Exploring the natural variation for reproductive thermotolerance in wild tomato species

Exploring the natural variation for reproductive thermotolerance in wild tomato species Euphytica (2018) 214:67 https://doi.org/10.1007/s10681-018-2150-2 Exploring the natural variation for reproductive thermotolerance in wild tomato species . . . . Nicky Driedonks Mieke Wolters-Arts Heidrun Huber Gert-Jan de Boer . . Wim Vriezen Celestina Mariani Ivo Rieu Received: 22 December 2017 / Accepted: 7 March 2018 / Published online: 13 March 2018 The Author(s) 2018 Abstract Climate change has become a serious germplasm screenings for thermotolerance have often threat for crop productivity worldwide. The increased used yield as the main measured trait. However, due to frequency of heat waves strongly affects reproductive the complex nature of yield and the relatively narrow success and thus yield for many crop species, implying genetic variation present in the cultivated germplasm that breeding for thermotolerant cultivars is critical for screened, there has been limited progress in under- food security. Insight into the genetic architecture of standing the genetic basis of reproductive heat toler- reproductive heat tolerance contributes to our funda- ance. Extending the screening to wild accessions of mental understanding of the stress sensitivity of this related species that cover a range of climatic condi- process and at the same time may have applied value. tions might be an effective approach to find novel, In the case of tomato (Solanum lycopersicum), more tolerant genetic resources. The purpose of this study was to provide insight into the sensitivity of individual reproductive key traits (i.e. the number of Electronic supplementary material The online version of pollen per flower, pollen viability and style protrusion) this article (https://doi.org/10.1007/s10681-018-2150-2) con- to heat-wave like long-term mild heat (LTMH), and tains supplementary material, which is available to authorized users. determine the extent to which genetic variation exists for these traits among wild tomato species. We found N. Driedonks  M. Wolters-Arts  C. Mariani that these traits were highly variable among the I. Rieu (&) screened accessions. Although no overall thermotol- Department of Molecular Plant Physiology, Institute for Water and Wetland Research, Radboud University, erant species were identified, several S. pimpinelli- 6525 AJ Nijmegen, The Netherlands folium individuals outperformed the best performing e-mail: i.rieu@science.ru.nl cultivar in terms of pollen viability under LTMH. Furthermore, we reveal that there has been local H. Huber Department of Experimental Plant Ecology, Institute for adaptation of reproductive heat tolerance, as acces- Water and Wetland Research, Radboud University, sions from lower elevations and higher annual 6525 AJ Nijmegen, The Netherlands temperature are more likely to show high pollen viability under LTMH. G.-J. de Boer Enza Zaden Research and Development B.V, 1600 AA Enkhuizen, The Netherlands Keywords Heat tolerance  Pollen  Male fertility Wild tomato W. Vriezen Bayer Vegetable Seeds, 6080 AA Haelen, The Netherlands 123 67 Page 2 of 12 Euphytica (2018) 214:67 Introduction of reproductive thermotolerance under long-term mild heat (LTMH). Ambient temperatures are rising as part of the current Secondly, germplasm used in thermotolerance global climate change, threatening agricultural output screening has mainly consisted of S. lycopersicum (IPCC 2007, 2013). High temperatures cause mor- cultivars (Abdul-Baki and Stommel 1995; Dane et al. phological, physiological, biochemical and molecular 1991; Grilli et al. 2007; Kugblenu et al. 2013a; Opena changes in plants that affect growth, and is particularly et al. 1992). However, as a result of domestication and detrimental during the reproductive stages (Wahid intensive breeding, the cultivated tomato germplasm et al. 2007). This leads to reduced yields in crop has a rather narrow genetic base (Bergougnoux 2014), species, and thus has a large impact on global food meaning that only a subset of the genes and alleles production (Barnabas et al. 2008; Hedhly et al. 2009). available in the wild progenitor gene pool are still For example, during cultivation, tomato is often present among crop cultivars (Godfray et al. 2010; exposed to high temperature either in the greenhouse Ladizinsky 1985; Olsen and Wendel 2013). Espe- or in the field, and consequently, fruit set is reduced in cially, as breeding efforts have mainly targeted yield at many S. lycopersicum cultivars. more or less optimal cultivation conditions, it seems Exploration of natural variation may offer insight likely that abiotic stress tolerance traits have been lost into the genetics of stress tolerance, and can provide (Ladizinsky 1985; Paran and Van Der Knaap 2007). genetic diversity useful for breeding (Grandillo et al. This implies that the potential gain in heat tolerance 2011). This also applies to reproductive heat tolerance, level from cultivated germplasm is likely to be limited. but so far, screening for variation in tomato heat A broader genetic diversity can be found in species sensitivity has yielded only a few genotypes consid- related to tomato, and could serve as an alternative ered to be thermotolerant (Opena et al. 1992), and source of plant thermotolerance traits (Vı´quez- these have limited applicability for pre-breeding Zamora et al. 2013). Wild tomato species are found (Grilli et al. 2007). There seem to be at least two in a variety of habitats ranging from sea level to above major reasons for this. Firstly, in previous germplasm 3000 m in altitude and from temperate deserts to wet screenings fruit set was the main trait of interest. tropical rainforests, and thus face a range of environ- However, fruit set is a complex trait, i.e. it represents mental challenges. As a result of natural selection, the sum of multiple sub-traits (yield components). these wild species vary broadly in terms of morphol- Thus, there may be a relatively small chance that ogy, physiology, biochemistry and stress tolerance optimal sub-traits combine to generate a strongly levels (Dolferus 2014; Grandillo et al. 2011; Madu- outperforming genotype. Furthermore, the complexity raimuthu and Prasad 2014). There is also variation in of the relation among different traits involved in fruit mating systems among wild accessions, i.e. self- set complicates genetic analysis. As an alternative compatible (SC) versus self-incompatible (SI), which approach, it might be more effective to analyse the is likely to affect reproductive traits and putatively various contributing sub-traits individually and com- their performance under LTMH (Arroyo 1973; Baker bine them afterwards in a breeding context. For 1955; Cruden 1977; Georgiady and Lord 2002; Peralta example, decreases in tomato fruit set under long-term et al. 2008). mildly elevated temperatures has been shown to Here, we hypothesised that higher levels of repro- correlate with a decrease in pollen viability (Dane ductive thermotolerance are present in wild relatives et al. 1991; Firon et al. 2006; Kinet and Peet 1997; of tomato than in the cultivated tomato germplasm. Levy et al. 1978; Peet et al. 1998; Pressman 2002; We investigated the performance of 64 accessions Pressman et al. 2006; Sato et al. 2000, 2006; Xu et al. across 13 wild species and 7 S. lycopersicum cultivars, 2017b). Also, style protrusion may affect reproductive including a subset known for relatively good repro- success under high temperature (Charles and Harris ductive thermotolerance under control temperature 1972; Dane et al. 1991; Rick and Dempsey 1969; and long-term mild heat. We focused on reproductive Rudich et al. 1977; Saeed et al. 2007; Xu et al. 2017b). traits generally assumed to contribute to overall Investigation of these traits separately might provide a fertility, i.e. the number of pollen per flower, pollen more effective strategy to determine the genetic basis viability and the distance between the top of the anther and the stigma (style protrusion). In addition, we 123 Euphytica (2018) 214:67 Page 3 of 12 67 tested whether the mating system influenced these from the vegetative to the generative phase occurred, traits under LTMH, and determined whether local flower buds were removed and the cuttings were adaption to thermotolerance had occurred. treated similarly as the mother plants. Phenotypic assessment Materials and methods To determine pollen quality, anthers of the three most Plant material and screening procedure recently opened flowers were cut into 4 equal trans- verse sections. After addition of 200 lL peroxidase Sixty-four accessions belonging to 13 wild species (S. indicator (Rodriguez-Riano and Dafni 2000) consist- arcanum, S. cheesmaniae, S. chilense, S. chmielewskii, ing of 1 vial peroxidase indicator (Sigma 3901-10VL) S. corneliomulleri, S. galapagense, S. habrochaites, S. in 0.012% (v/v) H O and 10% (v/v) Trizmal buffer 2 2 huaylasense, S. lycopersicum, S. neorickii, S. pennel- (903C; Sigma–Aldrich, St Louis, MO, USA). Pollen lii, S. peruvianum and S. pimpinellifolium) and 7 S. were considered viable when roundly shaped and lycopersicum cultivars (‘‘Hotset’’, ‘‘Malintka101’’, stained dark. In order to determine the pollen viability ‘‘Moneyberg’’, ‘‘Nagcarlang’’, ‘‘NCHS-1’’, ‘‘Salad- (PV, in %), 100 pollen were assessed per flower. To ette’’ and ‘‘Tof Hamlet’’) were obtained from various determine the number of pollen per flower (PN) the sources (Table S1). Seeds were incubated in 2.5% number of pollen was counted in 25 chambers hypochlorite for 30 min at room temperature to (0.04 mm ) of a haemocytometer. In addition, style improve germination and reduce pathogen load (Rick protrusion (SP in mm) was measured. For PN, PV and and Borgino, TGRC, http://tgrc.ucdavis.edu/seed_ SP, three flowers were analysed per plant. germ.aspx), followed by germination on potting soil (Horticoop, Lentse Potgrond, Slingerland Potgrond) Climate data covered with vermiculite (Agra-Vermiculite) under standard greenhouse conditions. Seedlings were Climatic data sets for the earth land surface area were transferred to 0.5 L pots after 2 weeks and, after downloaded from CHELSA (Karger et al. 2017). 1 month, placed in 12 L pots, containing potting soil Using the R package ‘‘raster’’ version 2.5–8 (Hijmans -1 and 4 g L Osmocote Exact Standard 3–4 M and van Etten 2012), all 19 bioclimatic variable data (Everris). When the transition from the vegetative to (BIO1 to BIO19) were extracted for the period the generative phase occurred, flower buds were 1979-2013 for each accession according to the GPS removed and the plants were transferred to a climate coordinates of the original collection site (Table S1). chamber maintaining a 14/10 h day/night photoperiod -1 -2 (* 300 lmol s m at plant height; Philips Statistical analysis D-Papillon daylight spectrum 340 W lamps and Phi- lips MastergreenPower TLD58 W/840 fluorescent All statistical analyses were performed using trans- tubes) and humidity of 70–80% at either control formed data, value’ = Log(value?1), except for temperature of 25/19 C (CT) or long-term mild heat PV, to which a logit transformation was applied, of 32/26 C (LTMH) for at least 14 days. Plants were value’ = LN((value?1)/(101-value)). The relation grown and analysed in a staggered manner over a time between traits was determined by a Pearson correla- course of 4 months in batches of 15 individuals, with tion analysis. To assess the heritability of the traits, a complete randomisation of accessions. To determine Pearson correlation analysis of the means of clones the influence of the genotype on the studied traits, (cuttings) and their corresponding mother plant were cuttings were taken from several plants. In order to set performed using a paired sample correlation analysis. roots, the cuttings were put in potting soil (Horticoop, Broad-sense heritability was calculated for PN and PV Lentse Potgrond, Slingerland Potgrond) and kept in by dividing the variance among clones by the total the greenhouse in a plastic container to maintain a high variance among and within clones (i.e. variance humidity. After 2 weeks, cuttings were placed in 12 L among clones/total variance). To test for variation in -1 pots, containing potting soil and 4 g L Osmocote heat tolerance among species, a two-way ANOVA Exact Standard 3–4 M (Everris). When the transition was performed at species level (using mean values of 123 67 Page 4 of 12 Euphytica (2018) 214:67 accessions and temperature treatment). Differences in clones of PN and PV measurements of the cuttings performance under LTMH between species (using revealed a broad-sense heritability of 0.78 and 0.85, mean values of accessions) were assessed by a one- respectively (Table 1). way ANOVA followed by Tukey’s HSD as post hoc To evaluate relationships between the traits of test. At accession level (using mean values of plants), interest, Pearson correlation analyses between the trait differences between accessions within species were means per species were performed. In addition, to assessed by a one-way ANOVA using Tukey’s HSD correct for putative species-specific effects, a as post hoc test. For PN and PV, the wild accessions between-species Pearson correlation analysis was were compared to the best tomato cultivar by a one- performed. In both cases, no significant correlations way ANOVA with LSD post hoc test. To test whether were detected (Table 2; data not shown). In addition, the best performing genotypes from the best wild when considering all accessions as independent accession outperformed Nagcarlang, a one-way (n = 71), no significant correlations were detected ANOVA with LSD post hoc test was performed, between any of the traits under CT and LTMH (data using clones as replicates. To analyse the effect of not shown). Together, this suggests that the three traits temperature treatment and mating system, a two-way under study are largely independently inherited in ANOVA with temperature treatment and mating these species and accessions. system class variables was performed. The different Variation in the number of pollen per flower geographical characteristics were correlated with the physiological plant traits under LTMH by Pearson under LTMH correlation analysis. All statistical analyses were performed using IBM SPSS Statistics version 21. Temperature treatment and species both had a signif- icant effect on the number of pollen per flower (PN), and no interaction was found between them (Table 3). Results Exposure to LTMH reduced PN by 76.3% on average. PN of S. corneliomulleri was significantly higher than To assess reproductive performance under long-term that of S. chmielewskii, which had the lowest PN under mild heat (LTMH) in the wild tomato germplasm, 64 LTMH (Table S2). For the S. chilense, S. chmielews- wild accessions belonging to 13 wild species and 7 S. kii, S. galapagense, S. huaylasense, S. pennellii, S. lycopersicum cultivars were screened. Accessions peruvianum and S. lycopersicum cultivars, significant were selected to roughly encompass the spatial and differences in PN among accessions within the species elevation distribution of accessions of each species, were detected under LTMH (Table S3). However, and where possible, including accessions with anno- none of the wild accessions performed better, i.e. had a tations related to high temperature or other abiotic significantly higher PN, than the best performing stresses (Table S1). None of the wild accessions cultivar, NCHS-1 (Fig. 2a; Table S3). screened in this study were previously determined to be heat tolerant. In total, 201 and 317 plants were Variation in pollen viability under LTMH exposed to control (CT) or LTMH conditions, respec- tively, and three reproductive traits, the number of In response to LTMH, PV was reduced by 85.6% on pollen per flower (PN), pollen viability (PV) and style average, at the species level. No significant differences were detected between any of the screened species, protrusion (SP), were analysed. nor was there a significant interactive effect between Trait heritability and inter-trait relations species and treatment (Table 3). Within S. corne- liomulleri, S. neorickii, S. pimpinellifolium and the S. To determine the influence of genotype on the selected lycopersicum cultivars, significant differences were traits, cuttings from several individuals were grown detected among accessions (Table S3). However, also and exposed to LTMH. Significant correlations for this trait, none of the wild accession performed between mother plants and cuttings were detected better under LTMH, i.e. had a significant higher PV, for all traits (Fig. 1). The division of the variance than the best performing cultivar, Nagcarlang among clones by the total variance among and within (Fig. 2b; Table S3). As wild accessions may exhibit 123 Euphytica (2018) 214:67 Page 5 of 12 67 Fig. 1 Correlations of traits between mother plant and cuttings LTMH for a The number of pollen per flower (*1000), b pollen under long-term mild heat. Mother plants of 4 accessions (from viability (%), and c style protrusion (mm). Trait values for the S. corneliomulleri, S. peruvianum, S. pimpinellifolium and S. cuttings represent the average (n = 2–15). The Pearson’s pennellii) and two tomato cultivars (Moneyberg and Nagcar- correlation coefficient is given in each graph (r). Significance lang) were selected based on their difference in pollen viability level (two-tailed): *P \ 0.05; **P \ 0.01; ***P \ 0.001 under LTMH. Cuttings were generated and phenotyped under Table 1 Broad-sense heritability of the number of pollen per Table 3 P-values of a two-way ANOVA testing for the effects flower and pollen viability of temperature treatment and species on key reproductive sub- traits PN PV Trait Treatment Species Treatment * Species Genotype 0.65 0.79 PN \ 0.001 \ 0.001 0.060 Cutting 0.18 0.14 PV \ 0.001 0.214 0.272 Error 0.17 0.06 SP \ 0.001 \ 0.001 0.165 Broad-sense heritability 0.78 0.85 PN pollen number, PV pollen viability, SP style protrusion. For The values given for genotype and cutting represent the details per species see Table S2 variance among and within cuttings, respectively. Error represents the unexplained variance PN number of pollen per flowers, PV pollen viability Variation in style protrusion under LTMH SP was significantly increased by LTMH, and differed Table 2 Pearson’s correlation coefficients among traits of species under long-term mild heat significantly among species, but no interactive effect between temperature treatment and species was Trait PN PV SP detected, indicating that the different species respond PN 1 similarly to LTMH with respect to SP (Table 3 and PV 0.08 1 Table S2). Within species, significant differences SP 0.17 0.15 1 under LTMH were observed only in the cases of S. pimpinellifolium and the tomato cultivars (Table S3). PN pollen number per flower, PV pollen viability, SP style protrusion. None of the correlations (two-tailed) were None of the wild accessions outperformed the culti- significant vars under LTMH (i.e. had lower SP), as cultivar Saladette did not show any protrusion (Table S3). Many wild accessions had higher SP then the cultivars, genotypic diversity, we tested whether the best already under control temperature. performing genotypes from the best wild accession (S. pimpinellifolium LA1630) outperformed Nagcar- lang, using multiple clones per genotype. Indeed, four of the five tested genotypes had significantly higher PV under LMTH than Nagcarlang (Fig. 3). 123 67 Page 6 of 12 Euphytica (2018) 214:67 Fig. 2 Cultivars and the three best performing wild accessions interquartile range (IQR), with indication of the median. Lower with respect to pollen number per flower and pollen viability and upper whiskers represent the smallest and largest observa- under long-term mild heat. a Pollen number per flower (PN), and tions smaller than or equal to lower and upper hinge ± 1.5 * b pollen viability (PV). Box of boxplot represents the IQR, respectively. Each dot represents an individual plant Comparison between self-compatible and self- incompatible accessions To test the effect of an accession’s mating system on reproductive traits, self-compatible (SC) and self- incompatible (SI) accessions were compared. SI accessions had significantly higher PN and SP than SC accessions under both temperature treatments (Fig. 4). None of the traits showed significant inter- action between the two factors. Relationship between trait performance and climatic parameters at site of origin Due to the wide variation in geographical origin of the accessions, ranging from a latitude of - 24.211 to 0.867, longitude of - 91.417 to - 43.083, and elevation from 0 up to 3450 meters above sea level (Fig. 5), accessions habitats were also diverse and Fig. 3 Comparison between Nagcarlang and best performing varied from very dry to wet locations and from sandy genotypes of S. pimpinellifolium LA1630 concerning pollen viability (PV) under long-term mild heat coastal areas to high up in the mountains. To assess whether adaptation to local conditions had occurred, a Pearson correlation analysis between the phenotypic data and various geographical and climatic parameters 123 Euphytica (2018) 214:67 Page 7 of 12 67 Fig. 4 Trait values in self-compatible and self-incompatible ***P \ 0.001, n.s., not significant. a Number of pollen number accession under control and long-term mild heat. To compare per flower (PN), b pollen viability (PV), and c style protrusion genotypes, multiple cuttings per genotype were evaluated. (SP). SC self-compatible, SI self-incompatible, CT control Values represent the mean ± standard deviation (n = 5–20 temperature, LTMH long-term mild heat. Values represent the plants). Differences were assessed by a one-way ANOVA mean ± standard deviation (n = 33 and 37 for SI and SC followed by LSD post hoc test. Asterisks above the bars indicate accessions, respectively). Significance of treatment/mating a significant difference between the respective genotype and the system/interaction was determined by a two-way ANOVA: cultivar Nagcarlang. Significance level: *P \ 0.05; ***P \ 0.001; n.s., not significant Fig. 5 Geographical origin of accessions screened. Elevation is in meters above sea level was performed. A significant negative correlation was temperature parameters (Table S4). Thus, accessions found between the accessions’ PV and the elevation at derived from lower elevations and warmer climates the site of origin (Table 4). A significant positive are more likely to be tolerant to LTMH with respect to correlation was found with the annual mean temper- PV. ature, and the same trend was visible for related 123 67 Page 8 of 12 Euphytica (2018) 214:67 Table 4 Pearson’s correlation coefficients among geographi- Villareal et al. 1978). However, it is a complex trait, cal and physiological traits under long-term mild heat and higher heritability may be found by separating the underlying individual key traits affecting fruit set. Trait EL TEMP PREC Indeed, we have recently reported two major QTLs for PN 0.048 - 0.080 - 0.102 PN and PV in an S. lycopersicum mapping population ** * PV - 0.372 0.279 - 0.102 (Xu et al. 2017a). In the current study, screening of SP 0.103 - 0.106 - 0.281 clones of individuals for PN and PV under LTMH in climate chambers indicated that a large fraction of the Significance level (two-tailed): *P \ 0.05; **P \ 0.01. Additional correlations with bioclimatic variables are total phenotypic variance was explained by the genetic presented in Table S4 variance. Whether these sub-traits also express in PN number of pollen per flower, PV pollen viability, SP style other genetic backgrounds and environmental condi- protrusion; EL elevation, TEMP mean annual temperature in tions, such as field conditions, remains to be period 1979–2013, PREC mean annual precipitation in period determined. 1979–2013. n = 59–69 accessions Our study did not detect overall thermotolerance of yield-contributing sub-traits in wild species compared to the performance of cultivars, but we show that Discussion several genotypes from the accession LA1630 outper- form the best performing cultivar in terms of PV under The reduction in tomato yield under long-term mild LTMH. Given the previously reported correlation heat (LTMH) may be attributed to the plant’s vulner- between PV and fruit set under LTMH (e.g. Dane et al. ability during reproductive development, resulting in a 1991; Sato et al. 2000; Xu et al. 2017b), we conclude lower number of pollen per flower (PN) and pollen that wild germplasm might indeed be a valuable viability (PV) (Dane et al. 1991; Firon et al. 2006; resource to enrich domesticated germplasm for repro- Kinet and Peet 1997; Levy et al. 1978; Peet et al. 1998; ductive thermotolerance. Pressman 2002; Pressman et al. 2006; Sato et al. 2000, 2006; Xu et al. 2017b). In this study, we Mating system advantages under LTMH analysed the natural variation of reproductive thermo- tolerance in wild tomato species, which may serve as In the tomato clade, the mating system ranges between gene sources for cultivated tomato. self-incompatible (SI) to self-compatible (SC) cross- ers (Miller and Tanksley 1990; Rick et al. 1977). In Superior heat tolerant wild genotypes with regard general, flowers of SI plant species produce more to pollen viability pollen than closely related SC species, probably because a much smaller fraction of the pollen will Yield screenings of cultivated S. lycopersicum under reach a compatible stigma in the case of SI (Arroyo high temperatures have shown phenotypic variation, 1973; Baker 1955; Cruden 1977; Georgiady and Lord but only a few cultivars, including Nagcarlang, Hotset 2002). Indeed, this study indicated that PN was and Saladette seem to perform relatively well under significantly higher in SI compared to SC accessions. such conditions (Abdul-Baki and Stommel 1995; Importantly, no interaction with temperature treatment Dane et al. 1991; Kugblenu et al. 2013b; Levy et al. was found. Thus, SI accessions seem to be a good 1978; Rudich et al. 1977; Villareal et al. 1978; Xu et al. source for a high PN under LTMH and could be 2017b). Indeed, the Asian Vegetable Research and interesting for thermotolerance breeding purposes. In Development Center (AVRDC; now World contrast to PN, PV was not significantly different Vegetable Center) concluded from screenings of [ between the mating types in either temperature 4000 wild and cultivated accessions under hot treatment. conditions that less than 1% could be considered Several studies indicated that protrusion of the style highly heat tolerant for fruit set (Opena et al. 1992; from the antheridial cone of [ 1 mm prevents fruit set Villareal et al. 1978). Fruit set under high temperature from self-fertilisation (Dane et al. 1991; Rudich et al. has been reported to have low narrow-sense heritabil- 1977; Saeed et al. 2007). As reported previously ity (El Ahmadi and Stevens 1979; Hanson et al. 2002; (Grandillo et al. 2011; Peralta et al. 2008), SP was 123 Euphytica (2018) 214:67 Page 9 of 12 67 reduced in SC accessions and almost absent in some temperature profile (Li et al. 2015). Such local cultivars, probably due to strong trait selection. SP was adaptation may also be seen at the molecular level, enhanced under high temperature, and although SC as the heat stress response of Arabidopsis and accessions still showed less protrusion of the style in Chenopodium album accessions, as measured by LTMH, in many accessions the distance between induction of heat shock proteins, was more strongly anther and style was likely too large to allow self- induced in accessions originating from cooler rather pollination. The tomato cultivars performed relatively than warmer environments (Barua et al. 2008; Zhang well for SP, suggesting wild relatives are less useful et al. 2015). for improving this trait. By enhancing SP under high temperature, SC plants seem to mimic the constitutive SI phenotype. Stimu- Conclusion lating cross-pollination under LTMH via increased SP, and lower PN and PV, might increase the chance We conclude that PN and PV are variable among wild that SC individuals are fertilised by another plant. This and cultivated tomato accessions, and that this vari- fits with the idea that genetic recombination may be ation is adaptive to the local environment in the case of beneficial under stress conditions, allowing the cre- PV. The absence of overall thermotolerant accessions ation of more adapted genotypes (Hedhly et al. 2004; with regards to PV suggests that selective pressure is ¨ not very strong, or that there is a trade-off with an Muller and Rieu 2016). unknown, beneficial trait. Although the best perform- Local adaptation ing wild accessions were equally thermotolerant to the best performing cultivars in terms of PN and PV, the Wild tomatoes occur over a wide range of ecological genetic background of these traits in the wild acces- and climatic conditions, but individual species and sions may be novel and could thus be valuable for accessions are often adapted to particular microcli- thermotolerance breeding of tomato, especially if the mates (Bauchet and Causse 2012; Zuriaga et al. 2009). traits show additivity. In the case of PV, several The diversity of conditions is expressed at the outperforming individuals were identified. Interspeci- morphological, physiological, sexual and molecular fic QTL analysis with S. lycopersicum would be a levels (Bauchet and Causse 2012; Peralta and Spooner logical step towards characterisation and application 2005). We explored whether variation in environment of the traits. Phenotypic improvement from QTLs at the sites of origin of accessions has resulted in depend on unpredictable interactions with the genetic variation in thermotolerance and found that the mean background, probably because variation often PV of accessions correlated negatively with elevation. involves additional, undetected small-effect loci In line with our results, chilling tolerance in tomato (Mackay et al. 2009). Moreover, for successful has also been shown to correlate with elevation: for application, it will be important to consider the geographical populations of L. hirsutum (S. habro- environmental context dependency of the expression chaites), chilling tolerance, including traits such as of QTLs (Collins et al. 2008). In the end, reproductive seedling survival rate and pollen tube growth, was success depends on multiple traits and we hypothesize greatest in those derived from the higher elevations that combining optimal variants of all these traits will (Patterson et al. 1978; Zamir et al. 1981). It seems be needed to significantly improve tolerance of likely that the effects of elevation are mainly due to reproduction to LTMH. Traits may be transferred local temperature profiles. Indeed, we found that and stacked through marker assisted breeding or by temperature variables such as mean annual tempera- application of newly developed genetic modification ture correlated positively to PV under LTMH. Simi- methods (Sander and Joung 2014; Woo et al. 2015). larly, seedling survival and root growth at high Acknowledgement This work was supported by the temperature for natural populations of Arabidopsis Technological Top Institute Green Genetics (grant thaliana correlated to temperature parameters at the number 4CFD047RP, to IR) and the Dutch Topsector site of origin (Zhang et al. 2015). In rice, the presence Horticulture and Starting Materials (grant number 2013-H320, of a major quantitative trait locus (QTL) for thermo- to IR). We are thankful to Jacob Monash for his kind help in proofreading the manuscript. tolerance, TT1, has also been linked to the local 123 67 Page 10 of 12 Euphytica (2018) 214:67 Open Access This article is distributed under the terms of the Sci Hortic 109:212–217. https://doi.org/10.1016/j.scienta. 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Exploring the natural variation for reproductive thermotolerance in wild tomato species

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Abstract

Euphytica (2018) 214:67 https://doi.org/10.1007/s10681-018-2150-2 Exploring the natural variation for reproductive thermotolerance in wild tomato species . . . . Nicky Driedonks Mieke Wolters-Arts Heidrun Huber Gert-Jan de Boer . . Wim Vriezen Celestina Mariani Ivo Rieu Received: 22 December 2017 / Accepted: 7 March 2018 / Published online: 13 March 2018 The Author(s) 2018 Abstract Climate change has become a serious germplasm screenings for thermotolerance have often threat for crop productivity worldwide. The increased used yield as the main measured trait. However, due to frequency of heat waves strongly affects reproductive the complex nature of yield and the relatively narrow success and thus yield for many crop species, implying genetic variation present in the cultivated germplasm that breeding for thermotolerant cultivars is critical for screened, there has been limited progress in under- food security. Insight into the genetic architecture of standing the genetic basis of reproductive heat toler- reproductive heat tolerance contributes to our funda- ance. Extending the screening to wild accessions of mental understanding of the stress sensitivity of this related species that cover a range of climatic condi- process and at the same time may have applied value. tions might be an effective approach to find novel, In the case of tomato (Solanum lycopersicum), more tolerant genetic resources. The purpose of this study was to provide insight into the sensitivity of individual reproductive key traits (i.e. the number of Electronic supplementary material The online version of pollen per flower, pollen viability and style protrusion) this article (https://doi.org/10.1007/s10681-018-2150-2) con- to heat-wave like long-term mild heat (LTMH), and tains supplementary material, which is available to authorized users. determine the extent to which genetic variation exists for these traits among wild tomato species. We found N. Driedonks  M. Wolters-Arts  C. Mariani that these traits were highly variable among the I. Rieu (&) screened accessions. Although no overall thermotol- Department of Molecular Plant Physiology, Institute for Water and Wetland Research, Radboud University, erant species were identified, several S. pimpinelli- 6525 AJ Nijmegen, The Netherlands folium individuals outperformed the best performing e-mail: i.rieu@science.ru.nl cultivar in terms of pollen viability under LTMH. Furthermore, we reveal that there has been local H. Huber Department of Experimental Plant Ecology, Institute for adaptation of reproductive heat tolerance, as acces- Water and Wetland Research, Radboud University, sions from lower elevations and higher annual 6525 AJ Nijmegen, The Netherlands temperature are more likely to show high pollen viability under LTMH. G.-J. de Boer Enza Zaden Research and Development B.V, 1600 AA Enkhuizen, The Netherlands Keywords Heat tolerance  Pollen  Male fertility Wild tomato W. Vriezen Bayer Vegetable Seeds, 6080 AA Haelen, The Netherlands 123 67 Page 2 of 12 Euphytica (2018) 214:67 Introduction of reproductive thermotolerance under long-term mild heat (LTMH). Ambient temperatures are rising as part of the current Secondly, germplasm used in thermotolerance global climate change, threatening agricultural output screening has mainly consisted of S. lycopersicum (IPCC 2007, 2013). High temperatures cause mor- cultivars (Abdul-Baki and Stommel 1995; Dane et al. phological, physiological, biochemical and molecular 1991; Grilli et al. 2007; Kugblenu et al. 2013a; Opena changes in plants that affect growth, and is particularly et al. 1992). However, as a result of domestication and detrimental during the reproductive stages (Wahid intensive breeding, the cultivated tomato germplasm et al. 2007). This leads to reduced yields in crop has a rather narrow genetic base (Bergougnoux 2014), species, and thus has a large impact on global food meaning that only a subset of the genes and alleles production (Barnabas et al. 2008; Hedhly et al. 2009). available in the wild progenitor gene pool are still For example, during cultivation, tomato is often present among crop cultivars (Godfray et al. 2010; exposed to high temperature either in the greenhouse Ladizinsky 1985; Olsen and Wendel 2013). Espe- or in the field, and consequently, fruit set is reduced in cially, as breeding efforts have mainly targeted yield at many S. lycopersicum cultivars. more or less optimal cultivation conditions, it seems Exploration of natural variation may offer insight likely that abiotic stress tolerance traits have been lost into the genetics of stress tolerance, and can provide (Ladizinsky 1985; Paran and Van Der Knaap 2007). genetic diversity useful for breeding (Grandillo et al. This implies that the potential gain in heat tolerance 2011). This also applies to reproductive heat tolerance, level from cultivated germplasm is likely to be limited. but so far, screening for variation in tomato heat A broader genetic diversity can be found in species sensitivity has yielded only a few genotypes consid- related to tomato, and could serve as an alternative ered to be thermotolerant (Opena et al. 1992), and source of plant thermotolerance traits (Vı´quez- these have limited applicability for pre-breeding Zamora et al. 2013). Wild tomato species are found (Grilli et al. 2007). There seem to be at least two in a variety of habitats ranging from sea level to above major reasons for this. Firstly, in previous germplasm 3000 m in altitude and from temperate deserts to wet screenings fruit set was the main trait of interest. tropical rainforests, and thus face a range of environ- However, fruit set is a complex trait, i.e. it represents mental challenges. As a result of natural selection, the sum of multiple sub-traits (yield components). these wild species vary broadly in terms of morphol- Thus, there may be a relatively small chance that ogy, physiology, biochemistry and stress tolerance optimal sub-traits combine to generate a strongly levels (Dolferus 2014; Grandillo et al. 2011; Madu- outperforming genotype. Furthermore, the complexity raimuthu and Prasad 2014). There is also variation in of the relation among different traits involved in fruit mating systems among wild accessions, i.e. self- set complicates genetic analysis. As an alternative compatible (SC) versus self-incompatible (SI), which approach, it might be more effective to analyse the is likely to affect reproductive traits and putatively various contributing sub-traits individually and com- their performance under LTMH (Arroyo 1973; Baker bine them afterwards in a breeding context. For 1955; Cruden 1977; Georgiady and Lord 2002; Peralta example, decreases in tomato fruit set under long-term et al. 2008). mildly elevated temperatures has been shown to Here, we hypothesised that higher levels of repro- correlate with a decrease in pollen viability (Dane ductive thermotolerance are present in wild relatives et al. 1991; Firon et al. 2006; Kinet and Peet 1997; of tomato than in the cultivated tomato germplasm. Levy et al. 1978; Peet et al. 1998; Pressman 2002; We investigated the performance of 64 accessions Pressman et al. 2006; Sato et al. 2000, 2006; Xu et al. across 13 wild species and 7 S. lycopersicum cultivars, 2017b). Also, style protrusion may affect reproductive including a subset known for relatively good repro- success under high temperature (Charles and Harris ductive thermotolerance under control temperature 1972; Dane et al. 1991; Rick and Dempsey 1969; and long-term mild heat. We focused on reproductive Rudich et al. 1977; Saeed et al. 2007; Xu et al. 2017b). traits generally assumed to contribute to overall Investigation of these traits separately might provide a fertility, i.e. the number of pollen per flower, pollen more effective strategy to determine the genetic basis viability and the distance between the top of the anther and the stigma (style protrusion). In addition, we 123 Euphytica (2018) 214:67 Page 3 of 12 67 tested whether the mating system influenced these from the vegetative to the generative phase occurred, traits under LTMH, and determined whether local flower buds were removed and the cuttings were adaption to thermotolerance had occurred. treated similarly as the mother plants. Phenotypic assessment Materials and methods To determine pollen quality, anthers of the three most Plant material and screening procedure recently opened flowers were cut into 4 equal trans- verse sections. After addition of 200 lL peroxidase Sixty-four accessions belonging to 13 wild species (S. indicator (Rodriguez-Riano and Dafni 2000) consist- arcanum, S. cheesmaniae, S. chilense, S. chmielewskii, ing of 1 vial peroxidase indicator (Sigma 3901-10VL) S. corneliomulleri, S. galapagense, S. habrochaites, S. in 0.012% (v/v) H O and 10% (v/v) Trizmal buffer 2 2 huaylasense, S. lycopersicum, S. neorickii, S. pennel- (903C; Sigma–Aldrich, St Louis, MO, USA). Pollen lii, S. peruvianum and S. pimpinellifolium) and 7 S. were considered viable when roundly shaped and lycopersicum cultivars (‘‘Hotset’’, ‘‘Malintka101’’, stained dark. In order to determine the pollen viability ‘‘Moneyberg’’, ‘‘Nagcarlang’’, ‘‘NCHS-1’’, ‘‘Salad- (PV, in %), 100 pollen were assessed per flower. To ette’’ and ‘‘Tof Hamlet’’) were obtained from various determine the number of pollen per flower (PN) the sources (Table S1). Seeds were incubated in 2.5% number of pollen was counted in 25 chambers hypochlorite for 30 min at room temperature to (0.04 mm ) of a haemocytometer. In addition, style improve germination and reduce pathogen load (Rick protrusion (SP in mm) was measured. For PN, PV and and Borgino, TGRC, http://tgrc.ucdavis.edu/seed_ SP, three flowers were analysed per plant. germ.aspx), followed by germination on potting soil (Horticoop, Lentse Potgrond, Slingerland Potgrond) Climate data covered with vermiculite (Agra-Vermiculite) under standard greenhouse conditions. Seedlings were Climatic data sets for the earth land surface area were transferred to 0.5 L pots after 2 weeks and, after downloaded from CHELSA (Karger et al. 2017). 1 month, placed in 12 L pots, containing potting soil Using the R package ‘‘raster’’ version 2.5–8 (Hijmans -1 and 4 g L Osmocote Exact Standard 3–4 M and van Etten 2012), all 19 bioclimatic variable data (Everris). When the transition from the vegetative to (BIO1 to BIO19) were extracted for the period the generative phase occurred, flower buds were 1979-2013 for each accession according to the GPS removed and the plants were transferred to a climate coordinates of the original collection site (Table S1). chamber maintaining a 14/10 h day/night photoperiod -1 -2 (* 300 lmol s m at plant height; Philips Statistical analysis D-Papillon daylight spectrum 340 W lamps and Phi- lips MastergreenPower TLD58 W/840 fluorescent All statistical analyses were performed using trans- tubes) and humidity of 70–80% at either control formed data, value’ = Log(value?1), except for temperature of 25/19 C (CT) or long-term mild heat PV, to which a logit transformation was applied, of 32/26 C (LTMH) for at least 14 days. Plants were value’ = LN((value?1)/(101-value)). The relation grown and analysed in a staggered manner over a time between traits was determined by a Pearson correla- course of 4 months in batches of 15 individuals, with tion analysis. To assess the heritability of the traits, a complete randomisation of accessions. To determine Pearson correlation analysis of the means of clones the influence of the genotype on the studied traits, (cuttings) and their corresponding mother plant were cuttings were taken from several plants. In order to set performed using a paired sample correlation analysis. roots, the cuttings were put in potting soil (Horticoop, Broad-sense heritability was calculated for PN and PV Lentse Potgrond, Slingerland Potgrond) and kept in by dividing the variance among clones by the total the greenhouse in a plastic container to maintain a high variance among and within clones (i.e. variance humidity. After 2 weeks, cuttings were placed in 12 L among clones/total variance). To test for variation in -1 pots, containing potting soil and 4 g L Osmocote heat tolerance among species, a two-way ANOVA Exact Standard 3–4 M (Everris). When the transition was performed at species level (using mean values of 123 67 Page 4 of 12 Euphytica (2018) 214:67 accessions and temperature treatment). Differences in clones of PN and PV measurements of the cuttings performance under LTMH between species (using revealed a broad-sense heritability of 0.78 and 0.85, mean values of accessions) were assessed by a one- respectively (Table 1). way ANOVA followed by Tukey’s HSD as post hoc To evaluate relationships between the traits of test. At accession level (using mean values of plants), interest, Pearson correlation analyses between the trait differences between accessions within species were means per species were performed. In addition, to assessed by a one-way ANOVA using Tukey’s HSD correct for putative species-specific effects, a as post hoc test. For PN and PV, the wild accessions between-species Pearson correlation analysis was were compared to the best tomato cultivar by a one- performed. In both cases, no significant correlations way ANOVA with LSD post hoc test. To test whether were detected (Table 2; data not shown). In addition, the best performing genotypes from the best wild when considering all accessions as independent accession outperformed Nagcarlang, a one-way (n = 71), no significant correlations were detected ANOVA with LSD post hoc test was performed, between any of the traits under CT and LTMH (data using clones as replicates. To analyse the effect of not shown). Together, this suggests that the three traits temperature treatment and mating system, a two-way under study are largely independently inherited in ANOVA with temperature treatment and mating these species and accessions. system class variables was performed. The different Variation in the number of pollen per flower geographical characteristics were correlated with the physiological plant traits under LTMH by Pearson under LTMH correlation analysis. All statistical analyses were performed using IBM SPSS Statistics version 21. Temperature treatment and species both had a signif- icant effect on the number of pollen per flower (PN), and no interaction was found between them (Table 3). Results Exposure to LTMH reduced PN by 76.3% on average. PN of S. corneliomulleri was significantly higher than To assess reproductive performance under long-term that of S. chmielewskii, which had the lowest PN under mild heat (LTMH) in the wild tomato germplasm, 64 LTMH (Table S2). For the S. chilense, S. chmielews- wild accessions belonging to 13 wild species and 7 S. kii, S. galapagense, S. huaylasense, S. pennellii, S. lycopersicum cultivars were screened. Accessions peruvianum and S. lycopersicum cultivars, significant were selected to roughly encompass the spatial and differences in PN among accessions within the species elevation distribution of accessions of each species, were detected under LTMH (Table S3). However, and where possible, including accessions with anno- none of the wild accessions performed better, i.e. had a tations related to high temperature or other abiotic significantly higher PN, than the best performing stresses (Table S1). None of the wild accessions cultivar, NCHS-1 (Fig. 2a; Table S3). screened in this study were previously determined to be heat tolerant. In total, 201 and 317 plants were Variation in pollen viability under LTMH exposed to control (CT) or LTMH conditions, respec- tively, and three reproductive traits, the number of In response to LTMH, PV was reduced by 85.6% on pollen per flower (PN), pollen viability (PV) and style average, at the species level. No significant differences were detected between any of the screened species, protrusion (SP), were analysed. nor was there a significant interactive effect between Trait heritability and inter-trait relations species and treatment (Table 3). Within S. corne- liomulleri, S. neorickii, S. pimpinellifolium and the S. To determine the influence of genotype on the selected lycopersicum cultivars, significant differences were traits, cuttings from several individuals were grown detected among accessions (Table S3). However, also and exposed to LTMH. Significant correlations for this trait, none of the wild accession performed between mother plants and cuttings were detected better under LTMH, i.e. had a significant higher PV, for all traits (Fig. 1). The division of the variance than the best performing cultivar, Nagcarlang among clones by the total variance among and within (Fig. 2b; Table S3). As wild accessions may exhibit 123 Euphytica (2018) 214:67 Page 5 of 12 67 Fig. 1 Correlations of traits between mother plant and cuttings LTMH for a The number of pollen per flower (*1000), b pollen under long-term mild heat. Mother plants of 4 accessions (from viability (%), and c style protrusion (mm). Trait values for the S. corneliomulleri, S. peruvianum, S. pimpinellifolium and S. cuttings represent the average (n = 2–15). The Pearson’s pennellii) and two tomato cultivars (Moneyberg and Nagcar- correlation coefficient is given in each graph (r). Significance lang) were selected based on their difference in pollen viability level (two-tailed): *P \ 0.05; **P \ 0.01; ***P \ 0.001 under LTMH. Cuttings were generated and phenotyped under Table 1 Broad-sense heritability of the number of pollen per Table 3 P-values of a two-way ANOVA testing for the effects flower and pollen viability of temperature treatment and species on key reproductive sub- traits PN PV Trait Treatment Species Treatment * Species Genotype 0.65 0.79 PN \ 0.001 \ 0.001 0.060 Cutting 0.18 0.14 PV \ 0.001 0.214 0.272 Error 0.17 0.06 SP \ 0.001 \ 0.001 0.165 Broad-sense heritability 0.78 0.85 PN pollen number, PV pollen viability, SP style protrusion. For The values given for genotype and cutting represent the details per species see Table S2 variance among and within cuttings, respectively. Error represents the unexplained variance PN number of pollen per flowers, PV pollen viability Variation in style protrusion under LTMH SP was significantly increased by LTMH, and differed Table 2 Pearson’s correlation coefficients among traits of species under long-term mild heat significantly among species, but no interactive effect between temperature treatment and species was Trait PN PV SP detected, indicating that the different species respond PN 1 similarly to LTMH with respect to SP (Table 3 and PV 0.08 1 Table S2). Within species, significant differences SP 0.17 0.15 1 under LTMH were observed only in the cases of S. pimpinellifolium and the tomato cultivars (Table S3). PN pollen number per flower, PV pollen viability, SP style protrusion. None of the correlations (two-tailed) were None of the wild accessions outperformed the culti- significant vars under LTMH (i.e. had lower SP), as cultivar Saladette did not show any protrusion (Table S3). Many wild accessions had higher SP then the cultivars, genotypic diversity, we tested whether the best already under control temperature. performing genotypes from the best wild accession (S. pimpinellifolium LA1630) outperformed Nagcar- lang, using multiple clones per genotype. Indeed, four of the five tested genotypes had significantly higher PV under LMTH than Nagcarlang (Fig. 3). 123 67 Page 6 of 12 Euphytica (2018) 214:67 Fig. 2 Cultivars and the three best performing wild accessions interquartile range (IQR), with indication of the median. Lower with respect to pollen number per flower and pollen viability and upper whiskers represent the smallest and largest observa- under long-term mild heat. a Pollen number per flower (PN), and tions smaller than or equal to lower and upper hinge ± 1.5 * b pollen viability (PV). Box of boxplot represents the IQR, respectively. Each dot represents an individual plant Comparison between self-compatible and self- incompatible accessions To test the effect of an accession’s mating system on reproductive traits, self-compatible (SC) and self- incompatible (SI) accessions were compared. SI accessions had significantly higher PN and SP than SC accessions under both temperature treatments (Fig. 4). None of the traits showed significant inter- action between the two factors. Relationship between trait performance and climatic parameters at site of origin Due to the wide variation in geographical origin of the accessions, ranging from a latitude of - 24.211 to 0.867, longitude of - 91.417 to - 43.083, and elevation from 0 up to 3450 meters above sea level (Fig. 5), accessions habitats were also diverse and Fig. 3 Comparison between Nagcarlang and best performing varied from very dry to wet locations and from sandy genotypes of S. pimpinellifolium LA1630 concerning pollen viability (PV) under long-term mild heat coastal areas to high up in the mountains. To assess whether adaptation to local conditions had occurred, a Pearson correlation analysis between the phenotypic data and various geographical and climatic parameters 123 Euphytica (2018) 214:67 Page 7 of 12 67 Fig. 4 Trait values in self-compatible and self-incompatible ***P \ 0.001, n.s., not significant. a Number of pollen number accession under control and long-term mild heat. To compare per flower (PN), b pollen viability (PV), and c style protrusion genotypes, multiple cuttings per genotype were evaluated. (SP). SC self-compatible, SI self-incompatible, CT control Values represent the mean ± standard deviation (n = 5–20 temperature, LTMH long-term mild heat. Values represent the plants). Differences were assessed by a one-way ANOVA mean ± standard deviation (n = 33 and 37 for SI and SC followed by LSD post hoc test. Asterisks above the bars indicate accessions, respectively). Significance of treatment/mating a significant difference between the respective genotype and the system/interaction was determined by a two-way ANOVA: cultivar Nagcarlang. Significance level: *P \ 0.05; ***P \ 0.001; n.s., not significant Fig. 5 Geographical origin of accessions screened. Elevation is in meters above sea level was performed. A significant negative correlation was temperature parameters (Table S4). Thus, accessions found between the accessions’ PV and the elevation at derived from lower elevations and warmer climates the site of origin (Table 4). A significant positive are more likely to be tolerant to LTMH with respect to correlation was found with the annual mean temper- PV. ature, and the same trend was visible for related 123 67 Page 8 of 12 Euphytica (2018) 214:67 Table 4 Pearson’s correlation coefficients among geographi- Villareal et al. 1978). However, it is a complex trait, cal and physiological traits under long-term mild heat and higher heritability may be found by separating the underlying individual key traits affecting fruit set. Trait EL TEMP PREC Indeed, we have recently reported two major QTLs for PN 0.048 - 0.080 - 0.102 PN and PV in an S. lycopersicum mapping population ** * PV - 0.372 0.279 - 0.102 (Xu et al. 2017a). In the current study, screening of SP 0.103 - 0.106 - 0.281 clones of individuals for PN and PV under LTMH in climate chambers indicated that a large fraction of the Significance level (two-tailed): *P \ 0.05; **P \ 0.01. Additional correlations with bioclimatic variables are total phenotypic variance was explained by the genetic presented in Table S4 variance. Whether these sub-traits also express in PN number of pollen per flower, PV pollen viability, SP style other genetic backgrounds and environmental condi- protrusion; EL elevation, TEMP mean annual temperature in tions, such as field conditions, remains to be period 1979–2013, PREC mean annual precipitation in period determined. 1979–2013. n = 59–69 accessions Our study did not detect overall thermotolerance of yield-contributing sub-traits in wild species compared to the performance of cultivars, but we show that Discussion several genotypes from the accession LA1630 outper- form the best performing cultivar in terms of PV under The reduction in tomato yield under long-term mild LTMH. Given the previously reported correlation heat (LTMH) may be attributed to the plant’s vulner- between PV and fruit set under LTMH (e.g. Dane et al. ability during reproductive development, resulting in a 1991; Sato et al. 2000; Xu et al. 2017b), we conclude lower number of pollen per flower (PN) and pollen that wild germplasm might indeed be a valuable viability (PV) (Dane et al. 1991; Firon et al. 2006; resource to enrich domesticated germplasm for repro- Kinet and Peet 1997; Levy et al. 1978; Peet et al. 1998; ductive thermotolerance. Pressman 2002; Pressman et al. 2006; Sato et al. 2000, 2006; Xu et al. 2017b). In this study, we Mating system advantages under LTMH analysed the natural variation of reproductive thermo- tolerance in wild tomato species, which may serve as In the tomato clade, the mating system ranges between gene sources for cultivated tomato. self-incompatible (SI) to self-compatible (SC) cross- ers (Miller and Tanksley 1990; Rick et al. 1977). In Superior heat tolerant wild genotypes with regard general, flowers of SI plant species produce more to pollen viability pollen than closely related SC species, probably because a much smaller fraction of the pollen will Yield screenings of cultivated S. lycopersicum under reach a compatible stigma in the case of SI (Arroyo high temperatures have shown phenotypic variation, 1973; Baker 1955; Cruden 1977; Georgiady and Lord but only a few cultivars, including Nagcarlang, Hotset 2002). Indeed, this study indicated that PN was and Saladette seem to perform relatively well under significantly higher in SI compared to SC accessions. such conditions (Abdul-Baki and Stommel 1995; Importantly, no interaction with temperature treatment Dane et al. 1991; Kugblenu et al. 2013b; Levy et al. was found. Thus, SI accessions seem to be a good 1978; Rudich et al. 1977; Villareal et al. 1978; Xu et al. source for a high PN under LTMH and could be 2017b). Indeed, the Asian Vegetable Research and interesting for thermotolerance breeding purposes. In Development Center (AVRDC; now World contrast to PN, PV was not significantly different Vegetable Center) concluded from screenings of [ between the mating types in either temperature 4000 wild and cultivated accessions under hot treatment. conditions that less than 1% could be considered Several studies indicated that protrusion of the style highly heat tolerant for fruit set (Opena et al. 1992; from the antheridial cone of [ 1 mm prevents fruit set Villareal et al. 1978). Fruit set under high temperature from self-fertilisation (Dane et al. 1991; Rudich et al. has been reported to have low narrow-sense heritabil- 1977; Saeed et al. 2007). As reported previously ity (El Ahmadi and Stevens 1979; Hanson et al. 2002; (Grandillo et al. 2011; Peralta et al. 2008), SP was 123 Euphytica (2018) 214:67 Page 9 of 12 67 reduced in SC accessions and almost absent in some temperature profile (Li et al. 2015). Such local cultivars, probably due to strong trait selection. SP was adaptation may also be seen at the molecular level, enhanced under high temperature, and although SC as the heat stress response of Arabidopsis and accessions still showed less protrusion of the style in Chenopodium album accessions, as measured by LTMH, in many accessions the distance between induction of heat shock proteins, was more strongly anther and style was likely too large to allow self- induced in accessions originating from cooler rather pollination. The tomato cultivars performed relatively than warmer environments (Barua et al. 2008; Zhang well for SP, suggesting wild relatives are less useful et al. 2015). for improving this trait. By enhancing SP under high temperature, SC plants seem to mimic the constitutive SI phenotype. Stimu- Conclusion lating cross-pollination under LTMH via increased SP, and lower PN and PV, might increase the chance We conclude that PN and PV are variable among wild that SC individuals are fertilised by another plant. This and cultivated tomato accessions, and that this vari- fits with the idea that genetic recombination may be ation is adaptive to the local environment in the case of beneficial under stress conditions, allowing the cre- PV. The absence of overall thermotolerant accessions ation of more adapted genotypes (Hedhly et al. 2004; with regards to PV suggests that selective pressure is ¨ not very strong, or that there is a trade-off with an Muller and Rieu 2016). unknown, beneficial trait. Although the best perform- Local adaptation ing wild accessions were equally thermotolerant to the best performing cultivars in terms of PN and PV, the Wild tomatoes occur over a wide range of ecological genetic background of these traits in the wild acces- and climatic conditions, but individual species and sions may be novel and could thus be valuable for accessions are often adapted to particular microcli- thermotolerance breeding of tomato, especially if the mates (Bauchet and Causse 2012; Zuriaga et al. 2009). traits show additivity. In the case of PV, several The diversity of conditions is expressed at the outperforming individuals were identified. Interspeci- morphological, physiological, sexual and molecular fic QTL analysis with S. lycopersicum would be a levels (Bauchet and Causse 2012; Peralta and Spooner logical step towards characterisation and application 2005). We explored whether variation in environment of the traits. Phenotypic improvement from QTLs at the sites of origin of accessions has resulted in depend on unpredictable interactions with the genetic variation in thermotolerance and found that the mean background, probably because variation often PV of accessions correlated negatively with elevation. involves additional, undetected small-effect loci In line with our results, chilling tolerance in tomato (Mackay et al. 2009). Moreover, for successful has also been shown to correlate with elevation: for application, it will be important to consider the geographical populations of L. hirsutum (S. habro- environmental context dependency of the expression chaites), chilling tolerance, including traits such as of QTLs (Collins et al. 2008). In the end, reproductive seedling survival rate and pollen tube growth, was success depends on multiple traits and we hypothesize greatest in those derived from the higher elevations that combining optimal variants of all these traits will (Patterson et al. 1978; Zamir et al. 1981). It seems be needed to significantly improve tolerance of likely that the effects of elevation are mainly due to reproduction to LTMH. Traits may be transferred local temperature profiles. Indeed, we found that and stacked through marker assisted breeding or by temperature variables such as mean annual tempera- application of newly developed genetic modification ture correlated positively to PV under LTMH. Simi- methods (Sander and Joung 2014; Woo et al. 2015). larly, seedling survival and root growth at high Acknowledgement This work was supported by the temperature for natural populations of Arabidopsis Technological Top Institute Green Genetics (grant thaliana correlated to temperature parameters at the number 4CFD047RP, to IR) and the Dutch Topsector site of origin (Zhang et al. 2015). In rice, the presence Horticulture and Starting Materials (grant number 2013-H320, of a major quantitative trait locus (QTL) for thermo- to IR). We are thankful to Jacob Monash for his kind help in proofreading the manuscript. tolerance, TT1, has also been linked to the local 123 67 Page 10 of 12 Euphytica (2018) 214:67 Open Access This article is distributed under the terms of the Sci Hortic 109:212–217. https://doi.org/10.1016/j.scienta. 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