosyntheis in plants and is range of shell colors varying from several shades of yel- enhanced for acclimatization processes at different levels of lows, reds, greens and blues. The black specimens used photosynthetic photon flux [91–94]. Mutations in the gene in this study as a control phenotype for the individual coding for flavonols in angiosperms have shown to induce analysis, were selected randomly. The inter-individual dramatic color changes in the flower organs [95]. The gene gene expression variation detected within those samples we detected in P. magaritifera is related to the gene coding highlights the diversity of colors found in our pool of for this enzyme and although no information is available black phenotypes. This result supports the hypothesis about its role in animals, we can hypothesize that a hom- that SHEM 4, MP8, KRMP, CHIT and SERP are very ologous function to acclimatization to various levels of light likely to be involved not only in the origin of shell color can be found in P. margaritifera as the over-expression of but also in the variability of the shell color among this gene is correlated with the presence of a black mantle "black" specimens. The presence of these wide variations (which translates into melanin pigment biosynthesis in in expression patterns in the black phenotypes, used as these individuals). controls in this analysis, could also be responsible for The expression patterns of genes involved both in the preventing the detection of significant differential expres- biomineralization of the periostracum and the synthesis sion patterns in the individual analyses of albinos specimens of melanin suggest once again that the origin and poly- when those patterns were significant in the pooled analysis. morphism of the color in the nacre of P. margaritifera takes place in all the layers of the shell and in different Conclusions biological processes. Thegoalofthisstudy wastoidentifya subset of genes involved in the color of the nacreous layer of the pearl oyster Pinctada margaritifera, and assess Differential expression of genes not directly involved in their expression discrepancies. The role of the pro- biomineralization processes teins encoded by the genes showing differential ex- Two genes coding for proteins involved in metabolic pro- pression levels among the different tested phenotypes cesses showed significant differential expression among revealed that three main genetic processes were in- phenotypes: SERP and PDZ. The gene SERP coding for volved in color polymorphisms: the biomineralization Serine protease inhibitor or serpins (which is part of a of the nacreous and calcitic layers and the synthesis large family of enzymes that inhibit the activity of prote- of pigments such as melanin in the periostracum. ases) showed high expression levels in both albino condi- However, our results suggest that the color observed tions, in the pooled analysis. A limited number of serpins in the nacre is mainly under the influence of genes proteins has been reported in bivalves or mollusks and so involved in the biomineralization of the calcitic layer. far they are thought to be involved in an immune defen- A few genes involved in the formation of the aragon- sive role against invasive pathogens [94, 96]. The gene ite tablets of the nacre and in the biosynthesis chain coding for a PDZ domain protein (PDZ) showed signifi- of melanin also showed differential expression pat- cantly higher expression levels in black phenotypes than terns, suggesting that color polymorphism takes place in albino phenotypes, both in the pooled and individual at different levels in the shell structure. Color in vari- analyses. PDZ domains are one of the most frequently en- ous organisms like birds or flowers is known to be countered domains, consisting of approximately 90 amino under the influence of a combination of genes coding acid residues and identified as a region of sequence hom- for multiple pigments (melanin and carotenoid being ology among a diverse list of signaling proteins [97, 98]. the most commons in birds and anthocyanin in an- The reason why these genes exhibit differential expression giosperms) and surface structure [99, 95]. Different patterns across phenotypes remains unclear and will need gene expression arrangements seem to lead to the further explorations. extraordinary variety of colors and iridescence observed in Lemer et al. BMC Genomics (2015) 16:568 Page 12 of 14 bird feathers and flower organs [99, 95]. Similarly, it ap- Tahiti, French Polynesia. 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Identification of genes associated with shell color in the black-lipped pearl oyster, Pinctada margaritifera

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Lemer et al. BMC Genomics (2015) 16:568 DOI 10.1186/s12864-015-1776-x RESEARCH ARTICLE Open Access Identification of genes associated with shell color in the black-lipped pearl oyster, Pinctada margaritifera 1,2* 3 3,4 1 Sarah Lemer , Denis Saulnier , Yannick Gueguen and Serge Planes Background: Color polymorphism in the nacre of pteriomorphian bivalves is of great interest for the pearl culture industry. The nacreous layer of the Polynesian black-lipped pearl oyster Pinctada margaritifera exhibits a large array of color variation among individuals including reflections of blue, green, yellow and pink in all possible gradients. Although the heritability of nacre color variation patterns has been demonstrated by experimental crossing, little is known about the genes involved in these patterns. In this study, we identify a set of genes differentially expressed among extreme color phenotypes of P. margaritifera using a suppressive and subtractive hybridization (SSH) method comparing black phenotypes with full and half albino individuals. Results: Out of the 358 and 346 expressed sequence tags (ESTs) obtained by conducting two SSH libraries respectively, the expression patterns of 37 genes were tested with a real-time quantitative PCR (RT-qPCR) approach by pooling five individuals of each phenotype. The expression of 11 genes was subsequently estimated for each individual in order to detect inter-individual variation. Our results suggest that the color of the nacre is partially under the influence of genes involved in the biomineralization of the calcitic layer. A few genes involved in the formation of the aragonite tablets of the nacre layer and in the biosynthesis chain of melanin also showed differential expression patterns. Finally, high variability in gene expression levels were observed within the black phenotypes. Conclusions: Our results revealed that three main genetic processes were involved in color polymorphisms: the biomineralization of the nacreous and calcitic layers and the synthesis of pigments such as melanin, suggesting that color polymorphism takes place at different levels in the shell structure. The high variability of gene expression found within black phenotypes suggests that the present work should serve as a basis for future studies exploring more thoroughly the expression patterns of candidate genes within black phenotypes with different dominant iridescent colors. Keywords: Differential expression, Biomineralization, Nacre, Pearl, Pigmentation, Albino Background basis [3–5]. Most studies have reported a relatively sim- Color polymorphisms of mollusk shells have fascinated ple genetic basis for color polymorphisms involving one humans throughout history. Color polymorphism results or two loci with dominance [6–8], although more elabor- from a combination of multiple factors starting from ate polymorphisms are likely to arise from complex mul- inner genetic to mitigating environmental factors related tigenic systems [9, 10]. Recent studies have shown that to biochemistry, substrate or nutrition [1, 2]. Experimen- marine shellfish tend to display more complex patterns tal crosses have shown that shell color has a genetic of pigmentation than freshwater shellfish [11]. Examples of complexity are mostly found on shells displaying a nacreous layer (i.e. mother of pearl) such as the gastro- * Correspondence: [email protected] Laboratoire d’Excellence “CORAIL”, USR 3278 CNRS-CRIOBE- EPHE, Perpignan, pods in the family Haliotidae (abalone), the cephalopod France, Papetoai, Moorea, French Polynesia 2 family Nautlidae (chambered Nautilus), or the bivalves Present address: Department of Organismic and Evolutionary Biology, of the families Pteriidae, Mytilidae and Pinnidae. Because Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA of its high industrial value, the composition and Full list of author information is available at the end of the article © 2015 Lemer et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Lemer et al. BMC Genomics (2015) 16:568 Page 2 of 14 formation of the nacreous layer have been studied in multiple taxa at different levels of expertise: proteomics [12–21], genetics [22–26, 21], optics [27, 28], mineralogy [29] and chemistry [30–32]. The nacre is made of regu- lar layers of aragonite tiles and is unique from its irides- cence. Iridescence is usually attributed to a diffraction effect caused by the evenly grooved surface microstruc- ture, similar to that of a diffraction grating [33, 34, 27]. The rainbow-like diffraction colors arise from surface repetition of the nacreous layers (growth lines) and are named by gemologists “orient” [29]. However, not all iri- descences arise from surface regularities and the pig- ments present in the binding regions of the aragonite tiles and integrated in the biomineralization process also play a role in the color of the nacreous layer [29]. There- fore the process of biomineralization of the nacreous layer is of great economic interest to the pearl aquacul- ture industry. As a result, extensive studies have been conducted to identify proteins responsible for the nacre formation by screening proteins contained in the shell and genes specifically expressed in the calcifying tissues i.e. the mantle [35, 36]. A wide variety of proteins and genes have been identified and their functions in nacre formation have been partially characterized (reviewed in [35, 37–41]). Our knowledge of the “biomineralization toolkit” in- volved in the formation of the calcitic and aragonitic layers is expanding and we now know more about how the different shell layers are synthesized and how their production is genetically regulated [37]. Never- theless the precise molecular mechanism controlling color polymorphism in nacre remains unexplored and Fig. 1 Picture of dissected specimens of Pinctada margaritifera of the three studied phenotypes. a. Full albino (FA; white shell and is far from being understood. To address this key white mantle), b. half albino (HA; white shell and black mantle), c. question we searched for specific genes displaying dif- black phenotype (C; black shell and black mantle) ferential expression patterns among different shell color phenotypes of the Polynesian black-lipped pearl oyster, Pinctada margaritifera. The nacreous layer of albino specimens and black specimens is an ideal P. margaritifera exhibits a large array of color vari- method to identify the genetic processes directly ation among individuals, although the shell is usually linked to shell color polymorphisms. simply referred to as black. The heritability of nacre Albinism is induced by the absence of melanin or by color variation patterns in P. margaritifera have been non-functional melanin molecules as a result of one or demonstrated by experimental crossing and by main- more mutations in genes involved in the melanin bio- synthesis chain [43–49]. Melanin has been identified in taining single colored dominated lineages (CL. Ky personal communication). An interesting feature of P. many mollusk species, including cephalopods, gastro- margaritifera is the rare occurrence of albinos, 1 out pods, and bivalves [50]. In P. margaritifera, melanin is secreted in the epithelium of the median and internal of 10,000, displaying total or partial absence of color- ation. Albinos are characterized by a white shell (peri- fold of the mantle edge and in the pallial area [51]. The ostracum and calcitic layer) and a white mantle mantle edge is directly responsible for the biomineraliza- tion of the shell and more specifically of the nacre layer (Fig. 1a) in opposition to the common black shells (Fig.1c).Insomerare cases themantle ofalbino [21]. As a consequence, it is the chosen mantle part used specimens remains black (S. Planes and S. Lemer, by professional grafters in pearl culture manufacture to pers. observation; [42]; Fig. 1b); supporting the mul- isolate grafts , that will be transplanted together with a tiple genetic origins of the absence of color in the marble in the gonads of a recipient oyster. Three pro- shell. In the context of our study, the comparison of teins closely related to the tyrosinase, known to play a Lemer et al. BMC Genomics (2015) 16:568 Page 3 of 14 role in melanin biosynthesis, have previously been char- Total RNA was purified using the RNeasy Mini Kit acterized in the pallial or edge region of the mantle of (Qiagen, USA). Pinctada fucata: OT47, Pfty1 and Pfty2 [52, 53]. The The suppressive subtractive hybridization technique localization of these proteins suggests that they could be (SSH) [54] was used to characterize genes involved in the integrated in the shell layers including the nacre layer origin of shell color by comparing expression between the during the biomineralization process. phenotypes C and FA. For the forward library, cDNA from In this study, we aimed at identifying genes in- five phenotype FA samples was used as tester and cDNA volved in the origin and potentially the polymorphism from five phenotype C samples was used as driver, and of color in the nacreous layer of P. margaritifera.We vice versa for the reverse subtractive library. The construc- hypothesized that the variety of colors found in the tion of the libraries, clone sequencing, vector and adaptor nacre is a combined result of variation in expression trimming and differential screening using dot blot of genes involved in the formation of the shell’s hybridization were outsourced to Rx. Bioscience Ldt microstructure and of genes involved in pigmentation. (Maryland, USA). Briefly, both forward and reverse sub- In order to test this hypothesis, we identified and tracted libraries were produced from 2 ng mRNA. First compared coding genes differentially expressed be- and second strand cDNA synthesis, RsaI endonuclease tween normal black specimen of pearl oysters and enzyme digestion, adapter ligation, hybridization, and two types of albinos using a suppressive and subtract- PCR amplification were performed as described in the ive hybridization method, that we then tested and val- PCR-select cDNA subtraction kit manual (Clontech, Palo idated with RT-qPCR runs. We identified genes Alto, CA, USA) with modifications as described in [55] involved in the formation of the calcitic layer, the ara- for the Mirror Orientation Selection protocol. Differen- gonite tabletsof the nacreand in thebiosynthesis tially expressed PCR products were ligated into pJetBlunt chain of melanin suggesting that color polymorphism cloning vector (Fermentas) and transformed in Escheri- takes place at different levels of the shell structure. chia coli competent cells. For the differential screening by dot blotting, 1000 clones per library were randomly trans- ferred on two nylon membranes and hybridized with C Methods and FA cDNA probes, respectively. Nylon membranes Biological material were then auto-radiographed and superimposed for iden- In order to identify genes involved in the origin and po- tification of differentially expressed clones. Among clones tentially in the variation of shell color, mantle tissue of showing the most intense differential signal after Pinctada margaritifera from 3 phenotypes was obtained hybridization to cDNA probes, 960 were randomly se- from adult specimens of similar size and raised in the lected for sequencing in each library. same pearl farm in the Gambier archipelago (Mangareva, French Polynesia) in 2011. The three sampled pheno- types were: the normal black phenotype of P. margariti- Sequence analysis fera (black shell and black mantle) referred hereafter as For each library, the 960 clones were sequenced and vector phenotype C (Fig. 1b); the full albino phenotype (white trimmed by RxBioScience Ltd. (Maryland, USA). High- shell and white mantle) referred hereafter as phenotype quality expressed sequenced tag (ESTs) (>100 bp) were as- FA (Fig. 1a); the half albino phenotype (white shell and sembled into clusters or identified as unique sequences and black mantle) referred hereafter as phenotype HA.We used for database searches with the BlastX and BlastN pro- sampled five individuals of each phenotype. The mantle grams on the NCBI server (http://www.ncbi.nlm.nih.gov/ edge of each individual was dissected by a professional BLAST/) and UniProt. (http://www.uniprot.org/blast/) grafter following the method used for pearl production, Search of homology was also conducted in an EST bank of in order to insure the use of the specific part of the Pinctada margaritifera [21]. In addition, functional annota- mantle producing the calcitic and aragonite layers. Each tion was performed in Blast2Go (http://www.blast2go.com/) sample was immediately preserved in RNAlater RNA where gene information were obtain by blasting sequences Stabilization Reagent (Qiagen, USA) transported to the in Gene Ontology (http://www.geneontology.org/). Enriched laboratory and stored at −80 °C. Molecular function GO terms were then uploaded to REVIGO (Reduce + Visualize GeneOntologyhttp://revi- RNA isolation and subtractive cDNA library construction go.irb.hr/) for visualization. REVIGO summarizes the long Total RNA from mantle tissue of individuals of phenotypes list of GO terms by removing redundant terms and group- C, HA and FA was isolated using TRIzol reagent (Invitro- ing related terms based on semantic similarity [56]. The gen) according to the manufacturer’s instructions. The EST sequences used in this study have been submitted to RNA pellet was washed with 70 % ethanol and air-dried, the online database (Accession numbers: JZ845577- dissolved in DEPC-treated water and stored at −80 °C. JZ845610; JZ845790-JZ845792). Lemer et al. BMC Genomics (2015) 16:568 Page 4 of 14 Differential expression validated by quantitative RT-PCR RT-qPCR data analysis Quantitative real-time PCR (RT-qPCR) was used to analyze The Ct values of the replicates for each housekeeping the expression profiles of selected genes. Total RNA was gene and each candidate gene of each phenotype (C, FA extracted and reverse-transcribed (2 μg) using oligo dT and HA pooled or individual samples) were reported. primers and the Superscript II enzyme (Invitrogen). For each candidate gene, the level of transcription was At first, the cDNA from each of the five individuals normalized using the following calculation: for each phenotype (C, FA and HA) were pooled in ΔCt ¼ Ct −Ct Target M equal amounts (25 ng/μL each). Quantitative RT-PCR analyses were run on the three pooled samples for all se- where Ct is the mean Ct of the target gene in one Target lected genes. The genes showing the most significant dif- of the tested phenotype (C, FA and HA pooled or indi- ferential expression profile among the three phenotypes vidual samples) and Ct is the mean Ct of the house- were then selected to conduct subsequent RT-qPCR ana- keeping genes in that same phenotype. lysis on each individual sample separately in order to de- Relative quantification of gene expressions was estimated tect individual variability (Table 1). for each gene in each phenotype using the ΔΔCt method as Both pooled and individual RT-qPCR analysis were described in [59]. Relative quantification relates the PCR conducted as the following: in each 25 μl reaction, 10 μl signal of the target transcript (here FA or HA phenotypes) cDNA (diluted 1:100 in water) was mixed with 12.5 μl in a treatment group to that of another sample (here C Brillant® II SYBR Green Master Mix (Stratagene) and phenotype). We used the following equation: 0.2 μM of each primer (primer pairs listed in Table 1). ΔΔCt ¼ ΔCt − ΔCt This allowed for the consistent use of standardized ther- Target C mal cycling conditions performed using a Mx3000P where ΔCt is the ΔCt obtained for a target gene in Target Real-Time PCR System (Stratagene): 95 °C for 10 min, one of the tested phenotype (FA and HA pooled or indi- followed by 40 cycles of 95 °C for 30 s, 60 °C for 1 min vidual samples) and ΔCt is the ΔCt obtained for that and 72 °C for 1 min which were found to give efficien- same gene in the phenotype C. In the pooled analysis cies > 90 % (see below). The RT-qPCR reactions were ΔCt is the ΔCt of the pooled C individuals whereas in run in triplicate for the pooled analysis and in duplicate the individual analysis the ΔCt is the mean ΔCt ob- for the individual analyses. Melting curve profiles (95 to tained for each individual from the C phenotype. 55 °C decreasing by 1 °C every 30 s) were assayed to en- Finally the relative expression fold for each gene in sure that we were looking at a single product. Each run each tested condition in relation to the phenotype C was included three positive cDNA controls also used as estimated using the following equation: interplate calibrators, and three non-DNA template con- trols (water) for each primer pair and four housekeeping  ðÞ ΔΔCt E 2 genes: RNA and export factor binding protein 1 or REF 1 [57], 18S [58], Elongation factor 1 or EF1 (EF1S: where E is the reaction efficiency for each target gene. ATGCTGCCATGGTTGATATG/EF1: GTGGCCTCAGC For each target gene in each tested phenotype of the TTTCTCAAC) and Glyceraldehyde-3-Phosphate De- pooled analysis, a relative expression fold superior to 2 hydrogenase or GAPDH (GAPDH1S: AGGCTTGAT- was considered as significantly differentially expressed GACCACTGTCC / GAPDH1R: AGCCATTCCCGTCA [60, 61]. For the individual analysis, significances among ACTTC). EF1 and GADPH were chosen based on mean relative expression folds of each gene were deter- their ubiquitous and constitutive expression pattern in mined by an analysis of variance (ANOVA). P. margaritifera tissue. A 10-fold dilution series was created from a random Results pool of cDNA from our samples (including C, FA and HA Subtractive cDNA library construction and sequencing phenotypes), ranging from × 100 dilutions to × 100,000 di- Two SSH libraries were produced to identify differentially lutions. Triplicate RT-qPCR reactions were carried out as expressed genes between normally black colored and full described above for each gene at each dilution. Mean cycle albinos specimens of Pinctada margaritifera.Nine hun- threshold (Ct) values for each dilution were plotted dred and sixty (960) clones were sequenced from each li- against the log10 of the cDNA input for each gene to gen- brary, yielding 677 and 580 high quality cDNA sequences erate efficiency plots. The reaction efficiency of each gene (>100 bp), after removing low-quality regions and screen- assay was calculated using the following equation: E = ing for vector contamination, from the C and FA libraries 10(−1/slope) where E is the reaction efficiency and respectively. The ESTs from the C library generated 120 ‘slope’ is the slope of the line generated in the efficiency contigs and 238 singletons (358 total analyzed sequences plots. Only the genes yielding an efficiency comprised be- after discarding redundant sequences), indicating that the tween 90 % and 110 % were retained. overall redundancy of the library was 18 %. The ESTs from Lemer et al. BMC Genomics (2015) 16:568 Page 5 of 14 Table 1 Characteristics of the genes selected for the pooled and individual RT-qPCR analyses. The proteins, primers and efficiencies values are indicated. The genes selected for the individual analysis are indicated in bold Gene code Protein Primers Efficiencies % Genbank accession # KRMP Lysine (K)-rich mantle protein Joubert et al. 2014 95 JZ845792 SHEM 9B Shematrin 9 A_shem9b 5’- F CCCCGTATCCTCCATATCC 101 JZ845577 A_shem9b 3’- R GCTATTACCGGAGTACCCTACG SHEM 1B Shematrin 1 C_Shem1b F 5’- CGCTATCGTTGCTCTCATTG 101.00 JZ845578 C_Shem1b R 3’- TCCACCTCCTCCTCCTCTTC TYR 2A Tyrosinase 2 A_tyr2a F 5’- GCGGCTCTACTGTCAAATGG 112.00 JZ845579 A_tyr2b R 3’- CTGGACCTTTCAGGGACTGG TYR 2C Tyrosinase 2 A tyr2c F 5’- CCCGTGGCCTGGATAGTC 101.00 JZ845580 A tyr2c R 3’- TTTTCTTCCATCACTGCTACATTG FLAV Flavonol cinnamoyl CoA reductase-related A_flav aF 5’- AGCAGGGTTATCACGTCAGG 96.00 JZ845581 A_flav bR 3’- ATCCTTTGGTGCCTGTGC METH Methionine-rich nacre protein A_Meth bF 5’- ATGCGGAGGATACTGTGCTT 100.00 JZ845582 A_Meth bR 3’- CGGGGCTGGATAGACTCATA CLP3 Chitinase 3 protein A_clp3b F 5’- TCACAAAATGGATCATAACGTACC 94.00 JZ845583 A_clp3b R 3’- GGACTGCCTTTGAATGTCG PDZ PDZ domain protein A_PDZa F 5’- TGAGCTTCAGAGAGGTGACG 100.00 JZ845584 A_PDZa R 3’- AACATTTGGTGAGGGTTTGG MP10 Mantle protein 10 A_MP10d F 5’- ATTATGGACCGGGCAAGC 100.00 JZ845585 A_MP10c R 3’- GAGGACAGGAACATCAACAGG EGF Epidermal growth factor receptor A_EGF aF 5’- CATTCCCCATCTTCCTCAAA 108.00 JZ845586 A-EGF aR 3’- GGACTTCCTGGGATGTTGTC PHOS Phosphotyrosine protein phosphatase A_Phos bF 5’- AATTATGACACAGAGGGATCTAAGG 98.00 JZ845587 A_Phos aR 3’- CATAGAAACCACCACAACATCG YTH YTH domain family protein 2-like A_YTH aF 5’- AATTGAGACACATTCGGTTGG 93.00 JZ845588 A_YTH aR 3’- TTACCGCTCTCTGTCCTTGC PIF Pif177-like protein A_pif aF 5’- TTTTGAATTACACGACTGCTTTG 93.00 JZ845589 A_pif aR 3’- TTCAGTAGAACTAACGCTAAATCCAG CHIT Chitin synthase A_Chit aF 5’- AATCCAATTTCCCGCAGTC 90.00 JZ845590 A_Chit bR 3’- TTGTTGTAGACATTAGCGACGTATC YBOX Y-box binding protein A_Ybox aF 5’- CGTACATCAAACTGCCATTACC 97.00 JZ845591 A_Ybox aR 3’- CTACATCAAATTCCACCTTCTCC COLL Collagen alpha-1 (XI) chain precursor A_Coll bF 5’- CGTCCAATCAATCCAGGTG 105.00 JZ845592 A_Coll aR 3’- TGGGTCCAAAAGGTGAAATG GIGA Gigasin A_Giga aF 5’- TCTCTTCCGGTCAAAAATGC 91.00 JZ845593 A_Giga aR 3’- TCGCTGTACTATTTCCGTTCG SHEM 4 Shematrin 4 A_Shem4a F 5’- GCTTCCCATCGGTTTATGG 92.00 JZ845594 A_Shem4b 3’- R TGCCAACATTTCCGTATCC CAL Calreticulin A_Cal aF 5’- TCACCATCCATTTCATCATCC 102.00 JZ845595 A_Cal aR 3’- ACCAGAGGACTGGGACAAGC PEROX Peroxidase A_Perox aF 5’- TGCTGGGACTCACTCTATCCA 107.00 JZ845596 A_Perox bR 3’- TCAAGCCATCAAAGAAACATCTT MP8 Mantle protein 8 Joubert et al. 2014 101.00 JZ845791 ASP Aspein shell matrix protein Joubert et al. 2014 99.00 JZ845790 Lemer et al. BMC Genomics (2015) 16:568 Page 6 of 14 Table 1 Characteristics of the genes selected for the pooled and individual RT-qPCR analyses. The proteins, primers and efficiencies values are indicated. The genes selected for the individual analysis are indicated in bold (Continued) MAT Matrilin Cartilage matrix protein A_Mat aF 5’- TTGTTCTGGATGGGTCTTCG 109.00 JZ845597 A_Mat aR 3’- TCAAAGCCCGCACATCAG SHEM 8 Shematrin 8 C_Shem8 bF 5’- CTCCACCACCAATGACGATT 111.00 JZ845598 C_Shem8 aR 3’- TTTCGGGGGTGTTAACGTAG FER Ferritin C_Fer cF 5’- GGGCCTGATTGACAGACTTCT 96.00 JZ845599 C_Fer cR 3’- GAGGGCGCATTGTCCTTC PERL Perlin matrix protein C_Perl bF 5’- TTCCAGATATTACACCCTGTGCT 100.00 JZ845600 C_Perl aR 3’- CGTTACCGTTTCCACCAAAA PRISM Prism uncharacterized shell protein C_Prism aF 5’- TGGTTCCAAGTGTATTTGTCCA 96.00 JZ845601 C_Prism aR 3’- TCCATAGACGCACACCTTTG MP88 molluscan prismatic and nacreous layer 88 C_MP88a F 5’- TCTGTGAAAATTTTGATAAACTGAA 109.00 JZ845602 C_MP88a R 3’- GAATAAAAGTTTAATGTTCCATTCCT CHK1 Checkpoint-like protein C_CHK1a F 5’- CGGGCAGGTACTCATTCC 95.00 JZ845603 C_CHK1b R 3’- GCTACCACATCCAAGGAAGG LAMIN Laminin receptor precursor C_Lamin bF 5’- GAAGCTGAGAAAGAAGAACAGACC 101.00 JZ845604 C_Lamin aR 3’- TGTTGTGGTGGCTGTATTGC SEST Sestrin 1-like protein C_Sest aF 5’- TTTTTGCTACCAGGACTTTGC 91.00 JZ845605 C_Sest aR 3’- GTGTCAACATCTGTCATCTCACC FIBRO Fibronectin 3 C_Fibro aF 5’- AGTAGCTGATCGTGCATTCG 97.00 JZ845606 C_Fibro bR 3’- AACTCACAGGACACCAAAACG SHELL Nacre uncharacterize shell protein 6 C_Shell_cF 5’- CGTTCTATCCCTGGTCATCC 101.00 JZ845607 C_Shell_dR 3’- GGACGTGGATTTTCCTTGG SERP Serine protease inhibitor C_SerP_cF 5’- AGGTGTGTACCATTCTTCTACGG 92.00 JZ845608 C_SerP_cR 3’- GCAAACATCTCCTCCATCTCC ZINC Zinc metalloprotease C_Zinc bF 5’- CAGAGATGGTTTTGTGTTACTTACG 105.00 JZ845609 C_Zinc aR 3’- GCTTTGAGGCATTCATGTCC TRANS Translocon-associated protein subunit C_Trans aF 5’- TGCCCTAAAGAGGGAAAGC 102.00 JZ845610 gamma-like C_Trans aR 3’- CCACAATTAGCAGCACAAGG the FA library generated 82 contigs and 264 singletons the ESTs from the C library were related to the struc- (346 total analyzed sequences after discarding redundant tural constituent of ribosome category (See Additional sequences), indicating 54 % redundancy of this library. file 1). In both libraries, a large number of categories The average length of the ESTs was 521 bp and 781 bp in were attributed to binding (57 % and 66 % of the ESTs the C and FA libraries, respectively (Table 2). from the C and FA libraries respectively, Figs. 2 and 3); Singletons and consensus clusters were subjected to which includes protein and ion binding, organic cyclic BLATX searches in UniProt; 58 % of all the blasted se- and heterocyclic compound binding, transcription factor quences from the C library and 59 % from the FA library and DNA binding (Figs. 2 and 3). Overall, 23 categories obtained a sequence description. Most of the sequences of genes were solely found in the FA library, including of known gene function presented similarities to pre- functions such as transcription factor activity, extracellu- dicted proteins from unknown species (records that have lar matrix binding or metalloenzyme regulator activity no known mapping to the NCBI taxonomy) and P. (Fig. 3 and Additional file 2). margaritifera. The 358 sequences obtained from the C library and Confirmation of differentially expressed genes by the 346 sequences issued from the FA library were sep- RT-qPCR arately annotated and classified according to the terms Quantitative real-time PCR experiments were carried of the main Gene Ontology vocabulary for molecular out to validate gene differential expressions and to function level 3, using the Blast2GO software. 18.9 % of search for differential expression among the black (C), Lemer et al. BMC Genomics (2015) 16:568 Page 7 of 14 Table 2 General characteristics of the black (C) and Full Albino PIF, COLL, SHEM 4, MP8, ASP and PRISM). In addition, (FA) SSH libraries the expression of 2 more genes were down-regulated in CFA the HA phenotype (FLAV, ZINC) relatively to the C phenotype. Within the five up-regulated genes in the FA Sequenced clones 960 960 and HA phenotypes, two are known to play a role in the Retained sequences 677 580 shell calcitic layer biosynthesis (SHEM 1 and KRMP) Analyzed cDNA 358 346 and one is known to be involved in the shell nacreous Mean EST size (bp) 521 781 layer biosynthesis (SERP). Of the 10 down-regulated Contigs 120 82 genes found in the FA and HA phenotypes, four are EST in contigs 439 316 known to play a role in the shell calcitic layer biosyn- thesis (SHEM 9, SHEM 4, PRISM and FLAV) and one is Singletons 238 264 known to be involved in the shell nacreous layer biosyn- Redundancy (%) 18 54 thesis (PIF). The gene ZINC, involved in melanin bio- synthesis, was oppositely expressed in the two albino full albino (FA) and half albino (HA) phenotypes. Thirty condition: up-regulated in the FA condition and down- seven genes (37) associated with aragonite formation, regulated in the HA condition (Fig. 4). periostracum and calcite biosynthesis, shell protein Once the previous results were compiled, 9 genes were matrix formation, melanin biosynthesis, ion binding, oxi- finally selected for the subsequent individual RT-qPCR dase activity and transcription factors were selected to analysis based on their high differential expression level perform the expression analyses on the pooled individ- and their known or predicted role at different levels of uals (Table 1). the shell formation: SHEM 1, SHEM 4, PDZ, PIF, MP8, The RT-qPCR performed on the pooled individual in PRISM, SERP, ZINC and KRMP. In addition and al- each of the three phenotypes revealed that four genes though they did not show significant differential expres- were found to be significantly up-regulated in the FA sion in the pooled analysis, two more genes, CHIT and phenotype (SHEM 1, PEROX, SERP and ZINC) and two TYR 2a, were included in the individual RT-qPCR ana- in the HA phenotype (KRMP and SERP) relatively to the lysis because of their known role in melanin biosyn- C phenotype (one being shared with the FA phenotype; thesis. Some genes showing differential expression Fig. 4). The expression of eight other genes was down- patterns in the pooled analysis were however not in- regulated in the FA and HA phenotypes (SHEM 9, PDZ, cluded in the individual RT-qPCR analysis because they Fig. 2 TreeMap visualization of the summary of GO Molecular function found for the black phenotypes, obtained with a Revigo analysis. The size of each boxe is correlated to the frequencies of the occurence of the GO-term. Boxes with the same color are grouped by semantic similarity (SimRel, similarity = 0.7) and correspond to the same upper-hierarchy GO-term which is found in grey in the middle of each box Lemer et al. BMC Genomics (2015) 16:568 Page 8 of 14 Fig. 3 TreeMap visualization of the summary of GO Molecular function found for the full albinos phenotypes obtained with a Revigo analysis. The size of each boxe is correlated to the frequencies of the occurence of the GO-term. Boxes with the same color are grouped by semantic similarity (SimRel, similarity = 0.7) and correspond to the same upper-hierarchy GO-term which is found in grey in the middle of each box belonged to the same sub-family and showed similar ex- individual gene expression variability in the black speci- pression pattern as another gene selected for this ana- mens (1 being the value corresponding to an absence of lysis, e.g. SHEM 9, COLL or ASP. The genes SHEM 9 variation). and SHEM 4 displayed the same expression pattern and are part of the same shematrin sub-family; as a conse- Discussion quence we choose to analyze only SHEM 4 and SHEM 1 The nacreous layer of the Polynesian black-lipped pearl which both belong to two different shematrin sub- oyster Pinctada margaritifera exhibits a large array of families. The same reasoning was applied to the gene color variation among individuals. In order to study the COLL that is supposed to have a similar role as the gene corresponding molecular mechanisms behind the nacre CHIT. In addition we tried to select a somewhat equal color variation patterns, a selection of genes differentially number of genes involved in the different shell layers expressed between normal black specimens of pearl oyster (calcitic and aragonite) and in pigmentation, thus ex- and two types of albinos were identified using a SSH cluding the gene ASP, involved in the calcitic layer for- method and characterized. A subset of these genes encode mation and for which we already had selected 4 genes. for proteins involved in shell formation and in the melanin The RT-qPCR performed on each individual of each biosynthesis chain. phenotype separately, revealed the same expression pat- terns as the pooled analysis for most genes. The gene Origin of shell color induced by shell matrix protein SHEM 1 displayed a different expression pattern than expressed in the nacre and calcitic layers the one obtained from the pooled analysis: i.e. absence The genes PIF and COLL, directly involved in biominerali- of differential expression in any tested conditions. The zation of the nacreous layer, both showed higher expres- genes SHEM 4, MP8, SERP and KRMP failed to display sion in the black phenotype compared to both albino differentially significant expression patterns among phe- phenotypes in all RT-qPCR analyses for PIF and in the notypes because of high individual variability in the ex- pooled analysis for COLL. PIF codes for the gene Pmarg- pression of those genes in the C phenotype (which we Pif, previously identified by [21]. It is a homolog to Pif-177 used as a reference value, Fig. 5). In fact, individual RT- found in Pinctada fucata [62] and is known to specifically qPCR analyses revealed that the relative expression of bind to aragonite crystals. Pif-177 is an important compo- most targeted genes in the C phenotype significantly dif- nent of the nacreous layer and takes part in the initiation fered from 1 (except for SHEM 1, TYR 2a, PIF and of aragonite crystallization as well as subsequent stacking ZINC). This result highlights the high amount of inter- of aragonite tablets in the nacreous layer. Pif-177 also Lemer et al. BMC Genomics (2015) 16:568 Page 9 of 14 takes part in lamellar sheet formation and therefore it is an essential component of the organic matrix for normal growth of the nacreous layer [62]. Berland [24] showed that the down-regulation of the gene coding for Pif-177 led to a cessation of growth of the nacreous layer. The gene COLL (coding for Collagen alpha-1 XI chain precur- sor) is involved in the growth of collagen II fibrils in humans and seems to be related to chitin in bivalves [63]. Here, we show that both PIF and COLL are most likely in- 0.1 volved in the establishment of color in the shell of P. mar- garitifera. Because the proteins produced by these genes are specifically expressed in the organic matrix and regu- late the orientation of the c axis of the aragonite crystals [62, 64], we hypothesize that the color glints observed in 0.01 the nacre are entailed to the orientation of the aragonite crystals. We however encourage conducting in situ hybridization experiments in the near future to confirm this hypothesis. The weak expression of these genes in the albino phenotypes suggests that the arrangement of albino aragonite crystals differs from those of black shells and contribute to a colorless nacre. Differential expression patterns were however not de- tected for all genes involved in nacre formation. The genes coding for the proteins chitin and pearlin showed 0.1 low expression in the albino phenotypes but this differ- ential expression was not significant (0.6 instead of < 0.5 for under expression). Chitin is a polysaccharide widely distributed in living organisms and is considered to be a 0.01 fundamental template in biomineralization, as an alter- native to collagen [63]. In bivalves, chitin is involved in shell matrix scaffolding. It is the core of the organic framework surrounding the aragonite tablets in the nac- reous layer [64]. Pearlin is also known to play a direct role in the formation of the aragonite crystals in P. mar- agaritifera [12, 65–68]. The fact that these two proteins deeply involved in nacre formation are not showing dif- ferential expression patterns among phenotypes tends to indicate that the nacre is not the predominant shell layer 0.1 responsible for the color of the shell of P. margaritifera. However, this conclusion has to be regarded with cau- tion as there are many more genes involved in the for- mation of the nacreous layer that were not explored in the present work and that could display differential ex- 0.01 pression. In addition, other processes such as post- transcriptional mRNA treatments were not taken into Fig. 4 Relative gene expression levels obtained with the pooled account here. The different expression patterns observed analysis. For each tested gene, the white histogram represents the for COLL and CHIT also suggest that the two amplified expression in the full albino phenotype (FA), the grey histogram in the fragments code for two distinct proteins playing a differ- half albino (HA) and the black histogram in the black phenotype (C). TYR2a and TYR2c are 2 fragments of the same gene coding for ent role in the biomineralization process. Tyrosinase. Significant differential expressions (>2 folds) in phenotypes On the other hand, our analyses detected differential (FA) and (HA) relative to phenotype (C) are indicated by a red star expression of many genes known to be involved in the formation of the calcitic layer of P. margaritifera, sug- gesting that variation in shell color could primarily take place in the calcitic layer. These genes code for organic KRMP SHEM9 SHEM 1 TYR 2a TYR2c FLAV METH CLP3 PDZ MP10 EGF PHOS YTH YBOX COLL GIGA SHEM 4 CAL PEROX MP8 ASP MAT SHEM 8 FER PERL MP88 CHK1 LAMIN SEST FIBRO SHELL SERP ZINC TRANS PRISM PIF CHIT Normalized expression levels (log scale) Lemer et al. BMC Genomics (2015) 16:568 Page 10 of 14 formation of the calcitic layer: KRMP and SHEM 1.The two genes were found to be highly expressed in both al- bino phenotypes compared to the black phenotype, in the pooled analysis. SHEM 1 codes for another homolog 10 of the shematrin family: shematrin-1, which was classi- fied as belonging to a different sub-group of shematrins, 1 along with shematrin-2 and −3in P. fucata [69]. These differences imply that they may play a different role in 0.1 the formation of the calcitic layer and could explain the opposite expression pattern observed for SHEM 1 com- 0.01 pared to SHEM 4 and SHEM 9 in P. margaritifera. KRMP stands for lysine (K)-rich matrix protein and 0.001 codes for a matrix protein family participating in the framework formation of the prismatic layer [72]. Fig. 5 Relative gene expression levels obtained with the individual The high expression of PRISM, MP8, SHEM 4, SHEM analysis. For each tested gene, the white histogram represents the 9,and ASP and the weak expression of SHEM 1and expression in the full albino phenotype (FA), the grey histogram in the KRMP in the black phenotypes suggests that the origin half albino (HA) and the black histogram in the black phenotype (C). of color in the nacre is under the influence of genes dir- Significant differential expressions in phenotypes (FA)and (HA)relative to ectly involved in biomineralization of the calcitic layer. phenotype (C) (determined by an ANOVA) are indicated by a red star The two opposite expression patterns observed between genes highlights the complexity of this genetic process. matrix proteins produced in the mantle and were found to be highly expressed in the black specimens. Proteins Role of pigments in the origin of shell color produced in the mantle are secreted into the extrapallial A gene not directly linked to the calcification process, space, where calcium carbonate (CaCO ) crystallizes to ZINC displayed a significantly higher expression pattern build unusual microstructures [69]. The mechanism of in full albino (FA) phenotypes compared to half albino this process is unknown, but may involve interactions of (HA) and black phenotypes (C). ZINC stands for a gene the matrix proteins and inorganic ions present in the coding for a protein with a metallic zinc ion binding and extrapallial space, leading to crystallization of CaCO catalytic domain: Zinc metalloprotease. This gene is a and morphogenesis of the species-specific appearance of homolog of Tyrosinase related protein 1 [73] which is the shell [69]. Here, we detected high expressions of two known to be involved in the melanin biosynthesis path- genes in the black phenotype, PRISM and MP8, coding way: a tyrosinase-like protein detected in the periostra- for recently discovered proteins: uncharacterized shell cum layer of P. fucata by [52] and named OT47. matrix protein 18 and mantle protein 8, both believed to Tyrosinases are considered to be involved in many bio- be involved in the formation of the calcitic layer (B. logical activities of mollusks, including native immune Marin, pers. comm.). Three more genes, homologs of response [74, 75], formation of egg capsules [76–78], genes found in P. fucata and coding for proteins of the byssus [76, 79, 80], shell matrix proteins [76, 81, 82], and shematrin family, SHEM 4 and SHEM 9, and a gene periostracum [76, 83]. OT47 was found to be expressed coding for Aspein (ASP) also showed high expression in the middle fold of the mantle edge of P. fucata sup- patterns in the black phenotypes in the pooled analysis. porting the assumption that it contributes to the perios- Shematrin proteins form a family of mollusk proteins tracum development. The periostracum is an uncalcified exclusively expressed in the mantle, and particularly in layer covering the outer surface of mollusk shell. It is the edge region of the mantle [69, 20]. Recent findings among the strongest mechanically and the most chem- suggest that shematrin proteins are synthesized in the ically inert structures in the animal kingdom [76]. In mantle edge and secreted into the prismatic layer of the addition to the directly protective contribution to the or- shell, where they are thought to provide a framework for ganism, the periostracum is also thought to play a signifi- calcification and to be responsible for the toughness of cant role in shell biomineralization [84]. The periostracum the shell [69]. Aspein is an acidic protein that has a se- encloses the extrapallial space on the ventral side and iso- quence rich in aspartic acid and is expressed at the outer lates it from the external environment, enabling the forma- edge of the mantle [70]. It is the shell matrix protein tion of supersaturation conditions, which is a requisite for that controls the precipitation of calcite in P. fucata and the formation of calcified layers [85, 86]. Previous studies it is the most acidic protein found in molluscan shell also showed that it could serve as a substrate for the initial matrix protein [71]. The opposite pattern was observed deposition of calcium and even influence the prismatic for two genes coding for proteins participating in the layer formation [84, 85, 87–89]. SHEM 1 TYR 2 PDZ PIF SHEM 4 MP8 PRISM SERP ZINC KRMP CHIT Normalized expression levels (log scale) Lemer et al. BMC Genomics (2015) 16:568 Page 11 of 14 Albino phenotypes are supposed to be mainly the result Inter-individual variability in gene expression within black of the absence of melanin biosynthesis or of synthesis of phenotypes non- or partly functional melanin protein [44, 90]. The high One major outcome of the individual analysis performed expression of ZINC in the full albino phenotype compared on 11 genes is the existence of high gene expression to normal black and half albino phenotypes suggests that variability within the black phenotypes for most of the the full albino phenotype might overcompensate for a non- genes tested and mostly for SHEM 4, MP8, KRMP, CHIT functional melanin protein by over-expressing the gene and SERP, characterized by high standard deviation coding for this protein. Finally, FLAV (coding for Flavonol values and gene expression levels different than 1 (1 be- Cinnamoyl COA reductase related) was found to be highly ing the value corresponding to an absence of variation ; expressed in the black phenotypes, in the pooled analysis. Fig. 5). It is well known that what is commonly named Flavonol Cinnamoyl COA reductase is an enzyme that con- as the black-lipped pearl oyster is in fact a pool of a wide tributes in the phenylpropanoid biosyntheis in plants and is range of shell colors varying from several shades of yel- enhanced for acclimatization processes at different levels of lows, reds, greens and blues. The black specimens used photosynthetic photon flux [91–94]. Mutations in the gene in this study as a control phenotype for the individual coding for flavonols in angiosperms have shown to induce analysis, were selected randomly. The inter-individual dramatic color changes in the flower organs [95]. The gene gene expression variation detected within those samples we detected in P. magaritifera is related to the gene coding highlights the diversity of colors found in our pool of for this enzyme and although no information is available black phenotypes. This result supports the hypothesis about its role in animals, we can hypothesize that a hom- that SHEM 4, MP8, KRMP, CHIT and SERP are very ologous function to acclimatization to various levels of light likely to be involved not only in the origin of shell color can be found in P. margaritifera as the over-expression of but also in the variability of the shell color among this gene is correlated with the presence of a black mantle "black" specimens. The presence of these wide variations (which translates into melanin pigment biosynthesis in in expression patterns in the black phenotypes, used as these individuals). controls in this analysis, could also be responsible for The expression patterns of genes involved both in the preventing the detection of significant differential expres- biomineralization of the periostracum and the synthesis sion patterns in the individual analyses of albinos specimens of melanin suggest once again that the origin and poly- when those patterns were significant in the pooled analysis. morphism of the color in the nacre of P. margaritifera takes place in all the layers of the shell and in different Conclusions biological processes. Thegoalofthisstudy wastoidentifya subset of genes involved in the color of the nacreous layer of the pearl oyster Pinctada margaritifera, and assess Differential expression of genes not directly involved in their expression discrepancies. The role of the pro- biomineralization processes teins encoded by the genes showing differential ex- Two genes coding for proteins involved in metabolic pro- pression levels among the different tested phenotypes cesses showed significant differential expression among revealed that three main genetic processes were in- phenotypes: SERP and PDZ. The gene SERP coding for volved in color polymorphisms: the biomineralization Serine protease inhibitor or serpins (which is part of a of the nacreous and calcitic layers and the synthesis large family of enzymes that inhibit the activity of prote- of pigments such as melanin in the periostracum. ases) showed high expression levels in both albino condi- However, our results suggest that the color observed tions, in the pooled analysis. A limited number of serpins in the nacre is mainly under the influence of genes proteins has been reported in bivalves or mollusks and so involved in the biomineralization of the calcitic layer. far they are thought to be involved in an immune defen- A few genes involved in the formation of the aragon- sive role against invasive pathogens [94, 96]. The gene ite tablets of the nacre and in the biosynthesis chain coding for a PDZ domain protein (PDZ) showed signifi- of melanin also showed differential expression pat- cantly higher expression levels in black phenotypes than terns, suggesting that color polymorphism takes place in albino phenotypes, both in the pooled and individual at different levels in the shell structure. Color in vari- analyses. PDZ domains are one of the most frequently en- ous organisms like birds or flowers is known to be countered domains, consisting of approximately 90 amino under the influence of a combination of genes coding acid residues and identified as a region of sequence hom- for multiple pigments (melanin and carotenoid being ology among a diverse list of signaling proteins [97, 98]. the most commons in birds and anthocyanin in an- The reason why these genes exhibit differential expression giosperms) and surface structure [99, 95]. Different patterns across phenotypes remains unclear and will need gene expression arrangements seem to lead to the further explorations. extraordinary variety of colors and iridescence observed in Lemer et al. BMC Genomics (2015) 16:568 Page 12 of 14 bird feathers and flower organs [99, 95]. Similarly, it ap- Tahiti, French Polynesia. Present address: Ifremer, UMR 5244 IHPE, UPVD, CNRS, Université de Montpellier, CC 80, F-34095 Montpellier, France. pears that the genes involved in the origin and variation of color in the Polynesian black-lipped pearl oyster span a Received: 6 December 2014 Accepted: 14 July 2015 large range of functions from shell structure to pigment synthesis. Future studies, using RNA-Seq technology and in situ hybridization analyses of the genes identified in the References present work will help define more precisely the role of 1. Liu X, Wu F, Zhao H, Zang G, Guo X. A novel shell color variant of the Pacific abalone Haliotis discus hannai Ino subject to genetic control and the different shell layers in the origin of the color of the dietary influence. J Shellfish Res. 2009;28:419–24. nacre. The high variability of the gene expression found 2. Canales-Gómez E, Correa G, Viana MT. 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Journal

BMC Genomics – Springer Journals

Published: Dec 1, 2015

Keywords: life sciences, general; microarrays; proteomics; animal genetics and genomics; microbial genetics and genomics; plant genetics and genomics

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