Map Length Research Articles

English

For pearl millet production in the semi-arid tropics, Sclerospora graminicola is a major biotic constraint causes Downy mildew. The pathogen is heterothallic and frequent recombination leads to evolution of new virulent populations. Identification of resistance to new virulent isolates is a prerequisite for resistance breeding. A very good understanding of ability to manipulate oligogenic and quantitative traits is offered to plant breeders by recent advances in genetic marker technology. In the present investigation genotypic data generated for a total of 88 marker loci (39 Genomic SSRs and 49 EST SSRs) were used to construct a linkage map of the pearl millet mapping population of 188 F8 RIL progenies based on the cross ICMB 89111-P6 x ICMB 90111-P6. A skeleton linkage map of seven linkage groups with a total map length of 725.5 cM (Haldane units) was constructed using data from 74 marker loci for 188 RILs using JoinMap at LOD threshold value of 5.0 and MapMaker/Exp version 3.0b and map was drawn using Map Chart 2.2. The map length of individual linkage groups ranged from a minimum of 32.1cM (LG3) to a maximum of 140.2 cM (LG1). The average inter marker distance was 9.8 cM, with an average density of 0.102 markers/cM. The total number of mapped loci per linkage group (LG) ranged from 5 on LG3 to 23 on LG1.

Unleashing meiotic crossovers in hybrid plants

Meiotic crossovers shuffle parental genetic information, providing novel combinations of alleles on which natural or artificial selection can act. However, crossover events are relatively rare, typically one to three exchange points per chromosome pair. Recent work has identified three pathways limiting meiotic crossovers in Arabidopsis thaliana that rely on the activity of FANCM [Crismani W, et al. (2012) Science 336:1588-1590], RECQ4 [Séguéla-Arnaud M, et al. (2015) Proc Natl Acad Sci USA 112:4713-4718], and FIGL1 [Girard C, et al. (2015) PLoS Genet 11:e1005369]. Here we analyzed recombination in plants in which one, two, or all three of these pathways were disrupted in both pure line and hybrid contexts. The greatest effect was observed when combining recq4 and figl1 mutations, which increased the hybrid genetic map length from 389 to 3,037 cM. This corresponds to an unprecedented 7.8-fold increase in crossover frequency. Disrupting the three pathways did not further increase recombination, suggesting that some upper limit had been reached. The increase in crossovers is not uniform along chromosomes and rises from centromere to telomere. Finally, although in wild type recombination is much higher in male meiosis than in female meiosis (490 cM vs. 290 cM), female recombination is higher than male recombination in recq4 figl1 (3,200 cM vs. 2,720 cM), suggesting that the factors that make wild-type female meiosis less recombinogenic than male wild-type meiosis do not apply in the mutant context. The massive increase in recombination observed in recq4 figl1 hybrids opens the possibility of manipulating recombination to enhance plant breeding efficiency.

Construction of a SNP-based genetic linkage map for kiwifruit using next-generation restriction-site-associated DNA sequencing (RADseq)

Kiwifruit is a perennial horticultural crop species of the Actinidiaceae family and has high nutritional value. For a species with a long generation time, traditional breeding and genetic improvement is predicted to take more than 20 years to obtain superior cultivars. Thus, marker-assisted selection (MAS) should be used to accelerate the breeding process. Development of a genetic linkage map and molecular markers are pre-requisites for MAS of crop species. Here, we report a genome-wide SNP-based genetic map of kiwifruit by analysing next-generation restriction-site-associated DNA sequencing (RADseq) reads. To construct a genetic linkage map, a 102 F1 line mapping population of Actinidia chinensis (2n = 58) was derived by combining parents that had contrasting phenotypic traits. The maternal map contained 4112 SNP loci and spanned a distance of 3821 cM, with an average adjacent-marker interval length of 0.929 cM. The map length of the 29 linkage groups ranged from 78.3 to 169.9 cM, with an average length of 131.8 cM. High levels of collinearity between the 29 genetic maps with the kiwifruit reference genome were found. The genetic map developed in this study can serve as an important platform to improve kiwifruit research, including anchoring unmapped scaffolds of the kiwifruit genome sequence and mapping QTLs (quantitative trait loci) that control economically important traits.

Construction of a genetic map using EST-SSR markers and QTL analysis of major agronomic characters in hexaploid sweet potato (Ipomoea batatas (L.) Lam).

The Sweet potato, Ipomoea batatas (L.) Lam, is difficult to study in genetics and genomics because it is a hexaploid. The sweet potato study not have been performed domestically or internationally. In this study was performed to construct genetic map and quantitative trait loci (QTL) analysis. A total of 245 EST-SSR markers were developed, and the map was constructed by using 210 of those markers. The total map length was 1508.1 cM, and the mean distance between markers was 7.2 cM. Fifteen characteristics were investigated for QTLs analysis. According to those, the Four QTLs were identified, and The LOD score was 3.0. Further studies need to develop molecular markers in terms of EST-SSR markers for doing to be capable of efficient breeding. The genetic map created here using EST-SSR markers will facilitate planned breeding of sweet potato cultivars with various desirable traits.

Constructing high-density genetic maps for polyploid sugarcane (Saccharum spp.) and identifying quantitative trait loci controlling brown rust resistance

Sugarcane (Saccharum spp.) is an important economic crop for producing edible sugar and bioethanol. Brown rust has long been a major disease impacting sugarcane production worldwide. Resistance resource and markers linked to resistance are valuable tools for disease resistance improvement. An F1 segregating population derived from a cross between two hybrid sugarcane clones, brown rust-susceptible CP95-1039 and brown rust-resistant CP88-1762, were genotyped using genotyping by sequencing approach and also phenotyped in a replicated field trial. Single nucleotide polymorphism (SNP) and presence/absence markers were called with seven different pipelines to maximize reliable marker identification. High-density maps were constructed for both parental clones with a total map length of 4224.4 cM, and a marker density of one marker per 1.7 cM for CP95-1039, and a total map length of 4373.2 cM, and one marker per 2.0 cM for CP88-1762. Among the seven SNP callers, Tassel and Genome Analysis ToolKit performed better than other callers in single-dose SNP detection and contribution to genetic maps. Two major quantitative trait loci (QTL) controlling brown rust resistance were identified, which can explain 21 and 30% of the phenotypic variation, respectively. The genetic maps generated here will improve our understanding of sugarcane’s complex genome structure and discovery of underlying sequence variations controlling agronomic traits. The putative QTL controlling brown rust resistance can effectively be utilized in sugarcane breeding programs to expedite the selection process of brown rust resistance after validation.

Chromosomal inversion differences correlate with range overlap in passerine birds.

Chromosomal inversions evolve frequently but the reasons for this remain unclear. We used cytological descriptions of 411 species of passerine birds to identify large pericentric inversion differences between species, based on the position of the centromere. Within 81 small clades comprising 284 of the species, we found 319 differences on the 9 largest autosomes combined, 56 on the Zchromosome, and 55 on the Wchromosome. We also identified inversions present within 32 species. Using a new fossil-calibrated phylogeny, we examined the phylogenetic, demographic and genomic context in which these inversions have evolved. The number of inversion differences between closely related species is consistently predicted by whether the ranges of species overlap, even when time is controlled for as far as is possible. Fixation rates vary across the autosomes, but inversions are more likely to be fixed on the Zchromosome than the average autosome. Variable mutagenic input alone (estimated by chromosome size, map length, GC content or repeat density) cannot explain the differences between chromosomes in the number of inversions fixed. Together, these results support a model in which inversions increase because of their effects on recombination suppression in the face of hybridization. Other factors associated with hybridization may also contribute, including the possibility that inversions contain incompatibility alleles, making taxa less likely to collapse following secondary contact.

Effect of Co-segregating Markers on High-Density Genetic Maps and Prediction of Map Expansion Using Machine Learning Algorithms

Advances in sequencing and genotyping methods have enable cost-effective production of high throughput single nucleotide polymorphism (SNP) markers, making them the choice for linkage mapping. As a result, many laboratories have developed high-throughput SNP assays and built high-density genetic maps. However, the number of markers may, by orders of magnitude, exceed the resolution of recombination for a given population size so that only a minority of markers can accurately be ordered. Another issue attached to the so-called ‘large p, small n’ problem is that high-density genetic maps inevitably result in many markers clustering at the same position (co-segregating markers). While there are a number of related papers, none have addressed the impact of co-segregating markers on genetic maps. In the present study, we investigated the effects of co-segregating markers on high-density genetic map length and marker order using empirical data from two populations of wheat, Mohawk × Cocorit (durum wheat) and Norstar × Cappelle Desprez (bread wheat). The maps of both populations consisted of 85% co-segregating markers. Our study clearly showed that excess of co-segregating markers can lead to map expansion, but has little effect on markers order. To estimate the inflation factor (IF), we generated a total of 24,473 linkage maps (8,203 maps for Mohawk × Cocorit and 16,270 maps for Norstar × Cappelle Desprez). Using seven machine learning algorithms, we were able to predict with an accuracy of 0.7 the map expansion due to the proportion of co-segregating markers. For example in Mohawk × Cocorit, with 10 and 80% co-segregating markers the length of the map inflated by 4.5 and 16.6%, respectively. Similarly, the map of Norstar × Cappelle Desprez expanded by 3.8 and 11.7% with 10 and 80% co-segregating markers. With the increasing number of markers on SNP-chips, the proportion of co-segregating markers in high-density maps will continue to increase making map expansion unavoidable. Therefore, we suggest developers improve linkage mapping algorithms for efficient analysis of high-throughput data. This study outlines a practical strategy to estimate the IF due to the proportion of co-segregating markers and outlines a method to scale the length of the map accordingly.

Quantitative trait locus mapping under irrigated and drought treatments based on a novel genetic linkage map in mungbean (Vigna radiata L.).

A novel genetic linkage map was constructed using SSR markers and stable QTLs were identified for six drought tolerance related-traits using single-environment analysis under irrigation and drought treatments. Mungbean (Vigna radiata L.) is one of the most important leguminous food crops. However, mungbean production is seriously constrained by drought. Isolation of drought-responsive genetic elements and marker-assisted selection breeding will benefit from the detection of quantitative trait locus (QTLs) for traits related to drought tolerance. In this study, we developed a full-coverage genetic linkage map based on simple sequence repeat (SSR) markers using a recombinant inbred line (RIL) population derived from an intra-specific cross between two drought-resistant varieties. This novel map was anchored with 313 markers. The total map length was 1010.18cM across 11 linkage groups, covering the entire genome of mungbean with a saturation of one marker every 3.23cM. We subsequently detected 58 QTLs for plant height (PH), maximum leaf area (MLA), biomass (BM), relative water content, days to first flowering, and seed yield (Yield) and 5 for the drought tolerance index of 3 traits in irrigated and drought environments at 2 locations. Thirty-eight of these QTLs were consistently detected two or more times at similar linkage positions. Notably, qPH5A and qMLA2A were consistently identified in marker intervals from GMES5773 to MUS128 in LG05 and from Mchr11-34 to the HAAS_VR_1812 region in LG02 in four environments, contributing 6.40-20.06% and 6.97-7.94% of the observed phenotypic variation, respectively. None of these QTLs shared loci with previously identified drought-related loci from mungbean. The results of these analyses might facilitate the isolation of drought-related genes and help to clarify the mechanism of drought tolerance in mungbean.

A High-Density Linkage Map Reveals Sexual Dimorphism in Recombination Landscapes in Red Deer (Cervus elaphus).

High-density linkage maps are an important tool to gain insight into the genetic architecture of traits of evolutionary and economic interest, and provide a resource to characterize variation in recombination landscapes. Here, we used information from the cattle genome and the 50 K Cervine Illumina BeadChip to inform and refine a high-density linkage map in a wild population of red deer (Cervus elaphus). We constructed a predicted linkage map of 38,038 SNPs and a skeleton map of 10,835 SNPs across 34 linkage groups. We identified several chromosomal rearrangements in the deer lineage relative to sheep and cattle, including six chromosome fissions, one fusion, and two large inversions. Otherwise, our findings showed strong concordance with map orders in the cattle genome. The sex-averaged linkage map length was 2739.7 cM and the genome-wide autosomal recombination rate was 1.04 cM/Mb. The female autosomal map length was 1.21 longer than that of males (2767.4 cM vs. 2280.8 cM, respectively). Sex differences in map length were driven by high female recombination rates in peri-centromeric regions, a pattern that is unusual relative to other mammal species. This effect was more pronounced in fission chromosomes that would have had to produce new centromeres. We propose two hypotheses to explain this effect: (1) that this mechanism may have evolved to counteract centromeric drive associated with meiotic asymmetry in oocyte production; and/or (2) that sequence and structural characteristics suppressing recombination in close proximity to the centromere may not have evolved at neo-centromeres. Our study provides insight into how recombination landscapes vary and evolve in mammals, and will provide a valuable resource for studies of evolution, genetic improvement, and population management in red deer and related species.

High-Density Genetic Linkage Map Construction and Quantitative Trait Locus Mapping for Hawthorn (Crataegus pinnatifida Bunge)

Genetic linkage maps are an important tool in genetic and genomic research. In this study, two hawthorn cultivars, Qiujinxing and Damianqiu, and 107 progenies from a cross between them were used for constructing a high-density genetic linkage map using the 2b-restriction site-associated DNA (2b-RAD) sequencing method, as well as for mapping quantitative trait loci (QTL) for flavonoid content. In total, 206,411,693 single-end reads were obtained, with an average sequencing depth of 57× in the parents and 23× in the progeny. After quality trimming, 117,896 high-quality 2b-RAD tags were retained, of which 42,279 were polymorphic; of these, 12,951 markers were used for constructing the genetic linkage map. The map contained 17 linkage groups and 3,894 markers, with a total map length of 1,551.97 cM and an average marker interval of 0.40 cM. QTL mapping identified 21 QTLs associated with flavonoid content in 10 linkage groups, which explained 16.30–59.00% of the variance. This is the first high-density linkage map for hawthorn, which will serve as a basis for fine-scale QTL mapping and marker-assisted selection of important traits in hawthorn germplasm and will facilitate chromosome assignment for hawthorn whole-genome assemblies in the future.

Linkage Map of Lissotriton Newts Provides Insight into the Genetic Basis of Reproductive Isolation.

Linkage maps are widely used to investigate structure, function, and evolution of genomes. In speciation research, maps facilitate the study of the genetic architecture of reproductive isolation by allowing identification of genomic regions underlying reduced fitness of hybrids. Here we present a linkage map for European newts of the Lissotriton vulgaris species complex, constructed using two families of F2 L. montandoni × L. vulgaris hybrids. The map consists of 1146 protein-coding genes on 12 linkage groups, equal to the haploid chromosome number, with a total length of 1484 cM (1.29 cM per marker). It is notably shorter than two other maps available for salamanders, but the differences in map length are consistent with cytogenetic estimates of the number of chiasmata per chromosomal arm. Thus, large salamander genomes do not necessarily translate into long linkage maps, as previously suggested. Consequently, salamanders are an excellent model to study evolutionary consequences of recombination rate variation in taxa with large genomes and a similar number of chromosomes. A complex pattern of transmission ratio distortion (TRD) was detected: TRD occurred mostly in one family, in one breeding season, and was clustered in two genomic segments. This is consistent with environment-dependent mortality of individuals carrying L. montandoni alleles in these two segments and suggests a role of TRD blocks in reproductive isolation. The reported linkage map will empower studies on the genomic architecture of divergence and interactions between the genomes of hybridizing newts.

A high-density intraspecific SNP linkage map of pigeonpea (Cajanas cajan L. Millsp.).

Pigeonpea (Cajanus cajan (L.) Millsp.) is a major food legume cultivated in semi-arid tropical regions including the Indian subcontinent, Africa, and Southeast Asia. It is an important source of protein, minerals, and vitamins for nearly 20% of the world population. Due to high carbon sequestration and drought tolerance, pigeonpea is an important crop for the development of climate resilient agriculture and nutritional security. However, pigeonpea productivity has remained low for decades because of limited genetic and genomic resources, and sparse utilization of landraces and wild pigeonpea germplasm. Here, we present a dense intraspecific linkage map of pigeonpea comprising 932 markers that span a total adjusted map length of 1,411.83 cM. The consensus map is based on three different linkage maps that incorporate a large number of single nucleotide polymorphism (SNP) markers derived from next generation sequencing data, using Illumina GoldenGate bead arrays, and genotyping with restriction site associated DNA (RAD) sequencing. The genotyping-by-sequencing enhanced the marker density but was met with limited success due to lack of common markers across the genotypes of mapping population. The integrated map has 547 bead-array SNP, 319 RAD-SNP, and 65 simple sequence repeat (SSR) marker loci. We also show here correspondence between our linkage map and published genome pseudomolecules of pigeonpea. The availability of a high-density linkage map will help improve the anchoring of the pigeonpea genome to its chromosomes and the mapping of genes and quantitative trait loci associated with useful agronomic traits.

A Highly Dense Genetic Map for Ginkgo biloba Constructed Using Sequence-Based Markers.

Ginkgo biloba L. is a well-known living gymnosperm fossil that has medicinal and ornamental value. In this study, a high density genetic map was constructed with megagametophytes of 94 seeds from a single Ginkgo tree by employing the specific-locus amplified fragment (SLAF) sequencing technique. The average sequencing depth was 11.20×, which yielded 538,031 high-quality SLAFs. Among these SLAFs, 204,361 were heterozygous in the maternal tree and segregated in the progeny. The established map contained 12,263 SLAFs that were assigned to 12 linkage groups (LGs). The number of LGs on this map equaled the number of chromosomes in Ginkgo. The total map length was 1,671.77 cM, with an average distance of 0.89 cM between adjacent marker bins. Map evaluation based on the haplotype map and the heat map validated the high quality of the established map. Because Ginkgo is an economically and biologically important tree, strenuous efforts have been exerted to sequence its genome. This new map, built using sequence-based markers, will serve in the future as a fundamental platform for anchoring sequence assemblies along Ginkgo chromosomes. This map also provides a desirable platform for various genetic studies of Ginkgo, including gene/quantitative trait locus mapping and marker-aided selection.

A SNP-Based Genetic Linkage Map of Soybean Using the SoySNP6K Illumina Infinium BeadChip Genotyping Array

This study reports a high density genetic linkage map based on the ‘Maryland 96-5722’ by ‘Spencer’ recombinant inbred line (RIL) population of soybean [Glycine max (L.) Merr.] and constructed exclusively with single nucleotide polymorphism (SNP) markers. The Illumina Infinium SoySNP6K BeadChip genotyping array produced 5,376 SNPs in the mapping population, with a 96.75% success rate. Significant level of goodness-of-fit for each locus was tested based on the observed vs. expected ratio (1:1). Out of 5,376 markers, 1,465 SNPs fit the 1:1 segregation rate having ≤20% missing data plus heterozygosity among the RILs. Among this 1,456 just 657 were polymorphic between the parents DNAs tested. These 657 SNPs were mapped using the JoinMap 4.0 software and 550 SNPs were distributed on 16 linkage groups (LGs) among the 20 chromosomes of the soybean genome. The total map length was just 201.57 centiMorgans (cM) with an average marker density of 0.37 cM. This is one of the high density SNP-based genetic linkage maps of soybean that will be used by the scientific community to map quantitative trait loci (QTL) and identify candidate genes for important agronomic traits in soybean.

Characterization and mapping of Dt1 locus which co-segregates with CcTFL1 for growth habit in pigeonpea

Key messageWe report growth habit profiling following SEM, genetic mapping and QTL analysis. HighlightedCcTFL1, a candidate for determinacy in pigeonpea, since an Indel marker derived from this gene co-segregated withDt1locus.Pigeonpea (Cajanus cajan) is one of the most important legume crops grown in arid and semi-arid regions of the world. It is characterized with few unique features compared with other legume species, such as Lotus, Medicago, and Glycine. One of them is growth habit, an important agronomic trait. In the present study, identification of mutations affecting growth habit accompanied by a precise analysis of phenotype has been done which will shed more light upon developmental regulation in pigeonpea. A genetic study was conducted to examine the inheritance of growth habit and a genotyping by sequencing (GBS)-based genetic map constructed using F2 mapping population derived from crossing parents ICP 5529 and ICP 11605. Inheritance studies clearly demonstrated the dominance of indeterminate (IDT) growth habit over determinate (DT) growth habit in F2 and F2:3 progenies. A total of 787 SNP markers were mapped in the genetic map of 1454 cM map length. Growth habit locus (Dt1) was mapped on the CcLG03 contributing more than 61% of total phenotypic variations. Subsequently, QTL analysis highlighted one gene, CcTFL1, as a candidate for determinacy in pigeonpea, since an Indel marker derived from this gene co-segregated with the Dt1 locus. Ability of this Indel-derived marker to differentiate DT/IDT lines was also validated on 262 pigeonpea lines. This study clearly demonstrated that CcTFL1 is a candidate gene for growth habit in pigeonpea and a user-friendly marker was developed in the present study which will allow low-cost genotyping without need of automation.

Construction of two genetic linkage maps of taro using single nucleotide polymorphism and microsatellite markers

Linkage maps are needed for genetic studies and molecular breeding of taro (Colocasia esculenta). In this study, we used genotyping-by-sequencing (GBS) to identify single nucleotide polymorphism (SNP) loci on two mapping populations: F31 (HLB11 × VU006) composed of 266 progenies and F32 [HLB01 × (VU370×ID316)] composed of 292 progenies. SNP calling generated an initial set of 22,734 SNPs for F31 and 16,744 for F32. A large proportion of individuals and loci were later removed by filtering on the proportion of missing data and segregation distortions. Linkage maps were constructed with filtered SNPs in association with 14 simple sequence repeat (SSR) markers, using the maximum likelihood method. In both populations, loci were successfully grouped into 14 linkage groups (LGs) with an independence logarithm of odds (LOD) threshold of 11.0 and 8.0 for F31 and F32, respectively. LGs ranged in size from 90 to 15 markers for F31 and from 92 to 12 markers for F32. Bridge markers (459 SNPs and 9 SSRs) were identified and revealed homologous groups between families. Although our maps presented unprecedented chromosome coverage, the colinearity between homologous groups was low (except for LG07), and map lengths were globally inflated. Putative effects of missing data, segregation distortion, and genotyping errors on map accuracy are discussed. This research work led to the identification of a reliable set of SNPs potentially useful as a tool for a wide range of genetic studies in taro.

GBS-based single dosage markers for linkage and QTL mapping allow gene mining for yield-related traits in sugarcane

BackgroundSugarcane (Saccharum spp.) is predominantly an autopolyploid plant with a variable ploidy level, frequent aneuploidy and a large genome that hampers investigation of its organization. Genetic architecture studies are important for identifying genomic regions associated with traits of interest. However, due to the genetic complexity of sugarcane, the practical applications of genomic tools have been notably delayed in this crop, in contrast to other crops that have already advanced to marker-assisted selection (MAS) and genomic selection. High-throughput next-generation sequencing (NGS) technologies have opened new opportunities for discovering molecular markers, especially single nucleotide polymorphisms (SNPs) and insertion-deletion (indels), at the genome-wide level. The objectives of this study were to (i) establish a pipeline for identifying variants from genotyping-by-sequencing (GBS) data in sugarcane, (ii) construct an integrated genetic map with GBS-based markers plus target region amplification polymorphisms and microsatellites, (iii) detect QTLs related to yield component traits, and (iv) perform annotation of the sequences that originated the associated markers with mapped QTLs to search putative candidate genes.ResultsWe used four pseudo-references to align the GBS reads. Depending on the reference, from 3,433 to 15,906 high-quality markers were discovered, and half of them segregated as single-dose markers (SDMs) on average. In addition to 7,049 non-redundant SDMs from GBS, 629 gel-based markers were used in a subsequent linkage analysis. Of 7,678 SDMs, 993 were mapped. These markers were distributed throughout 223 linkage groups, which were clustered in 18 homo(eo)logous groups (HGs), with a cumulative map length of 3,682.04 cM and an average marker density of 3.70 cM. We performed QTL mapping of four traits and found seven QTLs. Our results suggest the presence of a stable QTL across locations. Furthermore, QTLs to soluble solid content (BRIX) and fiber content (FIB) traits had markers linked to putative candidate genes.ConclusionsThis study is the first to report the use of GBS for large-scale variant discovery and genotyping of a mapping population in sugarcane, providing several insights regarding the use of NGS data in a polyploid, non-model species. The use of GBS generated a large number of markers and still enabled ploidy and allelic dosage estimation. Moreover, we were able to identify seven QTLs, two of which had great potential for validation and future use for molecular breeding in sugarcane.

Construction of high-resolution recombination maps in Asian seabass

BackgroundA high-density genetic map is essential for de novo genome assembly, fine mapping QTL for important complex traits, comparative genomic studies and understanding the mechanisms of genome evolution. Although a number of genomic resources are available in Asian seabass (Lates calcarifer), a high-density linkage map is still lacking. To facilitate QTL mapping for marker-assisted selection and genome assembly, and to understand the genome-wide recombination rates, we constructed high density linkage maps using three families and genotyping by sequencing.ResultsA high-density consensus linkage map consisting of 8, 274 markers was constructed based on sex-averaged genetic maps. The genetic maps were then aligned and integrated with the current genome assembly of Asian seabass. More than 90% of the genome contig sequences were anchored onto the consensus genetic map. Evidence of assembly errors in the current genome assembly was identified. A fragment of up to 2.5 Mb belonging to LG14 was assembled into Chr15. The length of family-specific sex-averaged maps ranged from 1348.96 to 1624.65 cM. Female maps were slightly longer than male maps using common markers. Female-to-male ratios were highly variable both across chromosomes within each family and throughout three families for each chromosome. However, the distribution patterns of recombination along chromosomes were similar between sexes across the whole genome. The overall recombination rates were significantly correlated with genome-wide GC content and the correlations were revealed to be stronger in females than in males.ConclusionsThese high-density genetic maps provide not only essential tools for facilitating de novo genome assembly and comparative genomic studies in teleosts, but also critical resources for fine mapping QTL and genome-wide association mapping for economically important traits in Asian seabass.

Insights Into Upland Cotton (Gossypium hirsutum L.) Genetic Recombination Based on 3 High-Density Single-Nucleotide Polymorphism and a Consensus Map Developed Independently With Common Parents.

High-density linkage maps are vital to supporting the correct placement of scaffolds and gene sequences on chromosomes and fundamental to contemporary organismal research and scientific approaches to genetic improvement, especially in paleopolyploids with exceptionally complex genomes, eg, upland cotton (Gossypium hirsutum L., “2n = 52”). Three independently developed intraspecific upland mapping populations were analyzed to generate 3 high-density genetic linkage single-nucleotide polymorphism (SNP) maps and a consensus map using the CottonSNP63K array. The populations consisted of a previously reported F2, a recombinant inbred line (RIL), and reciprocal RIL population, from “Phytogen 72” and “Stoneville 474” cultivars. The cluster file provided 7417 genotyped SNP markers, resulting in 26 linkage groups corresponding to the 26 chromosomes (c) of the allotetraploid upland cotton (AD)1 arisen from the merging of 2 genomes (“A” Old World and “D” New World). Patterns of chromosome-specific recombination were largely consistent across mapping populations. The high-density genetic consensus map included 7244 SNP markers that spanned 3538 cM and comprised 3824 SNP bins, of which 1783 and 2041 were in the At and Dt subgenomes with 1825 and 1713 cM map lengths, respectively. Subgenome average distances were nearly identical, indicating that subgenomic differences in bin number arose due to the high numbers of SNPs on the Dt subgenome. Examination of expected recombination frequency or crossovers (COs) on the chromosomes within each population of the 2 subgenomes revealed that COs were also not affected by the SNPs or SNP bin number in these subgenomes. Comparative alignment analyses identified historical ancestral At-subgenomic translocations of c02 and c03, as well as of c04 and c05. The consensus map SNP sequences aligned with high congruency to the NBI assembly of Gossypium hirsutum. However, the genomic comparisons revealed evidence of additional unconfirmed possible duplications, inversions and translocations, and unbalance SNP sequence homology or SNP sequence/loci genomic dominance, or homeolog loci bias of the upland tetraploid At and Dt subgenomes. The alignments indicated that 364 SNP-associated previously unintegrated scaffolds can be placed in pseudochromosomes of the NBI G hirsutum assembly. This is the first intraspecific SNP genetic linkage consensus map assembled in G hirsutum with a core of reproducible mendelian SNP markers assayed on different populations and it provides further knowledge of chromosome arrangement of genic and nongenic SNPs. Together, the consensus map and RIL populations provide a synergistically useful platform for localizing and identifying agronomically important loci for improvement of the cotton crop.

Genomic Variation of Inbreeding and Ancestry in the Remaining Two Isle Royale Wolves.

Inbreeding, relatedness, and ancestry have traditionally been estimated with pedigree information, however, molecular genomic data can provide more detailed examination of these properties. For example, pedigree information provides estimation of the expected value of these measures but molecular genomic data can estimate the realized values of these measures in individuals. Here, we generate the theoretical distribution of inbreeding, relatedness, and ancestry for the individuals in the pedigree of the Isle Royale wolves, the first examination of such variation in a wild population with a known pedigree. We use the 38 autosomes of the dog genome and their estimated map lengths in our genomic analysis. Although it is known that the remaining wolves are highly inbred, closely related, and descend from only 3 ancestors, our analyses suggest that there is significant variation in the realized inbreeding and relatedness around pedigree expectations. For example, the expected inbreeding in a hypothetical offspring from the 2 remaining wolves is 0.438 but the realized 95% genomic confidence interval is from 0.311 to 0.565. For individual chromosomes, a substantial proportion of the whole chromosomes are completely identical by descent. This examination provides a background to use when analyzing molecular genomic data for individual levels of inbreeding, relatedness, and ancestry. The level of variation in these measures is a function of the time to the common ancestor(s), the number of chromosomes, and the rate of recombination. In the Isle Royale wolf population, the few generations to a common ancestor results in the high variance in genomic inbreeding.