One-factor sex determination evolves without linkage between feminizing and masculinizing mutations.

The evolution of separate sexes from cosexuality requires at least two mutations: a feminizing allele to cause female development and a masculinizing allele to cause male development. Classically, the double mutant is assumed to be sterile, which leads to two-factor sex determination where male and female sex chromosomes differ at two loci. However, several species appear to have one-factor sex determination where sexual development depends on variation at a single locus. We show that one-factor sex determination evolves when the double mutant develops as a male or a female. The feminizing allele fixes when the double mutant is male, and the masculinizing allele fixes when the double mutant is female. The other locus then gives XY or ZW sex determination based on dominance: for example, a dominant masculinizer becomes a Y chromosome. Although the resulting sex determination system differs, the conditions required for feminizers and masculinizers to spread are the same as in classical models, with the important difference that the two alleles do not need to be linked. Thus, we reveal alternative pathways for the evolution of sex determination and discuss how they can be distinguished using new data on the genetics of sex determination.

Cytogenetic identification and molecular mapping for the wheat-Thinopyrum ponticum introgression line with resistance to Fusarium head blight.

KEY MESSAGE: Xinong 511, a new wheat-Thinopyrum ponticum variety with excellent fusarium head blight resistance, the QTLs were mapped to the wheat chromosomes 5B and 7A with named QFhb.nwafu-5B and QFhb.nwafu-7A, respectively. Novel Fusarium head blight (FHB) resistance germplasms and genes are valuable for wheat improvement and breeding efforts. Thinopyrum ponticum, a wild relative of common wheat, is a valuable germplasm of disease resistance for wheat improvement and breeding. Xinong 511 (XN511) is a high-quality wheat variety widely cultivated in the Yellow and Huai Rivers Valley of China with stable FHB-resistance. Through analysis of pedigree materials of the wheat cultivar XN511, we found that the genetic material and FHB resistance from Th. ponticum were transmitted to the introgression line, indicating that the FHB resistance in XN511 likely originates from Th. ponticum. To further explore the genetic basis of FHB resistance in XN511, QTL mapping was conducted using the RILs population of XN511 and the susceptible line Aikang 58 (AK58). Survey with makers closely-linked to Fhb1, Fhb2, Fhb4, Fhb5, and Fhb7, indicated that both XN511 and the susceptible lines do not contain these QTL. Using bulked segregant analysis RNA-seq (BSR-Seq) and newly developed allele-specific PCR (AS-PCR) markers, QTLs in XN511 were successfully located on wheat chromosomes 5B and 7A. These findings are significant for further understanding and utilizing FHB resistance genes in wheat improvement.

Consensus genetic linkage map and QTL mapping allow to capture the genomic regions associated with agronomic traits in pearl millet.

MAIN CONCLUSION: A genetic linkage map representing the pearl millet genome was constructed with SNP markers. Major and stable QTL associated with flowering, number of productive tillers, ear head length, and test weight were mapped on chromosomes 1 and 3. Pearl millet (Pennisetum glaucum) is a major cereal and fodder crop in arid and semi-arid regions of Asia and Africa. Agronomic traits are important traits in pearl millet breeding and genetic and environmental factors highly influence them. In the present study, an F9 recombinant inbred line (RIL) population derived from a cross between PT6029 and PT6129 was evaluated for agronomic traits in three environments. Utilizing a genotyping by sequencing approach, a dense genetic map with 993 single nucleotide polymorphism markers covering a total genetic distance of 1035.4 cM was constructed. The average interval between the markers was 1.04 cM, and the seven chromosomes varied from 115.39 to 206.72 cM. Quantitative trait loci (QTL) mapping revealed 35 QTL for seven agronomic traits, and they were distributed on all pearl millet chromosomes. These QTL individually explained 11.35 to 26.71% of the phenotypic variation, with LOD values ranging from 2.74 to 5.80. Notably, four QTL (qDFF1.1, qNPT3.1, qEHL3.1, and qTW1.1) associated with days to fifty percent flowering, the number of productive tillers, ear head length, and test weight were found to be major and stable QTL located on chromosomes 1 and 3. Collectively, our results provide an important base for understanding the genetic architecture of agronomic traits in pearl millet, which is useful for accelerating the genetic gain toward crop improvement.

A catalogue of recombination coldspots in interspecific tomato hybrids.

Increasing natural resistance and resilience in plants is key for ensuring food security within a changing climate. Breeders improve these traits by crossing cultivars with their wild relatives and introgressing specific alleles through meiotic recombination. However, some genomic regions are devoid of recombination especially in crosses between divergent genomes, limiting the combinations of desirable alleles. Here, we used pooled-pollen sequencing to build a map of recombinant and non-recombinant regions between tomato and five wild relatives commonly used for introgressive tomato breeding. We detected hybrid-specific recombination coldspots that underscore the role of structural variations in modifying recombination patterns and maintaining genetic linkage in interspecific crosses. Crossover regions and coldspots show strong association with specific TE superfamilies exhibiting differentially accessible chromatin between somatic and meiotic cells. About two-thirds of the genome are conserved coldspots, located mostly in the pericentromeres and enriched with retrotransposons. The coldspots also harbor genes associated with agronomic traits and stress resistance, revealing undesired consequences of linkage drag and possible barriers to breeding. We presented examples of linkage drag that can potentially be resolved by pairing tomato with other wild species. Overall, this catalogue will help breeders better understand crossover localization and make informed decisions on generating new tomato varieties.

Inference of Locus-Specific Population Mixtures from Linked Genome-Wide Allele Frequencies.

Admixture between populations and species is common in nature. Since the influx of new genetic material might be either facilitated or hindered by selection, variation in mixture proportions along the genome is expected in organisms undergoing recombination. Various graph-based models have been developed to better understand these evolutionary dynamics of population splits and mixtures. However, current models assume a single mixture rate for the entire genome and do not explicitly account for linkage. Here, we introduce TreeSwirl, a novel method for inferring branch lengths and locus-specific mixture proportions by using genome-wide allele frequency data, assuming that the admixture graph is known or has been inferred. TreeSwirl builds upon TreeMix that uses Gaussian processes to estimate the presence of gene flow between diverged populations. However, in contrast to TreeMix, our model infers locus-specific mixture proportions employing a hidden Markov model that accounts for linkage. Through simulated data, we demonstrate that TreeSwirl can accurately estimate locus-specific mixture proportions and handle complex demographic scenarios. It also outperforms related D- and f-statistics in terms of accuracy and sensitivity to detect introgressed loci.

Genetic Analysis of an F2 Population Derived from the Cotton Landrace Hopi Identified Novel Loci for Boll Glanding.

Landraces are an important reservoir of genetic variation that can expand the narrow genetic base of cultivated cotton. In this study, quantitative trait loci (QTL) analysis was conducted using an F2 population developed from crosses between the landrace Hopi and inbred TM-1. A high-density genetic map spanning 2253.11 and 1932.21 cM for the A and D sub-genomes, respectively, with an average marker interval of 1.14 cM, was generated using the CottonSNP63K array. The linkage map showed a strong co-linearity with the physical map of cotton. A total of 21 QTLs were identified, controlling plant height (1), bract type (1), boll number (1), stem color (2), boll pitting (2), fuzz fiber development (2), boll shape (3), boll point (4), and boll glanding (5). In silico analysis of the novel QTLs for boll glanding identified a total of 13 candidate genes. Analysis of tissue-specific expression of the candidate genes suggests roles for the transcription factors bHLH1, MYB2, and ZF1 in gland formation. Comparative sequencing of open reading frames identified early stop codons in all three transcription factors in Hopi. Functional validation of these genes offers avenues to reduce glanding and, consequently, lower gossypol levels in cottonseeds without compromising the defense mechanisms of the plant against biotic stresses.

Meta-QTL and Candidate Gene Analyses of Agronomic Salt Tolerance and Related Traits in an RIL Population Derived from Solanum pimpinellifolium.

Breeding salt-tolerant crops is necessary to reduce food insecurity. Prebreeding populations are fundamental for uncovering tolerance alleles from wild germplasm. To obtain a physiological interpretation of the agronomic salt tolerance and better criteria to identify candidate genes, quantitative trait loci (QTLs) governing productivity-related traits in a population of recombinant inbred lines (RIL) derived from S. pimpinellifolium were reanalyzed using an SNP-saturated linkage map and clustered using QTL meta-analysis to synthesize QTL information. A total of 60 out of 85 QTLs were grouped into 12 productivity MQTLs. Ten of them were found to overlap with other tomato yield QTLs that were found using various mapping populations and cultivation conditions. The MQTL compositions showed that fruit yield was genetically associated with leaf water content. Additionally, leaf Cl- and K+ contents were related to tomato productivity under control and salinity conditions, respectively. More than one functional candidate was frequently found, explaining most productivity MQTLs, indicating that the co-regulation of more than one gene within those MQTLs might explain the clustering of agronomic and physiological QTLs. Moreover, MQTL1.2, MQTL3 and MQTL6 point to the root as the main organ involved in increasing productivity under salinity through the wild allele, suggesting that adequate rootstock/scion combinations could have a clear agronomic advantage under salinity.

The Garden of Forking Paths: Reinterpreting Haseman-Elston Regression for a Genotype-by-Environment Model.

Haseman-Elston regression (HE-reg) has been known as a classic tool for detecting an additive genetic variance component. However, in this study we find that HE-reg can capture GxE under certain conditions, so we derive and reinterpret the analytical solution of HE-reg. In the presence of GxE, it leads to a natural discrepancy between linkage and association results, the latter of which is not able to capture GxE if the environment is unknown. Considering linkage and association as symmetric designs, we investigate how the symmetry can and cannot hold in the absence and presence of GxE, and consequently we propose a pair of statistical tests, Symmetry Test I and Symmetry Test II, both of which can be tested using summary statistics. Test statistics, and their statistical power issues are also investigated for Symmetry Tests I and II. Increasing the number of sib pairs is important to improve statistical power for detecting GxE.

Increased Positive Selection in Highly Recombining Genes Does not Necessarily Reflect an Evolutionary Advantage of Recombination.

It is commonly thought that the long-term advantage of meiotic recombination is to dissipate genetic linkage, allowing natural selection to act independently on different loci. It is thus theoretically expected that genes with higher recombination rates evolve under more effective selection. On the other hand, recombination is often associated with GC-biased gene conversion (gBGC), which theoretically interferes with selection by promoting the fixation of deleterious GC alleles. To test these predictions, several studies assessed whether selection was more effective in highly recombining genes (due to dissipation of genetic linkage) or less effective (due to gBGC), assuming a fixed distribution of fitness effects (DFE) for all genes. In this study, I directly derive the DFE from a gene's evolutionary history (shaped by mutation, selection, drift, and gBGC) under empirical fitness landscapes. I show that genes that have experienced high levels of gBGC are less fit and thus have more opportunities for beneficial mutations. Only a small decrease in the genome-wide intensity of gBGC leads to the fixation of these beneficial mutations, particularly in highly recombining genes. This results in increased positive selection in highly recombining genes that is not caused by more effective selection. Additionally, I show that the death of a recombination hotspot can lead to a higher dN/dS than its birth, but with substitution patterns biased towards AT, and only at selected positions. This shows that controlling for a substitution bias towards GC is therefore not sufficient to rule out the contribution of gBGC to signatures of accelerated evolution. Finally, although gBGC does not affect the fixation probability of GC-conservative mutations, I show that by altering the DFE, gBGC can also significantly affect nonsynonymous GC-conservative substitution patterns.

Construction of a high-density genetic map and QTL localization of body weight and wool production related traits in Alpine Merino sheep based on WGR.

BACKGROUND: The Alpine Merino is a new breed of fine-wool sheep adapted to the cold and arid climate of the plateau in the world. It has been popularized in Northwest China due to its superior adaptability as well as excellent production performance. Those traits related to body weight, wool yield, and wool fiber characteristics, which are economically essential traits in Alpine Merino sheep, are controlled by QTL (Quantitative Trait Loci). Therefore, the identification of QTL and genetic markers for these key economic traits is a critical step in establishing a MAS (Marker-Assisted Selection) breeding program. RESULTS: In this study, we constructed the high-density genetic linkage map of Alpine Merino sheep by sequencing 110 F1 generation individuals using WGR (Whole Genome Resequencing) technology. 14,942 SNPs (Single Nucleotide Polymorphism) were identified and genotyped. The map spanned 2,697.86 cM, with an average genetic marker interval of 1.44 cM. A total of 1,871 high-quality SNP markers were distributed across 27 linkage groups, with an average of 69 markers per LG (Linkage Group). Among them, the smallest genetic distance is 19.62 cM for LG2, while the largest is 237.19 cM for LG19. The average genetic distance between markers in LGs ranged from 0.24 cM (LG2) to 3.57 cM (LG17). The marker density in the LGs ranged from LG14 (39 markers) to LG1 (150 markers). CONCLUSIONS: The first genetic map of Alpine Merino sheep we constructed included 14,942 SNPs, while 46 QTLs associated with body weight, wool yield and wool fiber traits were identified, laying the foundation for genetic studies and molecular marker-assisted breeding. Notably, there were QTL intervals for overlapping traits on LG4 and LG8, providing potential opportunities for multi-trait co-breeding and further theoretical support for selection and breeding of ultra-fine and meaty Alpine Merino sheep.

Altered chromatin topologies caused by balanced chromosomal translocation lead to central iris hypoplasia.

Despite the advent of genomic sequencing, molecular diagnosis remains unsolved in approximately half of patients with Mendelian disorders, largely due to unclarified functions of noncoding regions and the difficulty in identifying complex structural variations. In this study, we map a unique form of central iris hypoplasia in a large family to 6q15-q23.3 and 18p11.31-q12.1 using a genome-wide linkage scan. Long-read sequencing reveals a balanced translocation t(6;18)(q22.31;p11.22) with intergenic breakpoints. By performing Hi-C on induced pluripotent stem cells from a patient, we identify two chromatin topologically associating domains spanning across the breakpoints. These alterations lead the ectopic chromatin interactions between APCDD1 on chromosome 18 and enhancers on chromosome 6, resulting in upregulation of APCDD1. Notably, APCDD1 is specifically localized in the iris of human eyes. Our findings demonstrate that noncoding structural variations can lead to Mendelian diseases by disrupting the 3D genome structure and resulting in altered gene expression.

Chromosome-level genome assemblies of 2 hemichordates provide new insights into deuterostome origin and chromosome evolution.

Deuterostomes are a monophyletic group of animals that includes Hemichordata, Echinodermata (together called Ambulacraria), and Chordata. The diversity of deuterostome body plans has made it challenging to reconstruct their ancestral condition and to decipher the genetic changes that drove the diversification of deuterostome lineages. Here, we generate chromosome-level genome assemblies of 2 hemichordate species, Ptychodera flava and Schizocardium californicum, and use comparative genomic approaches to infer the chromosomal architecture of the deuterostome common ancestor and delineate lineage-specific chromosomal modifications. We show that hemichordate chromosomes (1N = 23) exhibit remarkable chromosome-scale macrosynteny when compared to other deuterostomes and can be derived from 24 deuterostome ancestral linkage groups (ALGs). These deuterostome ALGs in turn match previously inferred bilaterian ALGs, consistent with a relatively short transition from the last common bilaterian ancestor to the origin of deuterostomes. Based on this deuterostome ALG complement, we deduced chromosomal rearrangement events that occurred in different lineages. For example, a fusion-with-mixing event produced an Ambulacraria-specific ALG that subsequently split into 2 chromosomes in extant hemichordates, while this homologous ALG further fused with another chromosome in sea urchins. Orthologous genes distributed in these rearranged chromosomes are enriched for functions in various developmental processes. We found that the deeply conserved Hox clusters are located in highly rearranged chromosomes and that maintenance of the clusters are likely due to lower densities of transposable elements within the clusters. We also provide evidence that the deuterostome-specific pharyngeal gene cluster was established via the combination of 3 pre-assembled microsyntenic blocks. We suggest that since chromosomal rearrangement events and formation of new gene clusters may change the regulatory controls of developmental genes, these events may have contributed to the evolution of diverse body plans among deuterostomes.

Fine mapping and identification of a Fusarium wilt resistance gene FwS1 in pea.

KEY MESSAGE: A Fusarium wilt resistance gene FwS1 on pea chromosome 6 was identified and mapped to a 91.4 kb region by a comprehensive genomic-based approach, and the gene Psat6g003960 harboring NB-ARC domain was identified as the putative candidate gene. Pea Fusarium wilt, incited by Fusarium oxysporum f. sp. pisi (Fop), has always been a devastating disease that causes severe yield losses and economic damage in pea-growing regions worldwide. The utilization of pea cultivars carrying resistance gene is the most efficient approach for managing this disease. In order to finely map resistance gene, F2 populations were established through the cross between Shijiadacaiwan 1 (resistant) and Y4 (susceptible). The resistance genetic analysis indicated that the Fop resistance in Shijiadacaiwan 1 was governed by a single dominant gene, named FwS1. Based on the bulked segregant analysis sequencing analyses, the gene FwS1 was initially detected on chromosome 6 (i.e., linking group II, chr6LG2), and subsequent linkage mapping with 589 F2 individuals fine-mapped the gene FwS1 into a 91.4 kb region. The further functional annotation and haplotype analysis confirmed that the gene Psat6g003960, characterized by a NB-ARC (nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4) domain, was considered as the most promising candidate gene. The encoding amino acids were altered by a "T/C" single-nucleotide polymorphism (SNP) in the first exon of the Psat6g003960, and based on this SNP locus, the molecular marker A016180 was determined to be a diagnostic marker for FwS1 by validating its specificity in both pea accessions and genetic populations with different genetic backgrounds. The FwS1 with diagnostic KASP marker A016180 could facilitate marker-assisted selection in resistance pea breeding in pea. In addition, a comparison of the candidate gene Psat6g003960 in 74SN3B and SJ1 revealed the same sequences. This finding indicated that 74SN3B carried the candidate gene for FwS1, suggesting that FwS1 and Fwf may be closely linked or an identical resistant gene against Fusarium wilt.

Combining genome-wide association study and linkage mapping in the genetic dissection of amylose content in maize (Zea mays L.).

KEY MESSAGE: Integrated linkage and association analysis revealed genetic basis across multiple environments. The genes Zm00001d003102 and Zm00001d015905 were further verified to influence amylose content using gene-based association study. Maize kernel amylose is an important source of human food and industrial raw material. However, the genetic basis underlying maize amylose content is still obscure. Herein, we used an intermated B73 × Mo17 (IBM) Syn10 doubled haploid population composed of 222 lines and a germplasm set including 305 inbred lines to uncover the genetic control for amylose content under four environments. Linkage mapping detected 16 unique QTL, among which four were individually repeatedly identified across multiple environments. Genome-wide association study revealed 17 significant (P = 2.24E-06) single-nucleotide polymorphisms, of which two (SYN19568 and PZE-105090500) were located in the intervals of the mapped QTL (qAC2 and qAC5-3), respectively. According to the two population co-localized loci, 20 genes were confirmed as the candidate genes for amylose content. Gene-based association analysis indicated that the variants in Zm00001d003102 (Beta-16-galactosyltransferase GALT29A) and Zm00001d015905 (Sugar transporter 4a) affected amylose content across multi-environment. Tissue expression analysis showed that the two genes were specifically highly expressed in the ear and stem, respectively, suggesting that they might participate in sugar transport from source to sink organs. Our study provides valuable genetic information for breeding maize varieties with high amylose.

Diversity in bread and durum wheat stigma morphology and linkage of increased stigma length to dwarfing gene Rht14.

KEY MESSAGE: The dwarfing allele Rht14 of durum wheat associates with greater stigma length, an important trait for hybrid breeding, whilst major dwarfing alleles Rht-B1b and Rht-D1b showed little to no effect. Although much understudied in wheat, the stigma is a crucial component for attaining grain set, the fundamental basis for yield, particularly in hybrid production systems where successful grain set relies on wind-driven pollen dispersal by the male parent and effective pollen capture by the female parent. Females with long stigma that exsert early are thought to be advantageous. Using glasshouse-grown lines, we examined variation in Total Stigma Length (TSL) across diverse panels comprising 27 durum and 116 bread wheat genotypes. Contrasting genotypes were selected for population development and genetic analysis. Quantitative trait loci (QTL) analysis was performed on a durum F2 population and a bread wheat recombinant inbred line (RIL) population. Contrasting with studies of anther length, we found no large effect on TSL of the GA-insensitive semi-dwarfing genes Rht-B1 and Rht-D1 in either durum or bread wheat. However, in durum cultivar Italo, we identified a region on chromosome 6A which is robustly associated with larger TSL and contains the Rht14 allele for reduced plant height, a trait that is favourable for female line development in hybrid systems. This dual effect locus explained 25.2 and 19.2% of TSL phenotypic variation in experiments across two growing seasons, with preliminary results suggesting this locus may increase TSL when transferred to bread wheat. In a bread wheat, RIL population minor QTL on 1A and 2A was indicated, but the strongest association was with Ppd-B1. Methods developed here, and the identification of a TSL-enhancing locus provides advances and further opportunities in the study of wheat stigma.

Genome-wide association study and linkage mapping reveal TaqW-6B associated with water-extractable arabinoxylan content in wheat grain.

KEY MESSAGE: A novel QTL, TaqW-6B of water-extractable arabinoxylan content in the wheat grain on chromosome 6BL was identified and fine mapped in a narrow region 3.8 Mb. Water-extractable arabinoxylan (WE-AX), an important component of hemicellulose, is associated with various abundant health benefits. In this study, QTLs for WE-AX content were detected in two populations: (1) a recombinant inbred line (RIL) population with 164 lines derived from a cross between Avocet and Chilero (AC population) genotyped with diversity array technology (DArT), and (2) a natural population of 243 varieties (CH population) genotyped with the Axiom wheat 660 K single-nucleotide polymorphism (SNP) array. A stable QTL Qwe-ax.haust-6B, explaining 8.51-15.59% of the phenotypic variance, was mapped in the physical interval 459.38-572.09 Mb on the long arm of chromosome 6B in the AC population, tightly linked with DArT markers 3,944,740 and 4,991,038 under three experimental conditions. The Qwe-ax.haust-6B was further narrowed down to be delimited in the physical interval 516.47-571.58 Mb on chromosome 6BL, explaining 5.86-16.27% of the phenotypic variance in the CH population. Furthermore, we developed high-throughput kompetitive allele-specific PCR (KASP) markers to reconstruct the genetic linkage map in the AC population, and Qwe-ax.haust-6B was fine mapped into a narrow region named TaqW-6B, which was compressed between KASP-6B-3 and KASP-6B-6 at a physical distance of 3.8 Mb. In the meanwhile, the markers were also validated in a natural population of 160 wheat lines (NP population). Consequently, this study is of great importance to provide the theoretical basis for cloning the key gene and developing functional markers for molecular breeding.

Fine mapping and transcriptome profiling reveal CpAPRR2 to modulate immature fruit rind color formation in zucchini (Cucurbita pepo).

KEY MESSAGE: A large fragment deletion of CpAPRR2, encoding a two-component response regulator-like protein, which influences immature white rind color formation in zucchini (Cucurbita pepo). Fruit rind color is an important agronomic trait that affects commodity quality and consumer choice in zucchini (Cucurbita pepo). However, the molecular mechanism controlling rind color is unclear. We characterized two zucchini inbred lines: '19' (dark green rind) and '113' (white rind). Genetic analysis revealed white immature fruit rind color to be controlled by a dominant locus (CpW). Combining bulked segregant analysis sequencing (BSA-seq) and Kompetitive Allele-Specific PCR (KASP) markers, we mapped the CpW locus to a 100.4 kb region on chromosome 5 and then narrow down the candidate region to 37.5 kb using linkage analysis of 532 BC1 and 1613 F2 individuals, including 6 coding genes. Among them, Cp4.1LG05g02070 (CpAPRR2), encoding a two-component response regulator-like protein, was regarded to be a promising candidate gene. The expression level of CpAPRR2 in dark green rind was significantly higher than that in white rind and was induced by light. A deletion of 2227 bp at the 5' end of CpAPRR2 in '113' might explain the white phenotype. Further analysis of allelic diversity in zucchini germplasm resources revealed rind color to be associated with the deletion of CpAPRR2. Subcellular localization analysis indicated that CpAPRR2 was a nuclear protein. Transcriptome analysis using near-isogenic lines with dark green (DG) and white (W) rind indicated that genes involved in photosynthesis and porphyrin metabolism pathways were enriched in DG compared with W. Additionally, chlorophyll synthesis-related genes were upregulated in DG. These results identify mechanisms of zucchini rind color and provide genetic resources for breeding.

QTL mapping of downy mildew resistance in foxtail millet by SLAF­seq and BSR-seq analysis.

KEY MESSAGE: Key message Three major QTLs for resistance to downy mildew were located within an 0.78 Mb interval on chromosome 8 in foxtail millet. Downy mildew, a disease caused by Sclerospora graminicola, is a serious problem that jeopardizes the yield and quality of foxtail millet. Breeding resistant varieties represents one of the most economical and effective solutions, yet there is a lack of molecular markers related to the resistance. Here, a mapping population comprising of 158 F6:7 recombinant inbred lines (RILs) was constructed from the crossing of G1 and JG21. Based on the specific locus amplified fragment sequencing results, a high-density linkage map of foxtail millet with 1031 bin markers, spanning 1041.66 cM was constructed. Based on the high-density linkage map and the phenotype data in four environments, a total of nine quantitative trait loci (QTL) associated with resistance to downy mildew were identified. Further BSR-seq confirmed the genomic regions containing the potential candidate genes related to downy mildew resistance. Interestingly, a 0.78-Mb interval between C8M257 and C8M268 on chromosome 8 was highlighted because of its presence in three major QTL, qDM8_1, qDM8_2, and qDM8_4, which contains 10 NBS-LRR genes. Haplotype analysis in RILs and natural population suggest that 9 SNP loci on Seita8G.199800, Seita8G.195900, Seita8G.198300, and Seita.8G199300 genes were significantly correlated with disease resistance. Furthermore, we found that those genes were taxon-specific by collinearity analysis of pearl millet and foxtail millet genomes. The identification of these new resistance QTL and the prediction of resistance genes against downy mildew will be useful in breeding for resistant varieties and the study of genetic mechanisms of downy mildew disease resistance in foxtail millet.

Genome organization: Raison d'être of ancestral linkage groups.

The genomes of extant organisms contain conserved blocks of regions that can be traced back to ancient ancestors, yet the evolutionary pressures that maintained such genomic segments remain unclear. New research on a curious organism with two different genomes sheds light on why our genomes are organized as they are.

A high-density linkage map and sex-determination loci in Pacific white shrimp (Litopenaeus vannamei).

BACKGROUND: Expansion of genomic resources for the Pacific white shrimp (Litopenaeus vannamei), such as the construction of dense genetic linkage maps, is crucial for the application of genomic tools in order to improve economically relevant traits. Sexual dimorphism exists in Pacific white shrimp, and the mapping of the sex-determination region in this species may help in future reproductive applications. We have constructed male, female, and sex-averaged high-density genetic maps using a 50 K single-nucleotide polymorphism (SNP) array, followed by a genome-wide association study (GWAS) to identify genomic regions associated with sex in white shrimp. RESULTS: The genetic map yielded 15,256 SNPs assigned to 44 linkage groups (LG). The lengths of the male, female, and sex-averaged maps were 5,741.36, 5,461.20 and 5,525.26 cM, respectively. LG18 was found to be the largest for both sexes, whereas LG44 was the shortest for males and LG31 for females. A sex-determining region was found in LG31 with 21 statistically significant SNPs. The most important SNP was previously identified as a sex-linked marker and was able to identify 99% of the males and 88% of the females. Although other significant markers had a lower ability to determine sex, putative genes were intercepted or close to them. The oplophorus-luciferin 2-monooxygenase, serine/arginine repetitive matrix protein and spermine oxidase genes were identified as candidates with possible participation in important processes of sexual differentiation in shrimp. CONCLUSIONS: Our results provide novel genomic resources for shrimp, including a high-density linkage map and new insights into the sex-determining region in L. vannamei, which may be usefulfor future genetics and reproduction applications.