High-density Genetic Linkage Map Research Articles

Construction of a high-density genetic map and QTLs mapping with GBS from the interspecific F1 population of P. ostii ‘Fengdan Bai’ and P. suffruticosa ‘Xin Riyuejin’

Tree peony (Paeonia suffruticosa Andrews.) is an important ornamental and medicinal plant in China. However, the genetic mechanisms controlling the traits of interest have not been extensively investigated. In this study, an F1 hybrid population, derived from the cross of P. ostti (‘Feng Dan’) as the female parent and P. suffruticosa (‘Xin Riyuejin’) as the male parent, was used to construct the genetic linkage map using genotyping-by-sequencing (GBS) technique. From the high–throughput sequencing, a total of 243.35 Gb raw data were generated. And then 257,335,738 high-quality GBS markers were detected. Finally, an integrated genetic map containing 3868 markers was constructed, which distributed on 5 linkage groups (LGs). The genetic linkage map spanned 13175.5 cM with an average distance of 3.406 cM between adjacent markers. Based on this map, QTL analysis was performed. A total of 6 QTLs for 4 phenotypic traits of flower number, petal length, petal number and flowering stage were detected. This high-density genetic linkage map and QTL mapping for important flower-related traits using GBS with an F1 population of tree peony in this study will provide a solid foundation for gene positional cloning and molecular marker-assisted breeding in the near future.

Construction of a high-density genetic linkage map and QTL analysis for hazelnut breeding

Hazelnut breeding is a slow activity due to the long lifecycle of the plant, the long time between pollination and reliable phenotype observation in the progeny, the presence of self-incompatibility and the high level of heterozygosity. Improved knowledge on the hazelnut genome and the development of marker-assisted selection would greatly facilitate breeding programs. Objective of this study was the construction of high-density genetic maps and the detection of QTL (quantitative trait loci) related to phenological, vegetative and productive traits, by using an F1 progeny obtained by crossing ‘Tonda Gentile delle Langhe’ (female parent, hereafter TGdL) with ‘Merveille de Bollwiller’ (syn. ‘Hall’s Giant’ male parent, MB). Genotyping-by-sequencing (GBS), followed by SNP mining using a Samtools-based pipeline, yielded 7,980 SNP markers. Independent linkage maps were constructed for each parent on the basis of the double pseudo-testcross mapping strategy. The linkage map for TGdL consisted of 1,236 markers covering a total genetic length of 900.4 cM, with a mean inter-marker distance of 0.83 cM. A reciprocal translocation was detected in TGdL between two non-homologous chromosomes. The linkage map for MB was based on segregation at 1,211 markers covering a total genetic length of 899.1 cM, with a mean inter-marker distance of 0.82 cM. Plant and nut traits were recorded during five years and statistically analyzed with molecular data to identify marker/QTL associations for flowering and bud burst time, sucker habit and nut traits, such as nut and seed size, roundness index, percent kernel and ease of pellicle removal.

Identification of a Novel QTL for Panicle Length From Wild Rice (Oryza minuta) by Specific Locus Amplified Fragment Sequencing and High Density Genetic Mapping.

Wild rice possesses a large number of valuable genes that have been lost or do not exist in cultivated rice. To exploit the desirable gene controlling panicle length (PL) in wild rice Oryza minuta, a recombinant inbred line (RIL) population was constructed that was derived from a cross between the long panicle introgression line K1561 with Oryza minuta segments and a short panicle accession G1025. Specific Locus Amplified Fragment (SLAF) sequencing technology was used to uncover single nucleotide polymorphisms (SNPs) and construct the high-density genetic linkage map. Using 201 RIL populations, a high-density genetic map was developed, and spanned 2781.76 cM with an average genetic distance 0.45 cM. The genetic map was composed of 5, 521 markers on 12 chromosomes. Based on this high-density genome map, quantitative trait loci (QTL) for PL were analyzed for 2 years under four environments. Seven QTLs were detected, which were distributed within chromosomes 4, 9, and 10, respectively. pl4.1 was detected twice, and pl10.1 was only detected once. Although pl9.1 was only detected once, it was very near pl9.2 in the genetic map which was detected three times. Thus, we speculate one major QTL exists in the region of pl9.1 and pl9.2 to control PL (temporarily referred to as pl9). pl9 is a potentially novel allele derived from Oryza minuta, and it can be used for genetic improvement of cultivar rice.

QTL Detection for Kernel Size and Weight in Bread Wheat (Triticum aestivum L.) Using a High-Density SNP and SSR-Based Linkage Map.

High-density genetic linkage maps are essential for precise mapping quantitative trait loci (QTL) in wheat (Triticum aestivum L.). In this study, a high-density genetic linkage map consisted of 6312 SNP and SSR markers was developed to identify QTL controlling kernel size and weight, based on a recombinant inbred line (RIL) population derived from the cross of Shixin828 and Kenong2007. Seventy-eight putative QTL for kernel length (KL), kernel width (KW), kernel diameter ratio (KDR), and thousand kernel weight (TKW) were detected over eight environments by inclusive composite interval mapping (ICIM). Of these, six stable QTL were identified in more than four environments, including two for KL (qKL-2D and qKL-6B.2), one for KW (qKW-2D.1), one for KDR (qKDR-2D.1) and two for TKW (qTKW-5A and qTKW-5B.2). Unconditional and multivariable conditional QTL mapping for TKW with respect to TKW component (TKWC) revealed that kernel dimensions played an important role in regulating the kernel weight. Seven QTL-rich genetic regions including seventeen QTL were found on chromosomes 1A (2), 2D, 3A, 4B and 5B (2) exhibiting pleiotropic effects. In particular, clusters on chromosomes 2D and 5B possessing significant QTL for kernel-related traits were highlighted. Markers tightly linked to these QTL or clusters will eventually facilitate further studies for fine mapping, candidate gene discovery and marker-assisted selection (MAS) in wheat breeding.

A High-Density Genetic Map of an Allohexaploid Brassica Doubled Haploid Population Reveals Quantitative Trait Loci for Pollen Viability and Fertility.

A doubled haploid (DH) mapping population was obtained from microspore culture of an allohexaploid F1 from the cross between two recently-synthesized allohexaploid Brassica lines. We used single nucleotide polymorphism (SNP) genetic variation based on restriction-site associated DNA (RAD) sequencing to construct a high density genetic linkage map of the population. RAD libraries were constructed from the genomic DNA of both parents and 146 DH progenies. A total of 2.87 G reads with an average sequencing depth of 2.59 × were obtained in the parents and of 1.41 × in the progeny. A total of 290,422 SNPs were identified from clustering of RAD reads, from which we developed 7,950 high quality SNP markers that segregated normally (1:1) in the population. The linkage map contained all 27 chromosomes from the parental A, B and C genomes with a total genetic distance of 5725.19 cM and an average of 0.75 cM between adjacent markers. Genetic distance on non-integrated linkage groups was 1534.23 cM, or 21% of total genetic distance. Out of 146 DH progenies, 91 had a complete set of 27 chromosomes as expected of a hexaploid species, and 21 out of 27 chromosomes showed high collinearity between the physical and linkage maps. The loss of chromosome(s) or chromosome segment(s) in the DH population was associated with a reduction in pollen viability. Twenty-five additive QTL were associated with pollen viability and fertility-related traits (seed number, seed yield, pod length, plant height, 1000-seed weight). In addition, 44 intra-genomic and 18 inter-genomic epistatic QTL pairs were detected for 4 phenotypic traits. This provides confidence that the DH population may be selected for improved pollen viability and fertility in a future allohexaploid Brassica species.

A Genetic Map of Ostrich Z Chromosome and the Role of Inversions in Avian Sex Chromosome Evolution.

Recombination arrest is a necessary step for the evolution of distinct sex chromosomes. Structural changes, such as inversions, may represent the mechanistic basis for recombination suppression and comparisons of the structural organization of chromosomes as given by chromosome-level assemblies offer the possibility to infer inversions across species at some detail. In birds, deduction of the process of sex chromosome evolution has been hampered by the lack of a validated chromosome-level assembly from a representative of one of the two basal clades of modern birds, Paleognathae. We therefore developed a high-density genetic linkage map of the ostrich Z chromosome and used this to correct an existing assembly, including correction of a large chimeric superscaffold and the order and orientation of other superscaffolds. We identified the pseudoautosomal region as a 52 Mb segment (≈60% of the Z chromosome) where recombination occurred in both sexes. By comparing the order and location of genes on the ostrich Z chromosome with that of six bird species from the other major clade of birds (Neognathae), and of reptilian outgroup species, 25 Z-linked inversions were inferred in the avian lineages. We defined Z chromosome organization in an early avian ancestor and identified inversions spanning the candidate sex-determining DMRT1 gene in this ancestor, which could potentially have triggered the onset of avian sex chromosome evolution. We conclude that avian sex chromosome evolution has been characterized by a complex process of probably both Z-linked and W-linked inversions (and/or other processes). This study illustrates the need for validated chromosome-level assemblies for inference of genome evolution.

Construction of the first high-density genetic linkage map of pikeperch (Sander lucioperca) using specific length amplified fragment (SLAF) sequencing and QTL analysis of growth-related traits

High-density genetic linkage maps play an important role in genome assembly, quantitative trait locus (QTL) mapping, and marker-assisted selection. Pikeperch (Sander lucioperca) is a popular inland aquaculture species in Europe and Asia, but the genetic improvement of this species is still in its infancy. In this study, genomic DNA extracted from two pikeperch parents and 150 F1 offspring was subjected to high-throughput sequencing and specific length amplified fragment (SLAF) library construction. In total, 436.58 M reads (87.28 G data) were acquired, with average sequencing depths of 100.16-fold for the parents and 19.51-fold for each F1 offspring. A total of 237,687 high-quality SLAFs were detected, 34,774 of which were polymorphic and used for linkage map construction. The final merged linkage map consisted of 8159 SLAFs in 24 linkage groups (LGs) and spanned 3421.81 cM, with an average inter-marker distance of 0.46 cM. Based on this high-density genetic linkage map, 21 QTLs were identified for eight growth-related traits: total length, body weight, body length, head length, body height, body thickness, body girth, and slaughter weight. Three QTLs for each trait, including slaughter weight, body girth, body weight, and body thickness, were detected on LG6 or LG16. Moreover, five QTLs for body length were detected on LG6, LG9, and LG18. QTLs for total length, body height, and head length were detected on LG6. The linkage map and QTLs for growth-related traits identified in this study should be useful for improving pikeperch marker-assisted selection in the future.

Genome-Wide Linkage Mapping Reveals QTLs for Seed Vigor-Related Traits Under Artificial Aging in Common Wheat (Triticum aestivum).

Long-term storage of seeds leads to lose seed vigor with slow and non-uniform germination. Time, rate, homogeneity, and synchrony are important aspects during the dynamic germination process to assess seed viability after storage. The aim of this study is to identify quantitative trait loci (QTLs) using a high-density genetic linkage map of common wheat (Triticum aestivum) for seed vigor-related traits under artificial aging. Two hundred and forty-six recombinant inbred lines derived from the cross between Zhou 8425B and Chinese Spring were evaluated for seed storability. Ninety-six QTLs were detected on all wheat chromosomes except 2B, 4D, 6D, and 7D, explaining 2.9–19.4% of the phenotypic variance. These QTLs were clustered into 17 QTL-rich regions on chromosomes 1AL, 2DS, 3AS (3), 3BS, 3BL (2), 3DL, 4AS, 4AL (3), 5AS, 5DS, 6BL, and 7AL, exhibiting pleiotropic effects. Moreover, 10 stable QTLs were identified on chromosomes 2D, 3D, 4A, and 6B (QaMGT.cas-2DS.2, QaMGR.cas-2DS.2, QaFCGR.cas-2DS.2, QaGI.cas-3DL, QaGR.cas-3DL, QaFCGR.cas-3DL, QaMGT.cas-4AS, QaMGR.cas-4AS, QaZ.cas-4AS, and QaGR.cas-6BL.2). Our results indicate that one of the stable QTL-rich regions on chromosome 2D flanked by IWB21991 and IWB11197 in the position from 46 to 51 cM, presenting as a pleiotropic locus strongly impacting seed vigor-related traits under artificial aging. These new QTLs and tightly linked SNP markers may provide new valuable information and could serve as targets for fine mapping or markers assisted breeding.

Mapping QTLs for water-use efficiency reveals the potential candidate genes involved in regulating the trait in apple under drought stress

BackgroundImprovement of water-use efficiency (WUE) can effectively reduce production losses caused by drought stress. A better understanding of the genetic determination of WUE in crops under drought stress has great potential value for developing cultivars adapted to arid regions. To identify the genetic loci associated with WUE and reveal genes responsible for the trait in apple, we aim to map the quantitative trait loci (QTLs) for carbon isotope composition, the proxy for WUE, applying two contrasting irrigating regimes over the two-year experiment and search for the candidate genes encompassed in the mapped QTLs.ResultsWe constructed a high-density genetic linkage map with 10,172 markers of apple, using single nucleotide polymorphism (SNP) markers obtained through restriction site-associated DNA sequencing (RADseq) and a final segregating population of 350 seedlings from the cross of Honeycrisp and Qinguan. In total, 33 QTLs were identified for carbon isotope composition in apple under both well-watered and drought-stressed conditions. Three QTLs were stable over 2 years under drought stress on linkage groups LG8, LG15 and LG16, as validated by Kompetitive Allele-Specific PCR (KASP) assays. In those validated QTLs, 258 genes were screened according to their Gene Ontology functional annotations. Among them, 28 genes were identified, which exhibited significant responses to drought stress in ‘Honeycrisp’ and/or ‘Qinguan’. These genes are involved in signaling, photosynthesis, response to stresses, carbohydrate metabolism, protein metabolism and modification, hormone metabolism and transport, transport, respiration, transcriptional regulation, and development regulation. They, especially those for photoprotection and relevant signal transduction, are potential candidate genes connected with WUE regulation in drought-stressed apple.ConclusionsWe detected three stable QTLs for carbon isotope composition in apple under drought stress over 2 years, and validated them by KASP assay. Twenty-eight candidate genes encompassed in these QTLs were identified. These stable genetic loci and series of genes provided here serve as a foundation for further studies on marker-assisted selection of high WUE and regulatory mechanism of WUE in apple exposed to drought conditions, respectively.

A high-resolution genetic linkage map and QTL fine mapping for growth-related traits and sex in the Yangtze River common carp (Cyprinus carpio haematopterus)

BackgroundA high-density genetic linkage map is essential for QTL fine mapping, comparative genome analysis, identification of candidate genes and marker-assisted selection for economic traits in aquaculture species. The Yangtze River common carp (Cyprinus carpio haematopterus) is one of the most important aquacultured strains in China. However, quite limited genetics and genomics resources have been developed for genetic improvement of economic traits in such strain.ResultsA high-resolution genetic linkage map was constructed by using 7820 2b-RAD (2b-restriction site-associated DNA) and 295 microsatellite markers in a F2 family of the Yangtze River common carp (C. c. haematopterus). The length of the map was 4586.56 cM with an average marker interval of 0.57 cM. Comparative genome mapping revealed that a high proportion (70%) of markers with disagreed chromosome location was observed between C. c. haematopterus and another common carp strain (subspecies) C. c. carpio. A clear 2:1 relationship was observed between C. c. haematopterus linkage groups (LGs) and zebrafish (Danio rerio) chromosomes. Based on the genetic map, 21 QTLs for growth-related traits were detected on 12 LGs, and contributed values of phenotypic variance explained (PVE) ranging from 16.3 to 38.6%, with LOD scores ranging from 4.02 to 11.13. A genome-wide significant QTL (LOD = 10.83) and three chromosome-wide significant QTLs (mean LOD = 4.84) for sex were mapped on LG50 and LG24, respectively. A 1.4 cM confidence interval of QTL for all growth-related traits showed conserved synteny with a 2.06 M segment on chromosome 14 of D. rerio. Five potential candidate genes were identified by blast search in this genomic region, including a well-studied multi-functional growth related gene, Apelin.ConclusionsWe mapped a set of suggestive and significant QTLs for growth-related traits and sex based on a high-density genetic linkage map using SNP and microsatellite markers for Yangtze River common carp. Several candidate growth genes were also identified from the QTL regions by comparative mapping. This genetic map would provide a basis for genome assembly and comparative genomics studies, and those QTL-derived candidate genes and genetic markers are useful genomic resources for marker-assisted selection (MAS) of growth-related traits in the Yangtze River common carp.

Accounting for Errors in Low Coverage High-Throughput Sequencing Data When Constructing Genetic Maps Using Biparental Outcrossed Populations.

Next-generation sequencing is an efficient method that allows for substantially more markers than previous technologies, providing opportunities for building high-density genetic linkage maps, which facilitate the development of nonmodel species' genomic assemblies and the investigation of their genes. However, constructing genetic maps using data generated via high-throughput sequencing technology (e.g., genotyping-by-sequencing) is complicated by the presence of sequencing errors and genotyping errors resulting from missing parental alleles due to low sequencing depth. If unaccounted for, these errors lead to inflated genetic maps. In addition, map construction in many species is performed using full-sibling family populations derived from the outcrossing of two individuals, where unknown parental phase and varying segregation types further complicate construction. We present a new methodology for modeling low coverage sequencing data in the construction of genetic linkage maps using full-sibling populations of diploid species, implemented in a package called GUSMap. Our model is based on the Lander-Green hidden Markov model but extended to account for errors present in sequencing data. We were able to obtain accurate estimates of the recombination fractions and overall map distance using GUSMap, while most existing mapping packages produced inflated genetic maps in the presence of errors. Our results demonstrate the feasibility of using low coverage sequencing data to produce genetic maps without requiring extensive filtering of potentially erroneous genotypes, provided that the associated errors are correctly accounted for in the model.

Construction of a high-density genetic linkage map and QTL mapping of oleic acid content and three agronomic traits in sunflower (Helianthus annuus L.) using specific-locus amplified fragment sequencing (SLAF-seq).

High-density genetic linkage maps are particularly important for quantitative trait loci (QTL) mapping, genome assembly, and marker-assisted selection (MAS) in plants. In this study, a high-density genetic linkage map of sunflower (Helianthus annuus L.) was constructed using an F2 population generated from a cross between Helianthus annuus L. ‘86-1’ and ‘L-1-OL-1’ via specific-locus amplified fragment sequencing (SLAF-seq). After sequence preprocessing, 530.50 M reads (105.60 Gb) were obtained that contained a total of 343,197 SLAFs, of which 39,589 were polymorphic. Of the polymorphic SLAFs, 6,136 were organized into a linkage map consisting of 17 linkage groups (LGs) spanning 2,221.86 cM, with an average genetic distance of 0.36 cM between SLAFs. Based on this high-density genetic map, QTL analysis was performed that focused on four sunflower phenotypic traits: oleic acid content (OAC), plant height (PH), head diameter (HD), and stem diameter (SD). Subsequently, for these four traits eight QTLs were detected that will likely be useful for increasing our understanding of genetic factors underlying these traits and for use in marker-assisted selection (MAS) for future sunflower breeding.

TSPmap, a tool making use of traveling salesperson problem solvers in the efficient and accurate construction of high-density genetic linkage maps

BackgroundRecent advances in nucleic acid sequencing technologies have led to a dramatic increase in the number of markers available to generate genetic linkage maps. This increased marker density can be used to improve genome assemblies as well as add much needed resolution for loci controlling variation in ecologically and agriculturally important traits. However, traditional genetic map construction methods from these large marker datasets can be computationally prohibitive and highly error prone.ResultsWe present TSPmap, a method which implements both approximate and exact Traveling Salesperson Problem solvers to generate linkage maps. We demonstrate that for datasets with large numbers of genomic markers (e.g. 10,000) and in multiple population types generated from inbred parents, TSPmap can rapidly produce high quality linkage maps with low sensitivity to missing and erroneous genotyping data compared to two other benchmark methods, JoinMap and MSTmap. TSPmap is open source and freely available as an R package.ConclusionsWith the advancement of low cost sequencing technologies, the number of markers used in the generation of genetic maps is expected to continue to rise. TSPmap will be a useful tool to handle such large datasets into the future, quickly producing high quality maps using a large number of genomic markers.

Fine-mapping of QTLs for individual and total isoflavone content in soybean (Glycine max L.) using a high-density genetic map.

Fifteen stable QTLs were identified using a high-density soybean genetic map across multiple environments. One major QTL, qIF5-1, contributing to total isoflavone content explained phenotypic variance 49.38, 43.27, 46.59, 45.15 and 52.50%, respectively. Soybeans (Glycine max L.) are a major source of dietary isoflavones. To identify novel quantitative trait loci (QTL) underlying isoflavone content, and to improve the accuracy of marker-assisted breeding in soybean, a valuable mapping population comprised of 196 F7:8-10 recombinant inbred lines (RILs, Huachun 2×Wayao) was utilized to evaluate individual and total isoflavone content in plants grown in four different environments in Guangdong. A high-density genetic linkage map containing 3469 recombination bin markers based on 0.2×restriction site-associated DNA tag sequencing (RAD-seq) technology was used to finely map QTLs for both individual and total isoflavone contents. Correlation analyses showed that total isoflavone content, and that of five individual isoflavone, was significantly correlated across the four environments. Based on the high-density genetic linkage map, a total of 15 stable quantitative trait loci (QTLs) associated with isoflavone content across multiple environments were mapped onto chromosomes 02, 05, 07, 09, 10, 11, 13, 16, 17, and 19. Further, one of them, qIF5-1, localized to chromosomes 05 (38,434,171-39,045,620bp) contributed to almost all isoflavone components across all environments, and explained 6.37-59.95% of the phenotypic variance, especially explained 49.38, 43.27, 46.59, 45.15 and 52.50% for total isoflavone. The results obtained in the present study will pave the way for a better understanding of the genetics of isoflavone accumulation and reveals the scope available for improvement of isoflavone content through marker-assisted selection.

Fine mapping of growth-related quantitative trait loci in Yellow River carp (Cyprinus carpio haematoperus)

Genetic linkage map and quantitative trait loci (QTL) mapping are powerful tools for genetic analysis of a series of economic traits in aquatic animals. Yellow River carp (Cyprinus carpio haematoperus), one of the famous common carp strains, is widely cultured in northern China, however, genetic and genomic bases of growth-related traits have been rarely studied in this carp strain. In present study, an F2 family of Yellow River carp (8months old) was used for constructing genetic linkage map and detecting potential QTL for growth traits, including total length (TL), body length (BL), body height (BH), head length (HL) and body weight (BW). A high-density genetic linkage map was constructed mainly based on 2b-RAD technology, with 6230 SNPs and 65 SSRs assigned onto 50 linkage groups, covering 98.2% of the Yellow River carp genome (average 0.59cM/marker). Comparative mapping showed high levels of syntenic relationship between our map and the genomes of two cyprinid fish (Denio rerio and Ctenopharyngodon idellus), while a lower synteny was detected between our map and C. c. carpio genome. Twelve chromosome-wide and two genome-wide growth-associated QTL were detected in our study, with the phenotypic variance explained ranging from 11.9% to 16.2%. These QTL were scatteredly distributed on 8 LGs of our sex-averaged linkage map. Fifteen candidate genes (e.g. INSIG1, 3β-HSD, HGF) were identified by BLAST searching the sequences of QTL-linked markers in Yellow River carp against Denio rerio genome, and their functions involve in the regulation of development, cell-proliferation, energy metabolism, and so on. The identification of significant QTL and potential growth-related genes in this study provides valuable genomic resources for the study of genetic mechanism underlying growth and development, and would contribute to accelerate the progress of marker-assisted selection in Yellow River carp.

Detection of genomic loci associated with chromosomal recombination using high-density linkage mapping in Setaria

Meiotic recombination is essential to sexual reproduction and the generation of genetic diversity. Variation in recombination rates is presently of particular interest due to efforts being made to increase the rate of genetic gain in agricultural crops by breaking up large linkage disequilibrium blocks containing both beneficial and detrimental alleles. Here, a high-density genetic linkage map of Setaria was constructed using tunable genotyping by sequencing (tGBS) analysis of a population of recombinant inbred lines (RILs). Several regions of the Setaria genome exhibited significant levels of segregation distortion (SD), and recombination crossovers (COs) were also detected. The regions with high SD generally tended to have fewer COs, particularly for pericentromeric chromosomal areas. Recombination crossovers detected in Setaria were unevenly distributed across the genome and occurred more often in intergenic regions. Quantitative trait loci (QTLs) contributing towards the recombination frequency (Type I) and occurrence of COs in designated loci (Type II) were identified, and Type II QTLs garnered higher statistical power. The result of this study suggest that QTLs analysis of Type II traits using RILs might provide an opportunity to further understand meiotic recombination using high throughput genome sequencing and genotyping technologies.

Construction of high-density genetic linkage maps and QTL mapping in the golden pompano

The golden pompano (Trachinotus blochii) is an essential marine fish species of South China. In this study, a genetic map containing 24 linkage groups was compared with 102 F1 generation individuals (full-sib). A total of 23,558 single-nucleotide polymorphisms (SNPs) have been identified via restriction-site associated DNA sequencing (RAD-seq). Among these, 12,358 high-quality SNPs were utilized to construct a linkage map and consequently, we present the first reported genetic map of T. blochii. The total lengths of female, male and sex average linkage maps were 2559.12cm, 2827.48cm and 3810.03cm, respectively. These linkage maps were built via the application of quantitative trait loci (QTL) tags, which are related to the size and growth of T. blochii. We successfully detected 23 QTL of growth-related traits, including 4 QTL for body length, 5 QTL for body weight, and 14 QTL for body height. This newly developed, QTL facilitating, and high-resolution genetic map represents preliminary progress toward linking the traits of key conservation interest to DNA regions in T. blochii. Our research will provide guidance for the molecular breeding of the golden pompano.

High-density genetic linkage map construction by F2 populations and QTL analysis of early-maturity traits in upland cotton (Gossypium hirsutum L.).

Due to China’s rapidly increasing population, the total arable land area has dramatically decreased; as a consequence, the competition for farming land allocated for grain and cotton production has become fierce. Therefore, to overcome the existing contradiction between cotton grain and fiber production and the limited farming land, development of early-maturing cultivars is necessary. In this research, a high-density linkage map of upland cotton was constructed using genotyping by sequencing (GBS) to discover single nucleotide polymorphism (SNP) markers associated with early maturity in 170 F2 individuals derived from a cross between LU28 and ZHONG213. The high-density genetic map, which was composed of 3978 SNP markers across the 26 cotton chromosomes, spanned 2480 cM with an average genetic distance of 0.62 cM. Collinearity analysis showed that the genetic map was of high quality and accurate and agreed well with the Gossypium hirsutum reference genome. Based on this high-density linkage map, QTL analysis was performed on cotton early-maturity traits, including FT, FBP, WGP, NFFB, HNFFB and PH. A total 47 QTLs for the six traits were detected; each of these QTLs explained between 2.61% and 32.57% of the observed phenotypic variation. A major region controlling early-maturity traits in Gossypium hirsutum was identified for FT, FBP, WGP, NFFB and HNFFB on chromosome D03. QTL analyses revealed that phenotypic variation explained (PVE) ranged from 10.42% to 32.57%. Two potential candidate genes, Gh_D03G0885 and Gh_D03G0922, were predicted in a stable QTL region and had higher expression levels in the early-maturity variety ZHONG213 than in the late-maturity variety LU28. However, further evidence is required for functional validation. This study could provide useful information for the dissection of early-maturity traits and guide valuable genetic loci for molecular-assisted selection (MAS) in cotton breeding.

A High-Density Genetic Linkage Map and QTL Fine Mapping for Body Weight in Crucian Carp (Carassius auratus) Using 2b-RAD Sequencing.

A high-resolution genetic linkage map is essential for a wide range of genetics and genomics studies such as comparative genomics analysis and QTL fine mapping. Crucian carp (Carassius auratus) is widely distributed in Eurasia, and is an important aquaculture fish worldwide. In this study, a high-density genetic linkage map was constructed for crucian carp using 2b-RAD technology. The consensus map contains 8487 SNP markers, assigning to 50 linkage groups (LGs) and spanning 3762.88 cM, with an average marker interval of 0.44 cM and genome coverage of 98.8%. The female map had 4410 SNPs, and spanned 3500.42 cM (0.79 cM/marker), while the male map had 4625 SNPs and spanned 3346.33 cM (0.72 cM/marker). The average recombination ratio of female to male was 2.13:1, and significant male-biased recombination suppressions were observed in LG47 and LG49. Comparative genomics analysis revealed a clear 2:1 syntenic relationship between crucian carp LGs and chromosomes of zebrafish and grass carp, and a 1:1 correspondence, but extensive chromosomal rearrangement, between crucian carp and common carp, providing evidence that crucian carp has experienced a fourth round of whole genome duplication (4R-WGD). Eight chromosome-wide QTL for body weight at 2 months after hatch were detected on five LGs, explaining 10.1–13.2% of the phenotypic variations. Potential candidate growth-related genes, such as an EGF-like domain and TGF-β, were identified within the QTL intervals. This high-density genetic map and QTL analysis supplies a basis for genome evolutionary studies in cyprinid fishes, genome assembly, and QTL fine mapping for complex traits in crucian carp.

Construction of the First High-Density Genetic Linkage Map and Analysis of Quantitative Trait Loci for Growth-Related Traits in Sinonovacula constricta

The razor clam (Sinonovacula constricta) is an important aquaculture species, for which a high-density genetic linkage map would play an important role in marker-assisted selection (MAS). In this study, we constructed a high-density genetic map and detected quantitative trait loci (QTLs) for Sinonovacula constricta with an F1 cross population by using the specific locus amplified fragment sequencing (SLAF-seq) method. A total of 315,553 SLAF markers out of 467.71 Mreads were developed. The final linkage map was composed of 7516 SLAFs (156.60-fold in the parents and 20.80-fold in each F1 population on average). The total distance of the linkage map was 2383.85cM, covering 19 linkage groups with an average inter-marker distance of 0.32cM. The proportion of gaps less than 5.0cM was on average 96.90%. A total of 16 suggestive QTLs for five growth-related traits (five QTLs for shell height, six QTLs for shell length, three QTLs for shell width, one QTL for total body weight, and one QTL for soft body weight) were identified. These QTLs were distributed on five linkage groups, and the regions showed overlapping on LG9 and LG13. In conclusion, the high-density genetic map and QTLs for S. constricta provide a valuable genetic resource and a basis for MAS.