Standard Illumina libraries are biased toward sequences of intermediate GC-content. This results in an underrepresentation of GC- rich regions in sequencing projects of genomes with heterogeneous base composition, such as mammals and birds. We developed a simple, cost-effective protocol to enrich sheared genomic DNA in its GC-rich fraction by subtracting AT-rich DNA. This was achieved by heating DNA up to 90 C before applying Illumina library preparation. We tested the new approach on chicken DNA and found that heated DNA increased average coverage in the GC-richest chromosomes by a factor up to six. Using a Taq polymerase supposedly appropriate for PCR ampliﬁcation of GC-rich sequences had a much weaker effect. Our protocol should greatly facilitate sequencing and resequencing of the GC-richest regions of heterogeneous genomes, in combination with standard short-read and long-read technologies. Key words: GC content, GC enrichment, high-throughput sequencing, bird. Introduction including the human genome, have a local GC-content that High-throughput sequencing technologies have decreased the varies from 30% to >55% at the kilo-base scale (Landeretal. cost of sequencing by several orders of magnitude over the last 2001; Cohen et al. 2005; Duret et al. 2006), and a similar pat- few decades (Reuter et al. 2015). Short-read technologies have tern has been reported in honey bee (Apis mellifera)and several increased the depth of coverage to values typically >60 for species of ants (The Honeybee Genome Sequencing whole-genome sequencing and 15 for resequencing data Consortium et al. 2006; Smith et al. 2011). (Sims et al. 2014). Unfortunately, depth of coverage is often The genomes of birds are arguably among the most hetero- far from evenly distributed across the sequenced genome. geneous with respect to GC-content, both within and among Biases in PCR ampliﬁcation create uneven genomic represen- chromosomes. Birds show a particularly striking negative corre- tation in classical Illumina libraries (Dohm et al. 2008; Kozarewa lation between GC-content and chromosome size (Hillieretal. et al. 2009; Aird et al. 2011), PCR being sensitive to extreme 2004): the bird karyotype includes a number of very small-sized GC-content variation (Baskaran et al. 1996; Benita et al. 2003; chromosomes that are particularly GC-rich, underrepresented Oyola et al. 2012). In consequence, the GC-rich regions of in short-read sequence data, and difﬁcult to assemble. The orig- large, heterogeneous genomes are typically undercovered, inal draft chicken genome assembly, for instance, only included therefore inefﬁciently assembled, when libraries are prepared 29 out of the 38 autosomes with the smallest chromosomes following standard protocols (Hillier et al. 2004). A marked being missing (Hillier et al. 2004). Importantly, gene density is heterogeneity in GC-content has been identiﬁed in various strongly correlated with GC-content in birds (ﬁg. 1). The unas- genomes of relatively large size. In angiosperms, monocots sembled GC-rich regions actually contain a substantial and especially grasses (Poaceae) show a bimodal distribution portion—probably 15%—of the bird gene complement, of GC-content in protein-coding genes, with a class of very GC- which is currently missing from genome annotation databases, rich genes (Yu et al. 2002; Serres-Giardi et al. 2012; Clement as we recently demonstrated from transcriptome analyses et al. 2014; Glemin et al. 2014). Most mammalian genomes, (Botero-Castro et al. 2017, see also Hron et al. 2015). The Author(s) 2018. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact email@example.com 616 Genome Biol. Evol. 10(2):616–622. doi:10.1093/gbe/evy022 Advance Access publication January 27, 2018 Downloaded from https://academic.oup.com/gbe/article-abstract/10/2/616/4827694 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Illumina Library Preparation for Sequencing the GC-Rich Fraction of Heterogeneous Genomic DNA GBE 12 ● aiming at enriching genomic DNA in its GC-rich fraction prior to library preparation. We show that a simple heat- denaturation and sizing of fragmented DNA before the blunt-end repair step results in a substantially increase in av- ● erage GC-content of sequence reads. Applying this protocol ● ● ● to chicken DNA, we achieved a considerable increase in cov- ● ● erage depth of the GC-richest regions of the genome. The ● new approach is cheap, does not require high quantity or ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ●● ● ●● ● quality of DNA, and is complementary to the shotgun, mate ● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● pair and/or SMRT approaches. ● ●● ● ● ● ● ● ● ● ● ● ●● ● ● ●● ● ● ●● ●●● ● ● ● ● ● ●● ●●● ● ● ● ● ● ● ●● ● ● ● ● ● ●●● ●● ● ● ● ● ● ● ● ●● ● ●● ● ● ● ● ● ● ●●● ●● ● ● ● ● ●● ●●● ● ●● ● ● ● ● ● ●● ● ● ● ●● ● ●●● ● ● ● ●●● ● ●● ● ●● ● ●● ● ●●● ● ● ●● Materials and Methods ●●●●● ● ● ● ●● ● ●● ● ● ●●●● ●●●● ●●●●● ● ● ●●● ● ● ● ● ●●●● ●●● ●●● ● ● ●● ● ● ●●●●●● ●●●●●●●●● ●● ● ● ●● ●● ● ●●●●●●●●●●●●●●●●● ● ● ● ● ●●● ● ●● ●● ●● ● ●●●●●● ● ● ● ●● ●●●●●● ●●●●●●●●●●●●●●● ●● ● ● ● ● ● ● ●●●●●●●●●●●●●●● ●●●●●● ● ● ● ●●● ●● ● ● ● ●●●● ● ●●●●●● ●●●●●●●●●●●●●●●●●● ● ● ●●●●●●●●●●● ●●●● ●●●● ●●●●● ●●●●● ● ● ● ●● ●●●●●●●●●●●● ●●●●● ●●●●● ●● ● DNA Extraction and Treatment Post-Illumina Library ● ●● ●●●●●●●●●●●●●●●●●●●●●● ● ●●● ●● ●● ●● ● ●● ●●●●● ●● ●●●●● ●●●● ● ●● ●●● ●●● ●●●●●●● ●●●●●●●●●●●●●●●●●●●●●●● ●● ● ● ●● ●● ● ● ●● ● ●●●●●●●●●●●●●●●●●●●●●●● ●●● ●● ● ● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●●●●●● ● ● ●●● ● ●● ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●● ●●● ● ● ●●●●●●● ●●●●●●●●● ●●●●●●● ●●●●●●●●●●●●●●●● ●● ●● ● ●●●●●●● ●●●●●●●●●●●●●●●●●●●●●●● ●●●●●●● ● ● Preparation ● ●●●● ●●●●●●●●●●●●●●●●●● ● ● ●● ● ● ● ●●●●●●●●●●●●●●●●●● ●●●●●●●●● ●●●● ● ● ● ● ● ●●●●●●● ●●●●● ● ●●● ●●● ● ● ●●●●●●●●●● ●● ● ●●●● ●●●●●●●●● ● ● ● ● ● ●● ● ● Total genomic DNA was extracted from chicken tissue using DNAeasy Blood and Tissue kit (QIAGEN) following the manu- 0.4 0.5 facturer instructions. About 3 mg of total genomic DNA were GC sheared for 20 min using an ultrasonic cleaning unit (Elmasonic FIG.1.—Gene content and GC-content computed in 100-kb non- One). Sheared DNA was separated in six tubes of 50 ml contain- overlapping windows across the chicken genomes (Gallus_gallus-5.0). ing 500 ng of DNA each. We applied different temperatures to Linear and quadratic regression lines are shown in blue and red, the sheared DNA in order to denature it. Two samples (CHK2- respectively. 75 and CHK2-85) were heated 5 min to 75 C and 85 C, respectively. Three samples (CHK3-75, CHK3-85, and CHK- 90) were heated to 75 C, 85 C, and 90 C, respectively, There is, therefore, a clear need for DNA sequencing and submitted to a second step of shearing in an ultrasonic methods alleviating the GC bias. Single-molecule real- cleaning unit (Elmasonic One) during 5 min. One control sam- time (SMRT) sequencing technologies that do not rely ple (CHK1) was not heated. All samples were sized using on PCR have recently contributed to signiﬁcantly improve AMPure (Agencourt) immediately after treatments (see genome assembly in large genomes (Davey et al. 2016; table 1). Gordon et al. 2016; Bickhart et al. 2017; Korlach et al. 2017; Warren et al. 2017; Weissensteiner et al. 2017). In birds, the chicken, zebra ﬁnch (Taeniopygia guttata), Library Preparation and Sequencing Anna’s hummingbird (Calypte anna), and hooded crow (Corvus cornix) assemblies have been improved using Illumina library preparation followed the classical protocol in- PacBio technologies with a coverage from 50 to 96 volving blunt-end repair, adapter ligation, and adapter ﬁll-in (Korlach et al. 2017; Warren et al. 2017; Weissensteiner steps as developed by Meyer and Kircher (Meyer and Kircher et al. 2017). SMRT sequencing, however, remains rela- 2010) with slight modiﬁcations as explained by Tilak et al. tively costly and error prone, and requires high quantity (2015). The full protocol has been deposited in protocols.io and quality of DNA, so that in many projects sequencing dx.doi.org/10.17504/protocols.io.jxicpke. Libraries were depth is mainly contributed by PCR-dependent technolo- quantiﬁed using a Nanodrop ND-8000 spectrophotometer gies. Several attempts have been made to optimize PCR (Nanodrop technologies). About 5 ng of each library (except conditions, such as temperature ramp rate, denaturation CHK-90) were PCR indexed using Taq Phusion (Phusion High- time, chemical additives, and DNA polymerase, in order to Fidelity DNA Polymerase Thermo Scientiﬁc) and KAPA HiFi reduce the GC bias during library preparation (Aird et al. (2 KAPA HiFi HotStart ReadyMix KAPABIOSYSTEMS) poly- 2011; Oyola et al. 2012). Aird et al. (2011),for instance,im- merases because these ampliﬁcation enzymes could have dif- proved the homogeneity of coverage depth when applying ferent GC biases (Quail et al. 2011). CHK-90 was only optimized protocols to a mixture of bacterial DNA from three ampliﬁed with KAPA HIFI and 3% DMSO, so that 11 index distinct species but they concluded that not a single protocol libraries were generated—one for CHK-90 and two for each is appropriate in every situation. GC-rich and GC-poor DNA of the other ﬁve conditions. Indexed libraries were puriﬁed have distinct optimal PCR conditions, so that amplifying het- using AMPure (Agencourt) ratio 1.6, quantiﬁed with erogeneous DNA is intrinsically a difﬁcult problem. Nanodrop ND-800, and pooled in equimolar ratio. The pool Elaborating on this idea, we here suggest to isolate GC-rich of indexed libraries was single-read sequenced on one lane of DNA before sequencing it. We investigate a simple method Illumina HiSeq 2500 at GATC-Biotech (Konstanz, Germany). Genome Biol. Evol. 10(2):616–622 doi:10.1093/gbe/evy022 Advance Access publication January 27, 2018 617 Downloaded from https://academic.oup.com/gbe/article-abstract/10/2/616/4827694 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Gene density (gene per 100kb) Tilak et al. GBE Table 1 Pretreatments, Melting Temperature (Tm) and GC Content (FastQC) for Each Library Library Heating ( C) Shearing Polymerase DMSO (%) Tm ( C) GC Content (%) CHK1 No No Phusion 0 86 41 CHK1 No No Kapa 0 86 41 CHK2-75 75 No Phusion 0 86 41 CHK2-75 75 No Kapa 0 86 41 CHK3-75 75 5 min Phusion 0 86 41 CHK3-75 75 5 min Kapa 0 86 41 CHK2-85 85 No Phusion 0 88 51 CHK2-85 85 No Kapa 0 89.5 52 CHK3-85 85 5 min Phusion 0 88 51 CHK3-85 85 5 min Kapa 0 89.5 52 CHK-90 90 5 min Kapa 3 86, 91, 94 59 Fusion Curves Results We generated fusion curves in order to check the effect of We ﬁrst analyzed fusion curves in order to estimate the melt- pretreatments on the GC-content of the constructed libraries. ing temperature (Tm), which, is known to be positively corre- About 5 ng of each indexed PCR was mixed with ResoLight latedtoGC-content (Marmur and Doty 1962). Tm was not ROCHE 20 (ﬂuorescent molecule) for a ﬁnal volume of 10ml. notably different between CHK1, CHK2-75, and CHK3-75 The libraries were heated from 65 Cto 98 C with increasing regardless of the enzyme used for ampliﬁcation. These results ramp to 0.02 C per second and 25 acquisitions per degree using suggest that GC-content was nearly the same for these librar- the High Resolution Melting program of ROCHE Light Cycler ies. In contrast, the libraries constructed from DNA heated to 480. The melting curves were obtained for all libraries and their 85 C and 90 C had a signiﬁcantly increased Tm, compared negative ﬁrst-derivative (100 dF/dT) were calculated to esti- with CHK1, suggesting a GC enrichment (ﬁg. 2). There was mate the corresponding melting temperatures (Tm). no conspicuous difference in Tm between CHK2-85 and CHK3-85, suggesting that an additional 5-min DNA shearing after heating has no strong effect on GC-content. Sequence Analyses GC-content was estimated for each library using FastQC (table 1). In agreement with the analysis of melting curves, For a fair comparison between libraries, we generated 11 data GC-content was signiﬁcantly increased when DNA was sets of exactly eight millions of 101-bp reads each. This was heated to a temperature of 85 C orhigher(table 1): the achieved by randomly subsampling in fastq ﬁles prior to any average GC-content of reads was increased from 41% quality control or ﬁltering step (see command line in supple- (unheated) to 52% (85 C) andupto59% (90 C). In con- mentary material online). The quality and GC-content of the trast, GC-content was similar between CHK1, CHK2-75, and data obtained in this study were assessed using FastQC 0.11.4 CHK3-75. The choice of DNA polymerase (Taq Phusion or (Andrews 2010. Available at: https://www.bioinformatics. Kapa Hiﬁ) only had a weak effect on GC-content in treat- babraham.ac.uk/projects/fastqc/). Reads were cleaned with ments CHK2-85 and CHK3-85. Trimmomatic (Bolger et al. 2014) using parameters: Eight million reads from each of the 11 libraries were “LEADING: 3 TRAILING: 3 SLIDINGWINDOW: 4: 15 mapped to the chicken genome Gallus_gallus-5.0. Average MINLEN: 50.” Cleaned reads were mapped onto the refer- expected genome coverage is 0.67 per library. In agreement ence genome Gallus_gallus-5.0 using Bowtie2 with default with the Tm and FastQC results, the number of reads that parameters (Langmead and Salzberg 2012). The number of mapped onto reference genome was similar between libraries readsmappedtoeachchromosome and scaffold wascom- CHK1, CHK2-75, and CHK3-75, on one hand, and between puted using SAMtools. We also computed the number of CHK2-85 and CHK3-85, on the other hand. The results for reads mapped to small contigs that are not associated to libraries CHK1, CHK2-85, and CHK-90 are shown in table 2. any chromosome or linkage group (LG) in the Gallus_gallus- The average GC-content of mapped reads was also consider- 5.0 assembly. The size of these contigs varied from 200 to ably higher in the CHK2-85 and, particularly, CHK-90 libraries 209,746 bp, with an average of 8,964 bp. These contigs rep- when compared with that of CHK1 and this was true of all the resent the badly assembled regions of the chicken genome. groups of chromosomes. This indicates that heating libraries To analyze the relationship between depth of coverage and has not only improved depth of coverage in small, GC-rich GC-content, we sorted the contigs according to GC-content chromosomes but also for the GC-richest regions of large, and divided them in 29 bins of 623 contigs. Contigs with the GC-heterogeneous chromosomes. In addition, note that the 5% highest coverage were excluded from the analysis. 618 Genome Biol. Evol. 10(2):616–622 doi:10.1093/gbe/evy022 Advance Access publication January 27, 2018 Downloaded from https://academic.oup.com/gbe/article-abstract/10/2/616/4827694 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Illumina Library Preparation for Sequencing the GC-Rich Fraction of Heterogeneous Genomic DNA GBE FIG.2.—Melting curves of a standard (CHK1, blue) and three heated (CHK2-85, CHK3-85, CHK-90) Illumina libraries. Table 2 Mapping of Reads from Standard (CHK1) and Heated (CHK2-85, CHK-90) Libraries to the Reference Chicken Genome CHK1 CHK2-85 CHK-90 Chromosomes (%) Mapped (%) GC Mapped (%) Mapped (%) GC Mapped Coverage (%) Mapped (%) GC Mapped Coverage a a (% GC) Reads Reads Reads Reads Increase Reads Reads Increase 1–5 (40.3%) 58.5 39 43.4 51 <1 41.1 60 <1 6–10 (42.3%) 13.4 41 14 52 1 13.1 66 1 11–15 (42.8%) 7.6 43 10.4 53 1.4 10.9 67 1.4 16–20 (47.7%) 3.4 46 6.9 54 27.3 68 2.1 21–25 (50%) 2.2 48 5.2 55 2.4 5.2 68 2.4 26–31 (53%) 1.4 51 3.9 56 2.8 5.8 69 4.1 32–33 (54.9%) 0.06 53 0.4 58 6.6 0.4 70 6.6 W-Z-LGE64 (41.3%) 4.6 40 4.7 52 14.27 60 <1 Coverage increase was calculated by dividing the percentage of mapped reads of CHK2-85 (respectively, CHK-90) by that of CHK1. percentage of mapped reads was higher in heated than in small chicken contigs. These contigs represent the badly as- unheatedtreatmentsfor chromosomeshavinganaverage sembled regions of the chicken genome that are not assigned GC-content>42% (table 2). The proportion of reads mapped to any speciﬁc chromosome; some of them have a very high onto the different chromosomes clearly reﬂects the increased GC-content. Reads from CHK1 yielded a negative correlation average GC-content, and more homogeneous coverage, of between contig coverage and GC-content: depth of coverage heated libraries (ﬁg. 3). This result indicates that heating dropped by a factor of 2.5 as GC increased from 33% to 65% sheared DNA before library preparation makes it possible to (ﬁg. 4). In contrast, with CHK2-85 contigs coverage increased sequence GC-rich genomic DNA fragments that are otherwise with GC-content and reached a plateau 55% of GC for essentially out of reach when using the standard protocols. library CHK2-85 (ﬁg. 4). Calculating the average depth of coverage per group of chromosomes, we found that heated libraries yielded a higher Discussion coverage than unheated one for chromosomes with average Illumina library construction protocols are generally recog- GC-content >43%, with up to a 6-fold increase in the GC- nizedtobe biased towardfragments of intermediate richest ones (table 2). Finally, we analyzed the coverage of Genome Biol. Evol. 10(2):616–622 doi:10.1093/gbe/evy022 Advance Access publication January 27, 2018 619 Downloaded from https://academic.oup.com/gbe/article-abstract/10/2/616/4827694 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Tilak et al. GBE A) 0.20 0.15 Libraries CHK1 0.10 CHK2−85 CHK90 0.05 0.00 Chromosomes B) ●●● ●●● Libraries ●●● ●●● ● CHK1 ●●● ●●● ●●● ●●● ●●● ●●● ● CHK2−85 50 ●●● ●●● ●●● ●●● ●●● ●●● ●●● ● CHK90 ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● ●●● Chromosomes FIG.3.—(A) Proportion of reads mapped to the various chromosomes of the chicken genome. Colors represent the different libraries. Chromosomes are sorted according to average GC-content. (B) Average GC-content of chromosomes. GC-content, the GC-richest fraction of the target DNA being high-coverage, standard Illumina libraries, high-coverage, underrepresented (van Dijk et al. 2014). Here, we introduce a GC-enriched Illumina libraries, and medium-coverage SMRT simple, cheap protocol that leads to a substantial decrease of reads. Illumina reads would here be used to correct for se- this bias. Heating DNA to temperatures>85 C prior to library quencing errors in SMRT reads (Salmela and Rivals 2014), preparation increased coverage in the GC-richest fraction of and the GC-enriched library would ensure accurate correction the chicken genome by a factor of up to 6. We speculate that across all regions of the genome. We expect this approach to this happens because 1) AT-rich regions are underrepresented substantially improve the efﬁciency of de novo genome se- as double-stranded DNA in heated solutions due to their quencing in birds, but also in mammals, nonavian reptiles, lower melting temperature, and 2) adapter ligation and fur- hymenopterans, monocots, and presumably a number of ad- ther steps of library construction speciﬁcally target double- ditional taxa with GC-heterogeneous genomes. Our approach stranded DNA. should also facilitate the optimization of PCR conditions Our GC-enrichment protocol will complement existing (Baskaran et al. 1996; Aird et al. 2011; Oyola et al. 2012)by approaches for optimal sequencing of GC-heterogeneous decreasing the heterogeneity of matrix GC-content. genomes. We suggest that a promising strategy for, for Gene density is positively correlated to GC-content in birds example, bird genome sequencing would involve combining (Hillier et al. 2004; Axelsson et al. 2005). The unassembled/ 620 Genome Biol. Evol. 10(2):616–622 doi:10.1093/gbe/evy022 Advance Access publication January 27, 2018 Downloaded from https://academic.oup.com/gbe/article-abstract/10/2/616/4827694 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Chrom_2 Chrom_2 Chrom_3 Chrom_3 Chrom_4 Chrom_4 Chrom_1 Chrom_1 Chrom_Z Chrom_Z Chrom_5 Chrom_5 Chrom_7 Chrom_7 Chrom_6 Chrom_6 Chrom_8 Chrom_8 Chrom_11 Chrom_11 Chrom_9 Chrom_9 Chrom_10 Chrom_10 Chrom_12 Chrom_12 Chrom_13 Chrom_13 Chrom_15 Chrom_15 Chrom_14 Chrom_14 Chrom_W Chrom_W Chrom_20 Chrom_20 LGE64 LGE64 Chrom_18 Chrom_18 Chrom_19 Chrom_19 Chrom_22 Chrom_22 Chrom_21 Chrom_21 Chrom_17 Chrom_17 Chrom_24 Chrom_24 Chrom_23 Chrom_23 Chrom_27 Chrom_27 Chrom_26 Chrom_26 Chrom_28 Chrom_28 Chrom_31 Chrom_31 Chrom_16 Chrom_16 Chrom_25 Chrom_25 Chrom_33 Chrom_33 Chrom_32 Chrom_32 Chrom_30 Chrom_30 GC content Proportion of reads (# reads mapped / sum of reads) Illumina Library Preparation for Sequencing the GC-Rich Fraction of Heterogeneous Genomic DNA GBE Acknowledgments The authors thank Philippe Clair for helpful discussion and the Montpellier GenomiX qPCR core facility of University of Montpellier, France. The analyses beneﬁted from the Montpellier Bioinformatics Biodiversity platform services. This work was supported by Agence Nationale de la Recherche grant ANR-14-CE02-0002-01 “BirdIslandGenomic” to B.N. Literature Cited Aird D, et al. 2011. Analyzing and minimizing PCR ampliﬁcation bias in Illumina sequencing libraries. Genome Biol. 12(2):R18. Axelsson E, Webster MT, Smith NG, Burt DW, Ellegren H. 2005. Comparison of the chicken and turkey genomes reveals a higher rate of nucleotide divergence on microchromosomes than macrochro- mosomes. Genome Res. 15(1):120–125. Baskaran N, et al. 1996. Uniform ampliﬁcation of a mixture of deoxyribo- FIG.4.—Relationship between GC-content and coverage recorded on nucleic acids with varying GC content. Genome Res. 6(7):633–638. Benita Y, Oosting RS, Lok MC, Wise MJ, Humphery-Smith I. 2003. the small contigs of the chicken genome. 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