Mapping and linkage disequilibrium analysis with a genome-wide collection of SNPs that detect polymorphism in cultivated tomato

The history of tomato (Solanum lycopersicum L.) improvement includes genetic bottlenecks, wild species introgressions, and divergence into distinct market classes. This history makes tomato an excellent model to investigate the effects of selection on genome variation. A combination of linkage mapping in two F(2) populations and physical mapping with emerging genome sequence data was used to position 434 PCR-based markers including SNPs. Three-hundred-and-forty markers were used to genotype 102 tomato lines representing wild species, landraces, vintage cultivars, and contemporary (fresh market and processing) varieties. Principal component analysis confirmed genetic divergence between market classes of cultivated tomato (P <0.0001). A genome-wide survey indicated that linkage disequilibrium (LD) decays over 6-8 cM when all cultivated tomatoes, including vintage and contemporary, were considered together. Within contemporary processing varieties, LD decayed over 6-14 cM, and decay was over 3-16 cM within fresh market varieties. Significant inter-chromosomal (gametic phase) LD was detected in both fresh market and processing varieties between chromosomes 2 and 3, and 2 and 4, but in distinct chromosomal locations for each market class. Additional LD was detected between chromosomes 3 and 4, 3 and 11, and 4 and 6 in fresh market varieties and chromosomes 3 and 12 in processing varieties. These results suggest that breeding practices for market specialization in tomato have led to a genetic divergence between fresh market and processing types.     

SNP genotyping reveals genetic diversity between cultivated landraces and contemporary varieties of tomato

Background

The tomato (Solanum lycopersium L.) is the most widely grown vegetable in the world. It was domesticated in Latin America and Italy and Spain are considered secondary centers of diversification. This food crop has experienced severe genetic bottlenecks and modern breeding activities have been characterized by trait introgression from wild species and divergence in different market classes.

Results

With the aim to examine patterns of polymorphism, characterize population structure and identify putative loci under positive selection, we genotyped 214 tomato accessions (which include cultivated landraces, commercial varieties and wild relatives) using a custom-made Illumina SNP-panel. Most of the 175 successfully scored SNP loci were found to be polymorphic. Population structure analysis and estimates of genetic differentiation indicated that landraces constitute distinct sub-populations. Furthermore, contemporary varieties could be separated in groups (processing, fresh and cherry) that are consistent with the recent breeding aimed at market-class specialization. In addition, at the 95% confidence level, we identified 30, 34 and 37 loci under positive selection between landraces and each of the groups of commercial variety (cherry, processing and fresh market, respectively). Their number and genomic locations imply the presence of some extended regions with high genetic variation between landraces and contemporary varieties.

Conclusions

Our work provides knowledge concerning the level and distribution of genetic variation within cultivated tomato landraces and increases our understanding of the genetic subdivision of contemporary varieties. The data indicate that adaptation and selection have led to a genomic signature in cultivated landraces and that the subpopulation structure of contemporary varieties is shaped by directed breeding and largely of recent origin. The genomic characterization presented here is an essential step towards a future exploitation of the available tomato genetic resources in research and breeding programs.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-14-835) contains supplementary material, which is available to authorized users.     

High-Density SNP Genotyping of Tomato (Solanum lycopersicum L.) Reveals Patterns of Genetic Variation Due to Breeding

The effects of selection on genome variation were investigated and visualized in tomato using a high-density single nucleotide polymorphism (SNP) array. 7,720 SNPs were genotyped on a collection of 426 tomato accessions (410 inbreds and 16 hybrids) and over 97% of the markers were polymorphic in the entire collection. Principal component analysis (PCA) and pairwise estimates of F _st supported that the inbred accessions represented seven sub-populations including processing, large-fruited fresh market, large-fruited vintage, cultivated cherry, landrace, wild cherry, and S. pimpinellifolium. Further divisions were found within both the contemporary processing and fresh market sub-populations. These sub-populations showed higher levels of genetic diversity relative to the vintage sub-population. The array provided a large number of polymorphic SNP markers across each sub-population, ranging from 3,159 in the vintage accessions to 6,234 in the cultivated cherry accessions. Visualization of minor allele frequency revealed regions of the genome that distinguished three representative sub-populations of cultivated tomato (processing, fresh market, and vintage), particularly on chromosomes 2, 4, 5, 6, and 11. The PCA loadings and F _st outlier analysis between these three sub-populations identified a large number of candidate loci under positive selection on chromosomes 4, 5, and 11. The extent of linkage disequilibrium (LD) was examined within each chromosome for these sub-populations. LD decay varied between chromosomes and sub-populations, with large differences reflective of breeding history. For example, on chromosome 11, decay occurred over 0.8 cM for processing accessions and over 19.7 cM for fresh market accessions. The observed SNP variation and LD decay suggest that different patterns of genetic variation in cultivated tomato are due to introgression from wild species and selection for market specialization.     

Patterns of genetic diversity in the Andean gene pool of common bean reveal a candidate domestication gene

Most studies on the genetic diversity of common bean (Phaseolus vulgaris L.) have focussed on accessions from the Mesoamerican gene pool compared to the Andean gene pool. A deeper knowledge of the genetic structure of Argentinian germplasm would enable researchers to determine how the Andean domestication event affected patterns of genetic diversity in domesticated beans and to identify candidates for genes targeted by selection during the evolution of the cultivated common bean. A collection of 116 wild and domesticated accessions representing the diversity of the Andean bean in Argentina was genotyped by means of 114 simple sequence repeat (SSR) markers. Forty-seven Mesoamerican bean accessions and 16 Andean bean accessions representing the diversity of Andean landraces and wild accessions were also included. Using the Bayesian algorithm implemented in the software STRUCTURE we identified five major groups that correspond to Mesoamerican and Argentinian wild accessions and landraces and a group that corresponds to accessions from different Andean and Mesoamerican countries. The neighbour-joining algorithm and principal coordinate clustering analysis confirmed the genetic relationships among accessions observed with the STRUCTURE analysis. Argentinian accessions showed a substantial genetic variation with a considerable number of unique haplotypes and private alleles, suggesting that they may have played an important role in the evolution of the species. The results of statistical analyses aimed at identifying genomic regions with consistent patterns of variation were significant for 35 loci (~20 % of the SSRs used in the Argentinian accessions). One of these loci mapped in or near the genomic region of the glutamate decarboxylase gene. Our data characterize the population structure of the Argentinian germplasm. This information on its diversity will be very valuable for use in introgressing Argentinian genes into commercial varieties because the majority of present-day common bean varieties are of Andean origin.     

Unraveling the efficiency of RAPD and SSR markers in diversity analysis and population structure estimation in common bean

Increase in food production viz-a-viz quality of food is important to feed the growing human population to attain food as well as nutritional security. The availability of diverse germplasm of any crop is an important genetic resource to mine the genes that may assist in attaining food as well as nutritional security. Here we used 15 RAPD and 23 SSR markers to elucidate diversity among 51 common bean genotypes mostly landraces collected from the Himalayan region of Jammu and Kashmir, India. We observed that both the markers are highly polymorphic. The discriminatory power of these markers was determined using various parameters like; percent polymorphism, PIC, resolving power and marker index. 15 RAPDs produced 171 polymorphic bands, while 23 SSRs produced 268 polymorphic bands. SSRs showed a higher PIC value (0.300) compared to RAPDs (0.243). Further the resolving power of SSRs was 5.241 compared to 3.86 for RAPDs. However, RAPDs showed a higher marker index (2.69) compared to SSRs (1.279) that may be attributed to their higher multiplex ratio. The dendrograms generated with hierarchical UPGMA cluster analysis grouped genotypes into two main clusters with various degrees of sub clustering within the cluster. Here we observed that both the marker systems showed comparable accuracy in grouping genotypes of common bean according to their area of cultivation. The model based STRUCTURE analysis using 15 RAPD and 23 SSR markers identified a population with 3 sub-populations which corresponds to distance based groupings. High level of genetic diversity was observed within the population. These findings have further implications in common bean breeding as well as conservation programs.     

DNA based iPBS-retrotransposon markers for investigating the population structure of pea (Pisum sativum) germplasm from Turkey

Retrotransposons have been highly studied in monocots; however retrotransposon diversity in dicot crops has not been well documented. Our objective was to assess the diversity harbored by field pea landraces using retrotranposon markers. In this research, molecular characterization of 104 landraces and 34 field pea breeding lines was assessed using newly developed iPBS-retrotransposon markers. The 12 iPBS-retrotransposon primers generated a total 106 scorable bands, and 81 of these were found to be polymorphic (76.4%), with an average of 6.75 polymorphic fragments per primer. Polymorphism information content (PIC) ranged from 0.33 to 0.84 with an average of 0.61. It was evident that field pea landraces from the same geographical region were often placed in different groups in the neighbor joining analysis, indicating that grouping based on genetic parameters was not closely related to the geographical origin. The population structure was determined by using STRUCTURE software, and three populations at K = 3 and five populations at K = 5 were identified among landraces. The plentiful diversity present in Turkish field pea landraces could be used as genetic resource in designing breeding program, and may also contribute to worldwide pea breeding programs. Our data also suggested a role of iPBS-retrotransposons as ‘a universal marker’ for molecular characterization of pea germplasm.     

Genetic Diversity and Population Structure Among Pea (Pisum sativum L.) Cultivars as Revealed by Simple Sequence Repeat and Novel Genic Markers

Field pea (Pisum sativum L.) is an important cool season legume crop widely grown around the world. This research provides a basis for selection of pea germplasm across geographical regions in current and future breeding and genetic mapping efforts for pea improvement. Eleven novel genic markers were developed from pea expressed sequence tag (EST) sequences having significant similarity with gene calls from Medicago truncatula spanning at least one intron. In this study, 96 cultivars widely grown or used in breeding programs in the USA and Canada were analyzed for genetic diversity using 31 microsatellite or simple sequence repeat (SSR) and 11 novel EST-derived genic markers. The polymorphic information content varied from 0.01–0.56 among SSR markers and 0.04–0.43 among genic markers. The results showed that SSR and EST-derived genic markers displayed one or more highly reproducible, multi-allelic, and easy to score loci ranging from 200 to 700 bp in size. Genetic diversity was assessed through unweighted neighbor-joining method, and 96 varieties were grouped into three main clusters based on the dissimilarity matrix. Four subpopulations were determined through STRUCTURE analysis with no significant geographic separation of the subpopulations. The findings of the present study can be used to select diverse genotypes to be used as parents of crosses aimed for breeding improved pea cultivars.     

High-Throughput RAD-SNP Genotyping for Characterization of Sugar Beet Genotypes

High-throughput single-nucleotide polymorphism (SNP) genotyping provides a rapid way of developing resourceful sets of markers for delineating genetic structure and for understanding the basis of the taxonomic discrimination. In this paper, we present a panel of 192 SNPs for effective genotyping in sugar beet using a high-throughput marker array technology, QuantStudio 12K Flex system, coupled with Taqman OpenArray technology. The selected SNPs were evaluated for genetic diversity among a set of 150 individuals representing 15 genotypes (10 individuals each) from five cytoplasmic male steriles (CMSs), five pollinators, and five commercial varieties. We demonstrated that the proposed panel of 192 SNPs effectively differentiated the studied genotypes. A higher degree of polymorphism was observed among the CMSs as compared to pollinators and commercial varieties. PCoA and STRUCTURE analysis revealed that CMSs, pollinators, and varieties clustered into three distinct subpopulations. Our results demonstrate the utility of the identified panel of 192 SNPs coupled with TaqMan OpenArray technology as a wide set of markers for high-throughput SNP genotyping in sugar beet.     

凉山州马铃薯栽培品种的遗传多样性分析

利用SSR分子标记技术对凉山州马铃薯地方品种、自育品种和各地育成品种共计60份进行了遗传多样性分析,筛选出16对有效SSR引物。采用筛选的16对引物在供试材料中共检测到68个位点,其中多态性位点42个占总扩增位点的61.76%,平均PIC值为0.4717。通过NTSYSpc 2.0对所有材料间的遗传距离进行计算,平均遗传距离在2.43~3.96之间。使用MEGA4分析软件将供试材料分成3大类,2个凉山州地方品种在聚类图中最先被分离出来。利用PoPGEN将凉山州地方种和其余材料分成两类,。上述结果表明凉山州地方种与试验中其他品种的遗传基础差异较大,是较好的资源材料。该研究为凉山州地方马铃薯品种在今后育种中的应用提供了部分参考依据。     

Local genetic diversity of sorghum in a village in northern Cameroon: structure and dynamics of landraces

We present the first study of patterns of genetic diversity of sorghum landraces at the local scale. Understanding landrace diversity aids in deciphering evolutionary forces under domestication, and has applications in the conservation of genetic resources and their use in breeding programs. Duupa farmers in a village in Northern Cameroon distinguished 59 named sorghum taxa, representing 46 landraces. In each field, seeds are sown as a mixture of landraces (mean of 12 landraces per field), giving the potential for extensive gene flow. What level of genetic diversity underlies the great morphological diversity observed among landraces? Given the potential for gene flow, how well defined genetically is each landrace? To answer these questions, we recorded spatial patterns of planting and farmers’ perceptions of landraces, and characterized 21 landraces using SSR markers. Analysis using distance and clustering methods grouped the 21 landraces studied into four clusters. These clusters correspond to functionally and ecologically distinct groups of landraces. Within-landrace genetic variation accounted for 30% of total variation. The average F _is over landraces was 0.68, suggesting high inbreeding within landraces. Differentiation among landraces was substantial and significant (F _st = 0.36). Historical factors, variation in breeding systems, and farmers’ practices all affected patterns of genetic variation. Farmers’ practices are key to the maintenance, despite gene flow, of landraces with different combinations of agronomically and ecologically pertinent traits. They must be taken into account in strategies of conservation and use of genetic resources.     

Association mapping to discover significant marker-trait associations for resistance against fusarium wilt variant 2 in pigeonpea [<Emphasis Type="Italic">Cajanus cajan</Emphasis> (L.) Millspaugh] using SSR markers

Pigeonpea production is severely constrained by wilt disease caused by Fusarium udum. In the current study, we discover the putative genomic regions that control resistance response to variant 2 of fusarium wilt using association mapping approach. The association panel comprised of 89 diverse pigeonpea genotypes including seven varieties, three landraces and 79 germplasm lines. The panel was screened rigorously for 3 consecutive years (2013–14, 2014–15 and 2015–2016) against variant 2 in a wilt-sick field. A total of 65 pigeonpea specific hypervariable SSR markers (HASSRs) were screened representing seven linkage groups and 29 scaffolds of the pigeonpea genome. A total of 181 alleles were detected, with average values of gene diversity and polymorphism information content (PIC) of 0.55 and 0.47, respectively. Further analysis using model based (STRUCTURE) and distance based (clustering) approaches separated the entire pigeonpea collection into two distinct subgroups (K = 2). The marker trait associations (MTAs) were established based on three-year wilt incidence data and SSR dataset using a unified mixed linear model. Consequently, six SSR markers were identified, which were significantly associated with wilt resistance and explained up to 6% phenotypic variance (PV) across the years. Among these SSRs, HASSR18 was found to be the most stable and significant, accounting for 5–6% PV across the years. To the best of our knowledge, this is the first report of identification of favourable alleles for resistance to variant 2 of Fusarium udum in pigeonpea using association mapping. The SSR markers identified here will greatly facilitate marker assisted resistance breeding against fusarium wilt in pigeonpea.     

Genetic diversity analysis of traditional and improved Indonesian rice (Oryza sativa L.) germplasm using microsatellite markers

The archipelago of Indonesia has a long history of rice production across a broad range of rice-growing environments resulting in a diverse array of local Indonesian rice varieties. Although some have been incorporated into modern breeding programs, the vast majority of these landraces remain untapped. To better understand this rich source of genetic diversity we have characterized 330 rice accessions, including 246 Indonesian landraces and 63 Indonesian improved cultivars, using 30 fluorescently-labeled microsatellite markers. The landraces were selected across 21 provinces and include representatives of the classical subpopulations of cere, bulu, and gundil rices. A total of 394 alleles were detected at the 30 simple sequence repeat loci, with an average number of 13 alleles per locus across all accessions, and an average polymorphism information content value of 0.66. Genetic diversity analysis characterized the Indonesian landraces as 68% indica and 32% tropical japonica, with an indica gene diversity of 0.53 and a tropical japonica gene diversity of 0.56, and a Fst of 0.38 between the two groups. All of the improved varieties sampled were indica, and had an average gene diversity of 0.46. A set of high quality Indonesian varieties, including Rojolele, formed a separate cluster within the tropical japonicas. This germplasm presents a valuable source of diversity for future breeding and association mapping efforts.     

Population structure and marker–trait associations for pomological traits in peach and nectarine cultivars

Marker–trait associations based on populations from controlled crosses have been established in peach using markers mapped on the peach consensus map. In this study, we explored the utility of unstructured populations for association mapping to determine useful marker–trait associations in peach/nectarine cultivars. We used 94 peach cultivars representing local Spanish and modern cultivars from international breeding programs that are maintained at the Experimental Station of Aula Dei, Spain. This collection was characterized for pomological traits and was screened with 40 SSR markers that span the peach genome. Population structure analysis using STRUCTURE software identified two subpopulations, the local and modern cultivars, with admixture within both groups. The local Spanish cultivars were somewhat less diverse than modern cultivars. Marker–trait associations were determined in TASSEL with and without modelling coefficient of membership (Q) values as covariates. The results showed significant associations with pomological traits. We chose three markers on LG4 because of their proximity to the endoPG locus (freestone–melting flesh) that strongly affects pomological traits. Two genotypes of BPPCT015 marker showed significant associations with harvest date, flavonoids and sorbitol. Also, two genotypes of CPPCT028 showed associations with harvest date, total phenolics, RAC, and total sugars. Finally, two genotypes of endoPG1 showed associations with flesh firmness and total sugars. The analysis of linkage disequilibrium (LD) revealed a high level of LD up to 20 cM, and decay at farther distances. Therefore, association mapping could be a powerful tool for identifying marker–trait associations and would be useful for marker-assisted selection in peach breeding.     

Identifying loci influencing 1,000-kernel weight in wheat by microsatellite screening for evidence of selection during breeding

Chinese wheat mini core collection (262 accessions) was genotyped at 531 microsatellite loci representing a mean marker density of 5.1 cM. One-thousand-kernel weights (TKW) of lines were measured in five trials (three environments in four growing seasons). Structure analysis based on 42 unlinked SSR loci indicated that the materials formed two sub-populations, viz., landraces and modern varieties. A large difference in TKW (7.08 g, P<0.001) was found between the two sub-groups. Therefore, TKW is a major yield component that was improved in the past 6 decades; it increased from a mean 31.5 g in the 1940s to 44.64 g in the 2000s, representing a 2.19 g increase in each decade. Analyses based on a mixed linear model (MLM), population structure (Q) and relative kinship (K) revealed 22 SSR loci that were significantly associated with mean TKW (MTKW) of the five trials estimated by the best linear unbiased predictor (BLUP) method. They were mainly distributed on chromosomes of homoeologous groups 1, 2, 3, 5 and 7. Six loci, cfa2234-3A, gwm156-3B, barc56-5A, gwm234-5B, wmc17-7A and cfa2257-7A individually explained more than 11.84% of the total phenotypic variation. Favored alleles for breeding at the 22 loci were inferred according to their estimated effects on MTKW based on mean difference of varieties grouped by genotypes. Statistical simulation showed that these favored alleles have additive genetic effects. Frequency changes of alleles at loci associated with TKW are much more dramatic than those at neutral loci between the sub-groups. The numbers of favored alleles in modern varieties indicate there is still considerable genetic potential for their use as markers for genome selection of TKW in wheat breeding. Alleles that can be used globally to increase TKW were inferred according to their distribution by latitude and frequency of changes between landraces and the modern varieties.     

Population structure and association mapping on chromosome 7 using a diverse panel of Chinese germplasm of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The majority of 170 rice accessions used in this study were diverse landraces or varieties from a putative mini-core collection of Chinese germplasm along with some widely used parental lines in genetic analysis or breeding (a few from abroad). The population was genotyped using 84 SSR or InDel markers on chromosome 7 and 48 markers on other chromosomes. The phenotyping of heading date, plant height and panicle length were carried out in different locations for 2 years. Based on morphological characterization, distance-based clustering and model-based estimation of marker data, the population showed a predominant structure with two subpopulations in correspondence with indica and japonica subspecies. The estimation of linkage disequilibrium in 2 Mb windows varied along chromosome 7 and showed parallel changes with inter-subspecies differentiation of marker loci (Fst). Based on the mixed linear model considering population structure and family relatedness [i.e. the (Q + K) model], one to three associated markers (P ≤ 0.0001) per trait per experiment were scanned out on rice chromosome 7. Most significant loci were repeated for the data from both field experiments while two loci were associated with two or three traits. Marker-based allelic effects were shown in a couple of associated markers as examples. The application of association results in breeding program was also discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular evaluation of genetic diversity and association studies in rice (<Emphasis Type="BoldItalic">Oryza sativa</Emphasis> L.)

In the present study, we tested rice genotypes that included un(der)exploited landraces of Tamil Nadu along with indica and japonica test cultivars to ascertain their genetic diversity structure. Highly polymorphic microsatellite markers were used for generating marker segregation data. A novel measure, allele discrimination index, was used to determine subpopulation differentiation power of each marker. Phenotypic data were collected for yield and component traits. Pattern of molecular differentiation separated indica and japonica genotypes; indica genotypes had two subpopulations within. Landraces were found to have indica genome, but formed a separate subgroup with low linkage disequilibrium. The landraces further separated into distinct group in both hierarchical clustering analysis using neighbour-joining method as well as in the model based population structure analysis. Japonica and the remaining indica cultivars formed two other distinct groups. Linkage disequilibrium observed in the whole population was considerably reduced in subpopulations. Low linkage disequilibrium of landforms suggests their narrow adaptation in local geographical niche. Many population specific alleles could be identified particularly for japonica cultivars and landraces. Association analysis revealed nine marker-trait associations with three agronomic traits, of which 67% were previously reported. Although the testing landraces together with known cultivars had permitted genomewide association mapping, the experiment offers scope to study more landraces collected from the entire geographical region for drawing more reliable information.     

Assessment of genetic variation among commercial tomato (Solanum lycopersicum L) varieties using SSR markers and morphological characteristics

Simple sequence repeats (SSR) is one of the most suitable markers for variety identification as it has great discrimination power for varieties with limited genetic variation. Genetic characterization of commercial tomato varieties was investigated using 33 SSR markers and 22 morphological traits. Thirty three SSR primer pairs were screened for 63 tomato varieties. A total of 132 polymorphic amplified fragments were obtained by using 33 SSR markers. The average polymorphism information content (PIC) was 0.628 ranging from 0.210 to 0.880. One hundred thirty two SSR loci were used to calculate Jaccard's distance coefficients for UPGMA cluster analysis. A clustering group of varieties, based on the results of SSR analysis, were categorized into cherry and classic fruit type varieties. Almost all of the varieties were discriminated by SSR marker genotypes. The relationship between morphological and molecular data for 33 varieties out of 63 varieties was analyzed using Mantel matrix correspondence test. The correlation value between two methods was 0.644. However, SSR based dendrogram topology showed some similar form with morphological traits at the two main groups. Therefore, these markers may be used wide range of practical application in variety identification and pre-screening for distinctiveness test of tomato varieties.     

Characterization of genetic diversity and population structure in wheat using array based SNP markers

Genetic diversity is crucial for successful adaptation and sustained improvement in crops. India is bestowed with diverse agro-climatic conditions which makes it rich in wheat germplasm adapted to various niches. Germplasm repository consists of local landraces, trait specific genetic stocks including introgressions from wild relatives, exotic collections, released varieties, and improved germplasm. Characterization of genetic diversity is done using morpho-physiological characters as well as by analyzing variations at DNA level. However, there are not many reports on array based high throughput SNP markers having characteristics of genome wide coverage employed in Indian spring wheat germplasm. Amongst wheat SNP arrays, 35K Axiom Wheat Breeder’s Array has the highest SNP polymorphism efficiency suitable for genetic mapping and genetic diversity characterization. Therefore, genotyping was done using 35K in 483 wheat genotypes resulting in 14,650 quality filtered SNPs, that were distributed across the B (~?50%), A (~?39%), and D (~?10%) genomes. The total genetic distance coverage was 4477.85 cM with 3.27 SNP/cM and 0.49 cM/SNP as average marker density and average inter-marker distance, respectively. The PIC ranged from 0.09 to 0.38 with an average of 0.29 across genomes. Population structure and Principal Coordinate Analysis resulted in two subpopulations (SP1 and SP2). The analysis of molecular variance revealed the genetic variation of 2% among and 98% within subpopulations indicating high gene flow between SP1 and SP2. The subpopulation SP2 showed high level of genetic diversity based on genetic diversity indices viz. Shannon’s information index (I)?=?0.648, expected heterozygosity (He)?=?0.456 and unbiased expected heterozygosity (uHe)?=?0.456. To the best of our knowledge, this study is the first to include the largest set of Indian wheat genotypes studied exclusively for genetic diversity. These findings may serve as a potential source for the identification of uncharacterized QTL/gene using genome wide association studies and marker assisted selection in wheat breeding programs.

     

Genetic Diversity and Core Collection Evaluations in Common Wheat Germplasm from the Northwestern Spring Wheat Region in China

Fluorescence microsatellite markers were employed to reveal genetic diversity of 340 wheat accessions consisting of 229 landraces and 111 modern varieties from the Northwest Spring Wheat Region in China. The 340 accessions were chosen as candidate core collections for wheat germplasm in this region. A core collection representing the genetic diversity of these accessions was identified based on a cluster dendrogram of 78 SSR loci. A total of 967 alleles were detected with a mean of 13.6 alleles (5–32) per locus. Mean PIC was 0.64, ranged from 0.05 to 0.91. All loci were distributed relatively evenly in the A, B and D wheat genomes. Mean genetic richness of A, B and D genomes for both landraces and modern varieties was B > A > D. However, mean genetic diversity indices of landraces changed to B > D > A. As a whole, genetic diversity of the landraces was considerably higher than that of the modern varieties. The big difference of genetic diversity indices in the three genomes suggested that breeding has exerted greater selection pressure in the D than the A or B genomes in this region. Changes of allelic proportions represented in the proposed core collection at different sampling scales suggested that the sampling percentage of the core collection in the Northwest Spring Wheat Region should be greater than 4% of the base collection to ensure that more than 70% of the variation is represented by the core collection. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.