Genome-wide discovery of DNA polymorphism in Brassica rapa

Genetic diversity and population structure in Ethiopian mustard (Brassica carinata A. Braun) revealed by high-density DArTSeq SNP genotyping

BACKGROUND

Ethiopian mustard (Brassica carinata (A) Braun) is a promising oilseed crop with the potential for sustainable biofuel and bio-industrial applications. Despite the presence of diverse germplasms in Ethiopia, their genetic diversity remains largely unexplored. This study evaluated the genetic diversity and population structure of 188 B. carinata genotypes using high-density Single Nucleotide Polymorphism (SNP) markers generated though DArTseq™ Genotyping-by-Sequencing (GBS). Of the 15,515 identified DArTSeq SNPs, 3,793 high-quality markers were retained and used to analyze the genetic diversity and population structure.

RESULTS

The results from STRUCTURE, principal coordinate analysis (PCoA), and neighbor-joining tree analyses revealed two slightly distinct subpopulations, with Pop1 predominantly comprising genotypes from the Oromia and Amhara regions (86.17%), whereas Pop2 primarily consisted of released varieties, suggests the influence of targeted selection. Despite the presence of subpopulations, PCoA indicated a relatively limited overall genetic diversity among the genotypes. Analysis of Molecular Variance (AMOVA) revealed higher genetic variation within populations (65.19%) than between populations (34.81%), resulting in low genetic differentiation (PhiPT = 0.02) and high gene flow (Nm = 5.74). Notably, subpopulation formation was not strongly correlated with geographical origin, highlights that factors beyond geography, such as gene flow and selection pressure, may have played a significant role in shaping the observed genetic diversity. Genetic diversity indices revealed a slightly low-to-moderate variation within the B. carinata populations, as evidenced by the slightly low expected heterozygosity (He = 0.21) and moderate polymorphic information content (PIC = 0.36).

CONCLUSION

Overall, this study revealed a moderate level of genetic diversity within the evaluated B. carinata genotypes. The results offer valuable insights into the genetic structure of this species and highlight the need for targeted strategies to enhance genetic diversity in future breeding initiatives and conservation efforts.

Genetic Diversity and Population Structure of a Large USDA Sesame Collection

Sesame, Sesamum indicum L., is one of the oldest domesticated crops used for its oil and protein in many parts of the world. To build genomic resources for sesame that could be used to improve sesame productivity and responses to stresses, a USDA sesame germplasm collection of 501 accessions originating from 36 countries was used in this study. The panel was genotyped using genotyping-by-sequencing (GBS) technology to explore its genetic diversity and population structure and the relatedness among its accessions. A total of 24,735 high-quality single-nucleotide polymorphism (SNP) markers were identified over the 13 chromosomes. The marker density was 1900 SNP per chromosome, with an average polymorphism information content (PIC) value of 0.267. The marker polymorphisms and heterozygosity estimators indicated the usefulness of the identified SNPs to be used in future genetic studies and breeding activities. The population structure, principal components analysis (PCA), and unrooted neighbor-joining phylogenetic tree analyses classified two distinct subpopulations, indicating a wide genetic diversity within the USDA sesame collection. Analysis of molecular variance (AMOVA) revealed that 29.5% of the variation in this population was due to subpopulations, while 57.5% of the variation was due to variation among the accessions within the subpopulations. These results showed the degree of differentiation between the two subpopulations as well as within each subpopulation. The high fixation index (FST) between the distinguished subpopulations indicates a wide genetic diversity and high genetic differentiation among and within the identified subpopulations. The linkage disequilibrium (LD) pattern averaged 161 Kbp for the whole sesame genome, while the LD decay ranged from 168 Kbp at chromosome LG09 to 123 Kbp in chromosome LG05. These findings could explain the complications of linkage drag among the traits during selections. The selected accessions and genotyped SNPs provide tools to enhance genetic gain in sesame breeding programs through molecular approaches.

The Construction of lncRNA/circRNA-miRNA-mRNA Networks Reveals Functional Genes Related to Growth Traits in Schima superba

Schima superba is a precious timber and fire-resistant tree species widely distributed in southern China. Currently, there is little knowledge related to its growth traits, especially with respect to molecular breeding. The lack of relevant information has delayed the development of modern breeding. The purpose is to identify probable functional genes involved in S. superba growth through whole transcriptome sequencing. In this study, a total of 32,711 mRNAs, 525 miRNAs, 54,312 lncRNAs, and 1522 circRNAs were identified from 10 S. superba individuals containing different volumes of wood. Four possible regulators, comprising three lncRNAs, one circRNA, and eleven key miRNAs, were identified from the regulatory networks of lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA to supply information on ncRNAs. Several candidate genes involved in phenylpropane and cellulose biosynthesis pathways, including Ss4CL2, SsCSL1, and SsCSL2, and transcription factors, including SsDELLA2 (SsSLR), SsDELLA3 (SsSLN), SsDELLA5 (SsGAI-like2), and SsNAM1, were identified to reveal the molecular regulatory mechanisms regulating the growth traits of S. superba. The results not merely provide candidate functional genes related to S. superba growth trait and will be useful to carry out molecular breeding, but the strategy and method also provide scientists with an effective approach to revealing mechanisms behind important economic traits in other species.

Whole-genome sequencing and variant discovery of Citrus reticulata "Kinnow" from Pakistan

Citrus is a source of nutritional and medicinal advantages, cultivated worldwide with major groups of sweet oranges, mandarins, grapefruits, kumquats, lemons and limes. Pakistan produces all major citrus groups with mandarin (Citrus reticulata) being the prominent group that includes local commercial cultivars Feutral's Early, Dancy, Honey, and Kinnow. The present study designed to understand the genetic architecture of this unique variety of Citrus reticulata 'Kinnow.' The whole-genome resequencing and variant calling was performed to map the genomic variability that might be responsible for its particular characteristics like taste, seedlessness, juice content, thickness of peel, and shelf-life. A total of 139,436,350 raw sequence reads were generated with 20.9 Gb data in Fastq format having 98% effectiveness and 0.2% base call error rate. Overall, 3,503,033 SNPs, 176,949 MNPs, 323,287 INS, and 333,083 DEL were identified using the GATK4 variant calling pipeline against Citrus clementina. Furthermore, g:Profiler was applied for annotating the newly found variants, harbor genes/transcripts and their involved pathways. A total of 73,864 transcripts harbors 4,336,352 variants, most of the observed variants were predicted in non-coding regions and 1009 transcripts were found well annotated by different databases. Out of total aforementioned transcripts, 588 involved in biological processes, 234 in molecular functions and 167 transcripts in cellular components. In a nutshell, 18,153 high impact variants and 216 genic variants found in the current study, which may be used after its functional validation for marker-assisted breeding programs of "Kinnow" to propagate its valued traits for the improvement of contemporary citrus varieties in the region.

Evidence that miR168a contributes to salinity tolerance of Brassica rapa L. via mediating melatonin biosynthesis

Melatonin is a master regulator of diverse biological processes, including plant's abiotic stress responses and tolerance. Despite the extensive information on the role of melatonin in response to abiotic stress, how plants regulate endogenous melatonin content under stressful conditions remains largely unknown. In this study, we computationally mined Expressed Sequence Tag (EST) libraries of salinity-exposed Chinese cabbage (Brassica rapa) to identify the most reliable differentially expressed miRNA and its target gene(s). In light of these analyses, we found that miR168a potentially targets a key melatonin biosynthesis gene, namely O-METHYLTRANSFERASE 1 (OMT1). Accordingly, molecular and physiochemical evaluations were performed in a separate salinity experiment using contrasting B. rapa genotypes. Then, the association between B. rapa salinity tolerance and changes in measured molecular and physiochemical characteristics was determined. Results indicated that the expression profiles of miR168a and OMT1 significantly differed between B. rapa genotypes. Moreover, the expression profiles of miR168a and OMT1 significantly correlated with more melatonin content, robust antioxidant activities, and better ion homeostasis during salinity stress. Our results suggest that miR168a plausibly mediates melatonin biosynthesis, mainly through the OMT1 gene, under salinity conditions and thereby contributes to the salinity tolerance of B. rapa. To our knowledge, this is the first report on the role of miR168a and OMT1 in B. rapa salinity response.

Genetic Diversity and Population Structure of Schima superba From Southern China

The tree Schima superba is important for afforestation and fire prevention in southern China. The wood of this tree can also be used for furniture and buildings. However, the lack of genetic background and genomic information for this species has lowered wood yield speed and quality improvement. Here, we aimed to discover genome-wide single nucleotide polymorphisms (SNPs) in 302 S. superba germplasms collected from southern China and to use these SNPs to investigate the population structure. Using genotyping by sequencing, a total of 785 high-quality SNP markers (minor allele frequency [MAF] ≥ 0.05) were identified from 302 accessions collected from seven geographical locations. Population structure analyses and principal coordinate analyses (PCoAs) indicated that these germplasm resources can be clearly separated into different populations. The S. superba accessions originating from Yunnan (YN) and Guangxi (GX) fell into the same population, separate from the accessions originating from Guangdong (GD), which indicated that these two regions should be regarded as major provenances of this species. In addition, two independent core germplasm sets with abundant genetic polymorphisms were constructed to support the breeding work. The identification of SNP markers, analyses of population genetics, and construction of core germplasm sets will greatly promote the molecular breeding work of S. superba.

Development of GBTS and KASP Panels for Genetic Diversity, Population Structure, and Fingerprinting of a Large Collection of Broccoli (Brassica oleracea L. var. italica) in China

Broccoli (Brassica oleracea var. italica) is one of the most important and nutritious vegetables widely cultivated in China. In the recent four decades, several improved varieties were bred and developed by Chinese breeders. However, the efforts for improvement of broccoli are hindered by limited information of genetic diversity and genetic relatedness contained within the available germplasms. This study evaluated the genetic diversity, genetic relationship, population structure, and fingerprinting of 372 accessions of broccoli representing most of the variability of broccoli in China. Millions of SNPs were identified by whole-genome sequencing of 23 representative broccoli genotypes. Through several stringent selection criteria, a total of 1,167 SNPs were selected to characterize genetic diversity and population structure. Of these markers, 1,067 SNPs were genotyped by target sequencing (GBTS), and 100 SNPs were genotyped by kompetitive allele specific PCR (KASP) assay. The average polymorphism information content (PIC) and expected heterozygosity (gene diversity) values were 0.33 and 0.42, respectively. Diversity analysis revealed the prevalence of low to moderate genetic diversity in the broccoli accessions indicating a narrow genetic base. Phylogenetic and principal component analyses revealed that the 372 accessions could be clustered into two main groups but with weak groupings. STRUCTURE analysis also suggested the presence of two subpopulations with weak genetic structure. Analysis of molecular variance (AMOVA) identified 13% variance among populations and 87% within populations revealing very low population differentiation, which could be attributed to massive gene flow and the reproductive biology of the crop. Based on high resolving power, a set of 28 KASP markers was chosen for DNA fingerprinting of the broccoli accessions for seed authentication and varietal identification. To the best of our knowledge, this is the first comprehensive study to measure diversity and population structure of a large collection of broccoli in China and also the first application of GBTS and KASP techniques in genetic characterization of broccoli. This work broadens the understanding of diversity, phylogeny, and population structure of a large collection of broccoli, which may enhance future breeding efforts to achieve higher productivity.

Identification of genome-wide single-nucleotide polymorphisms among geographically diverse radish accessions

Radish (Raphanus sativus L.) is cultivated around the world as a vegetable crop and exhibits diverse morphological and physiological features. DNA polymorphisms are responsible for differences in traits among cultivars. In this study, we determined genome-wide single-nucleotide polymorphisms (SNPs) among geographically diverse radish accessions using the double-digest restriction site-associated DNA sequencing (ddRAD-Seq) method. A total of 52,559 SNPs was identified in a collection of over 500 radish accessions (cultivated and wild) from East Asia, South and Southeast Asia, and the Occident and Near East. In addition, 2,624 SNP sites without missing data (referred to as common SNP sites) were identified among 510 accessions. Genetic diversity analyses, based on the common SNP sites, divided the cultivated radish accessions into four main groups, each derived from four geographical areas (Japan, East Asia, South and Southeast Asia, and the Occident and Near East). Furthermore, we discuss the origin of cultivated radish and its migration from the West to East Asia. SNP data generated in this work will facilitate further genetic studies on the radish breeding and production of DNA markers.

Portuguese wild grapevine genome re-sequencing (Vitis vinifera sylvestris)

The first genome of Vitis vinifera vinifera (PN40024), published in 2007, boosted grapevine related studies. While this reference genome is a suitable tool for the overall studies in the field, it lacks the ability to unveil changes accumulated during V. v. vinifera domestication. The subspecies V. v. sylvestris preserves wild characteristics, making it a good material to provide insights into V. v. vinifera domestication. The difference in the reproductive strategy between both subspecies is one of the characteristics that set them apart. While V. v. vinifera flowers are hermaphrodite, V. v. sylvestris is mostly dioecious. In this paper, we compare the re-sequencing of the genomes from a male and a female individual of the wild sylvestris, against the reference vinifera genome (PN40024). Variant analysis reveals a low number but with high impact modifications in coding regions, essentially non-synonymous single nucleotide polymorphisms and frame shifts caused by insertions and deletions. The sex-locus was manually inspected, and the results obtained are in line with the most recent works related with wild grapevine sex. In this paper we also describe for the first time RNA editing in transcripts of 14 genes in the sex-determining region, including VviYABBY and VviPLATZ.

An extended KASP-SNP resource for molecular breeding in Chinese cabbage(Brassica rapa L. ssp. pekinensis)

Kompetitive allele-specific PCR (KASP) is a cost-effective single-step SNP genotyping technology, With an objective to enhance the marker repertoire and develop high efficient KASP-SNP markers in Chinese cabbage, we re-sequenced four Chinese cabbage doubled haploid (DH) lines, Y177-47, Y635-10, Y510-1 and Y510-9, and generated a total of more than 38.5 billion clean base pairs. A total of 827,720 SNP loci were identified with an estimated density of 3,217 SNPs/Mb. Further, a total of 387,354 SNPs with at least 30 bp to the next most adjacent SNPs on either side were selected as resource for KASP markers. From this resource, 258 (96.27%) of 268 SNP loci were successfully transformed into KASP-SNP markers using a Roche LightCycler 480-II instrument. Among these markers, 221 (85.66%) were co-dominant markers, 220 (85.27%) were non-synonymous SNPs, and 257 (99.6%) were newly developed markers. In addition, 53 markers were applied for genotyping of 34 Brassica rapa accessions. Cluster analysis separated these 34 accessions into three clusters based on heading types. The millions of SNP loci, a large set of resource for KASP markers, as well as the newly developed KASP markers in this study may facilitate further genetic and molecular breeding studies in Brassica rapa.

Detecting selection with a genetic cross

Distinguishing which traits have evolved under natural selection, as opposed to neutral evolution, is a major goal of evolutionary biology. Several tests have been proposed to accomplish this, but these either rely on false assumptions or suffer from low power. Here, I introduce an approach to detecting selection that makes minimal assumptions and only requires phenotypic data from ∼10 individuals. The test compares the phenotypic difference between two populations to what would be expected by chance under neutral evolution, which can be estimated from the phenotypic distribution of an F2 cross between those populations. Simulations show that the test is robust to variation in the number of loci affecting the trait, the distribution of locus effect sizes, heritability, dominance, and epistasis. Comparing its performance to the QTL sign test-an existing test of selection that requires both genotype and phenotype data-the new test achieves comparable power with 50- to 100-fold fewer individuals (and no genotype data). Applying the test to empirical data spanning over a century shows strong directional selection in many crops, as well as on naturally selected traits such as head shape in Hawaiian Drosophila and skin color in humans. Applied to gene expression data, the test reveals that the strength of stabilizing selection acting on mRNA levels in a species is strongly associated with that species' effective population size. In sum, this test is applicable to phenotypic data from almost any genetic cross, allowing selection to be detected more easily and powerfully than previously possible.

Genome-wide DNA polymorphisms in four Actinidia arguta genotypes based on whole-genome re-sequencing

Among the genus Actinidia, Actinidia arguta possesses the strongest cold resistance and produces fresh fruit with an intense flavor. To investigate genomic variation that may contribute to variation in phenotypic traits, we performed whole-genome re-sequencing of four A. arguta genotypes originating from different regions in China and identified the polymorphisms using InDel markers. In total, 4,710,650, 4,787,750, 4,646,026, and 4,590,616 SNPs and 1,481,002, 1,534,198, 1,471,304, and 1,425,393 InDels were detected in the ‘Ruby-3’, ‘Yongfeng male’, ‘Kuilv male’, and ‘Hongbei male’ genomes, respectively, compared with the reference genome sequence of cv ‘Hongyang’. A subset of 120 InDels were selected for re-sequencing validation. Additionally, genes related to non-synonymous SNPs and InDels in coding domain sequences were screened for functional analysis. The analysis of GO and KEGG showed that genes involved in cellular responses to water deprivation, sucrose transport, decreased oxygen levels and plant hormone signal transduction were significantly enriched in A. arguta. The results of this study provide insight into the genomic variation of kiwifruit and can inform future research on molecular breeding to improve cold resistance in kiwifruit.

Examination of Genomic and Transcriptomic Alterations in a Morphologically Stable Line, MU1, Generated by Intergeneric Pollination

Interspecific hybridization creates genetic variation useful for crop improvement. However, whether pollen from a different genus affects the genomic stability and/or transcriptome of the recipient species during intergeneric pollination has not been investigated. Here, we crossed japonica rice cv. Z12 with the maize accession B73 (pollen donor) and obtained a morphologically stable line, MU1, exhibiting moderate dwarfism, higher tiller number, and increased grain weight compared with Z12. To reveal the genetic basis of these morphological changes in MU1, we performed whole-genome resequencing of MU1 and Z12. Compared with Z12, MU1 showed 107,250 single nucleotide polymorphisms (SNPs) and 23,278 insertion/deletions (InDels). Additionally, 5'-upstream regulatory regions (5'UTRs) of 429 and 309 differentially expressed genes (DEGs) in MU1 contained SNPs and InDels, respectively, suggesting that a subset of these DEGs account for the variation in 5'UTRs. Transcriptome analysis revealed 2190 DEGs in MU1 compared with Z12. Genes up-regulated in MU1 were mainly involved in photosynthesis, generation of precursor metabolites, and energy and cellular biosynthetic processes; whereas those down-regulated in MU1 were involved in plant hormone signal transduction pathway and response to stimuli and stress processes. Quantitative PCR (qPCR) further identified the expression levels of the up- or down-regulated gene in plant hormone signal transduction pathway. The expression level changes of plant hormone signal transduction pathway may be significant for plant growth and development. These findings suggest that mutations caused by intergeneric pollination could be the important reason for changes of MU1 in agronomic traits.

Genetic Diversity and Population Structure of a Camelina sativa Spring Panel

There is a need to explore renewable alternatives (e.g., biofuels) that can produce energy sources to help reduce the reliance on fossil oils. In addition, the consumption of fossil oils adversely affects the environment and human health via the generation of waste water, greenhouse gases, and waste solids. Camelina sativa, originated from southeastern Europe and southwestern Asia, is being re-embraced as an industrial oilseed crop due to its high seed oil content (36-47%) and high unsaturated fatty acid composition (>90%), which are suitable for jet fuel, biodiesel, high-value lubricants and animal feed. C. sativa's agronomic advantages include short time to maturation, low water and nutrient requirements, adaptability to adverse environmental conditions and resistance to common pests and pathogens. These characteristics make it an ideal crop for sustainable agricultural systems and regions of marginal land. However, the lack of genetic and genomic resources has slowed the enhancement of this emerging oilseed crop and exploration of its full agronomic and breeding potential. Here, a core of 213 spring C. sativa accessions was collected and genotyped. The genotypic data was used to characterize genetic diversity and population structure to infer how natural selection and plant breeding may have affected the formation and differentiation within the C. sativa natural populations, and how the genetic diversity of this species can be used in future breeding efforts. A total of 6,192 high-quality single nucleotide polymorphisms (SNPs) were identified using genotyping-by-sequencing (GBS) technology. The average polymorphism information content (PIC) value of 0.29 indicate moderate genetic diversity for the C. sativa spring panel evaluated in this report. Population structure and principal coordinates analyses (PCoA) based on SNPs revealed two distinct subpopulations. Sub-population 1 (POP1) contains accessions that mainly originated from Germany while the majority of POP2 accessions (>75%) were collected from Eastern Europe. Analysis of molecular variance (AMOVA) identified 4% variance among and 96% variance within subpopulations, indicating a high gene exchange (or low genetic differentiation) between the two subpopulations. These findings provide important information for future allele/gene identification using genome-wide association studies (GWAS) and marker-assisted selection (MAS) to enhance genetic gain in C. sativa breeding programs.

Genome re-sequencing, SNP analysis, and genetic mapping of the parental lines of a commercial F1 hybrid cultivar of Chinese cabbage

The genome-wide characterization of single nucleotide polymorphism (SNP) between cultivars or between inbred lines contributes to the creation of genetic markers that are important for plant breeding. Functional markers derived from polymorphisms within genes that affect phenotypic variation are especially valuable in plant breeding. Here, we report on the genome re-sequencing and analysis of the two parental inbred lines of the commercial F₁ hybrid Chinese cabbage cultivar "W77". Through the genome-wide identification and classification of the SNPs and indels present in each parental line, we identified about 1,500 putative non-functional genes in each parent. We designed cleaved amplified polymorphic sequence (CAPS) markers using specific mutations found at Eco RI restriction sites in the parental lines and confirmed their Mendelian segregation by constructing a linkage map using 96 F₂ plants derived from the F₁ hybrid cultivar, "W77". Our results and data will be a useful genomic resource for future studies of gene function and metagenomic studies in Chinese cabbage.

Exploring the phylogeography of a hexaploid freshwater fish by RAD sequencing

The KwaZulu‐Natal yellowfish (Labeobarbus natalensis) is an abundant cyprinid, endemic to KwaZulu‐Natal Province, South Africa. In this study, we developed a single‐nucleotide polymorphism (SNP) dataset from double‐digest restriction site‐associated DNA (ddRAD) sequencing of samples across the distribution. We addressed several hidden challenges, primarily focusing on proper filtering of RAD data and selecting optimal parameters for data processing in polyploid lineages. We used the resulting high‐quality SNP dataset to investigate the population genetic structure of L. natalensis. A small number of mitochondrial markers present in these data had disproportionate influence on the recovered genetic structure. The presence of singleton SNPs also confounded genetic structure. We found a well‐supported division into northern and southern lineages, with further subdivision into five populations, one of which reflects north–south admixture. Approximate Bayesian Computation scenario testing supported a scenario where an ancestral population diverged into northern and southern lineages, which then diverged to yield the current five populations. All river systems showed similar levels of genetic diversity, which appears unrelated to drainage system size. Nucleotide diversity was highest in the smallest river system, the Mbokodweni, which, together with adjacent small coastal systems, should be considered as a key catchment for conservation.

IntroMap: a signal analysis based method for the detection of genomic introgressions

BACKGROUND

Breeding programs often rely on marker-assisted tests or variant calling of next generation sequence (NGS) data to identify regions of genomic introgression arising from the hybridization of two plant species. In this paper we present IntroMap, a bioinformatics pipeline for the screening of candidate plants through the application of signal processing techniques to NGS data, using alignment to a reference genome sequence (annotation is not required) that shares homology with the recurrent parental cultivar, and without the need for de novo assembly of the read data or variant calling.

RESULTS

We show the accurate identification of introgressed genomic regions using both in silico simulated genomes, and a hybridized cultivar data set using our pipeline. Additionally we show, through targeted marker-based assays, validation of the IntroMap predicted regions for the hybrid cultivar.

CONCLUSIONS

This approach can be used to automate the screening of large populations, reducing the time and labor required, and can improve the accuracy of the detection of introgressed regions in comparison to a marker-based approach. In contrast to other approaches that generally rely upon a variant calling step, our method achieves accurate identification of introgressed regions without variant calling, relying solely upon alignment.

A genome-wide SNP-based genetic map and QTL mapping for agronomic traits in Chinese cabbage

The aim of this work was to construct a high-resolution genetic map for the dissection of complex morphological and agronomic traits in Chinese cabbage (Brassica rapa L. syn. B. campestris). Chinese cabbage, an economically important vegetable, is a good model plant for studies on the evolution of morphologic variation. Herein, two high-generation inbred Chinese cabbage lines, 'Huangxiaoza' and 'Bqq094-11', were crossed. Then restriction-site-associated DNA sequencing (RAD-seq) was performed on the parents and 120 F₂ individuals. A genetic map containing 711 bins representing 3985 single nucleotide polymorphism (SNP) markers was constructed. By using WinQTL with composite interval mapping (CIM) and mixed-model based composite interval mapping (MCIM) analysis via QTLNetwork, quantitative trait loci (QTL) linked to 16 genetic traits related to plant size, color and leaf characteristics were mapped to 10 linkage groups. The high density genetic map and QTL identified for morphological and agronomic traits lay the groundwork for functional gene mapping, map-based cloning and marker-assisted selection (MAS) in Chinese cabbage.

Identification of candidate domestication regions in the radish genome based on high-depth resequencing analysis of 17 genotypes

This study provides high-quality variation data of diverse radish genotypes. Genome-wide SNP comparison along with RNA-seq analysis identified candidate genes related to domestication that have potential as trait-related markers for genetics and breeding of radish. Radish (Raphanus sativus L.) is an annual root vegetable crop that also encompasses diverse wild species. Radish has a long history of domestication, but the origins and selective sweep of cultivated radishes remain controversial. Here, we present comprehensive whole-genome resequencing analysis of radish to explore genomic variation between the radish genotypes and to identify genetic bottlenecks due to domestication in Asian cultivars. High-depth resequencing and multi-sample genotyping analysis of ten cultivated and seven wild accessions obtained 4.0 million high-quality homozygous single-nucleotide polymorphisms (SNPs)/insertions or deletions. Variation analysis revealed that Asian cultivated radish types are closely related to wild Asian accessions, but are distinct from European/American cultivated radishes, supporting the notion that Asian cultivars were domesticated from wild Asian genotypes. SNP comparison between Asian genotypes identified 153 candidate domestication regions (CDRs) containing 512 genes. Network analysis of the genes in CDRs functioning in plant signaling pathways and biochemical processes identified group of genes related to root architecture, cell wall, sugar metabolism, and glucosinolate biosynthesis. Expression profiling of the genes during root development suggested that domestication-related selective advantages included a main taproot with few branched lateral roots, reduced cell wall rigidity and favorable taste. Overall, this study provides evolutionary insights into domestication-related genetic selection in radish as well as identification of gene candidates with the potential to act as trait-related markers for background selection of elite lines in molecular breeding.

SNP diversity within and among Brassica rapa accessions reveals no geographic differentiation

Genetic diversity was studied in a collection of 61 accessions of Brassica rapa, which were mostly oil-type turnip rapes but also included two oil-type subsp. dichotoma and five subsp. trilocularis accessions, as well as three leaf-type subspecies (subsp. japonica, pekinensis, and chinensis) and five turnip cultivars (subsp. rapa). Two-hundred and nine SNP markers, which had been discovered by amplicon resequencing, were used to genotype 893 plants from the B. rapa collection using Illumina BeadXpress. There was great variation in the diversity indices between accessions. With STRUCTURE analysis, the plant collection could be divided into three groups that seemed to correspond to morphotype and flowering habit but not to geography. According to AMOVA analysis, 65% of the variation was due to variation within accessions, 25% among accessions, and 10% among groups. A smaller subset of the plant collection, 12 accessions, was also studied with 5727 GBS-SNPs. Diversity indices obtained with GBS-SNPs correlated well with those obtained with Illumina BeadXpress SNPs. The developed SNP markers have already been used and will be used in future plant breeding programs as well as in mapping and diversity studies.

Comparative analysis of disease-linked single nucleotide polymorphic markers from Brassica rapa for their applicability to Brassica oleracea

Numerous studies using single nucleotide polymorphisms (SNPs) have been conducted in humans, and other animals, and in major crops, including rice, soybean, and Chinese cabbage. However, the number of SNP studies in cabbage is limited. In this present study, we evaluated whether 7,645 SNPs previously identified as molecular markers linked to disease resistance in the Brassica rapa genome could be applied to B. oleracea. In a BLAST analysis using the SNP sequences of B. rapa and B. oleracea genomic sequence data registered in the NCBI database, 256 genes for which SNPs had been identified in B. rapa were found in B. oleracea. These genes were classified into three functional groups: molecular function (64 genes), biological process (96 genes), and cellular component (96 genes). A total of 693 SNP markers, including 145 SNP markers [BRH—developed from the B. rapa genome for high-resolution melt (HRM) analysis], 425 SNP markers (BRP—based on the B. rapa genome that could be applied to B. oleracea), and 123 new SNP markers (BRS—derived from BRP and designed for HRM analysis), were investigated for their ability to amplify sequences from cabbage genomic DNA. In total, 425 of the SNP markers (BRP-based on B. rapa genome), selected from 7,645 SNPs, were successfully applied to B. oleracea. Using PCR, 108 of 145 BRH (74.5%), 415 of 425 BRP (97.6%), and 118 of 123 BRS (95.9%) showed amplification, suggesting that it is possible to apply SNP markers developed based on the B. rapa genome to B. oleracea. These results provide valuable information that can be utilized in cabbage genetics and breeding programs using molecular markers derived from other Brassica species.

Construction of a reference genetic map of Raphanus sativus based on genotyping by whole-genome resequencing

This manuscript provides a genetic map of Raphanus sativus that has been used as a reference genetic map for an ongoing genome sequencing project. The map was constructed based on genotyping by whole-genome resequencing of mapping parents and F 2 population. Raphanus sativus is an annual vegetable crop species of the Brassicaceae family and is one of the key plants in the seed industry, especially in East Asia. Assessment of the R. sativus genome provides fundamental resources for crop improvement as well as the study of crop genome structure and evolution. With the goal of anchoring genome sequence assemblies of R. sativus cv. WK10039 whose genome has been sequenced onto the chromosomes, we developed a reference genetic map based on genotyping of two parents (maternal WK10039 and paternal WK10024) and 93 individuals of the F2 mapping population by whole-genome resequencing. To develop high-confidence genetic markers, ~83 Gb of parental lines and ~591 Gb of mapping population data were generated as Illumina 100 bp paired-end reads. High stringent sequence analysis of the reads mapped to the 344 Mb of genome sequence scaffolds identified a total of 16,282 SNPs and 150 PCR-based markers. Using a subset of the markers, a high-density genetic map was constructed from the analysis of 2,637 markers spanning 1,538 cM with 1,000 unique framework loci. The genetic markers integrated 295 Mb of genome sequences to the cytogenetically defined chromosome arms. Comparative analysis of the chromosome-anchored sequences with Arabidopsis thaliana and Brassica rapa revealed that the R. sativus genome has evident triplicated sub-genome blocks and the structure of gene space is highly similar to that of B. rapa. The genetic map developed in this study will serve as fundamental genomic resources for the study of R. sativus.

Genomic DNA enrichment using sequence capture microarrays: a novel approach to discover sequence nucleotide polymorphisms (SNP) in Brassica napus L

Targeted genomic selection methodologies, or sequence capture, allow for DNA enrichment and large-scale resequencing and characterization of natural genetic variation in species with complex genomes, such as rapeseed canola (Brassica napus L., AACC, 2n=38). The main goal of this project was to combine sequence capture with next generation sequencing (NGS) to discover single nucleotide polymorphisms (SNPs) in specific areas of the B. napus genome historically associated (via quantitative trait loci –QTL– analysis) to traits of agronomical and nutritional importance. A 2.1 million feature sequence capture platform was designed to interrogate DNA sequence variation across 47 specific genomic regions, representing 51.2 Mb of the Brassica A and C genomes, in ten diverse rapeseed genotypes. All ten genotypes were sequenced using the 454 Life Sciences chemistry and to assess the effect of increased sequence depth, two genotypes were also sequenced using Illumina HiSeq chemistry. As a result, 589,367 potentially useful SNPs were identified. Analysis of sequence coverage indicated a four-fold increased representation of target regions, with 57% of the filtered SNPs falling within these regions. Sixty percent of discovered SNPs corresponded to transitions while 40% were transversions. Interestingly, fifty eight percent of the SNPs were found in genic regions while 42% were found in intergenic regions. Further, a high percentage of genic SNPs was found in exons (65% and 64% for the A and C genomes, respectively). Two different genotyping assays were used to validate the discovered SNPs. Validation rates ranged from 61.5% to 84% of tested SNPs, underpinning the effectiveness of this SNP discovery approach. Most importantly, the discovered SNPs were associated with agronomically important regions of the B. napus genome generating a novel data resource for research and breeding this crop species.

Genomewide variation in an introgression line of rice-Zizania revealed by whole-genome re-sequencing

BACKGROUND

Hybridization between genetically diverged organisms is known as an important avenue that drives plant genome evolution. The possible outcomes of hybridization would be the occurrences of genetic instabilities in the resultant hybrids. It remained under-investigated however whether pollination by alien pollens of a closely related but sexually "incompatible" species could evoke genomic changes and to what extent it may result in phenotypic novelties in the derived progenies.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we have re-sequenced the genomes of Oryza sativa ssp. japonica cv. Matsumae and one of its derived introgressant RZ35 that was obtained from an introgressive hybridization between Matsumae and Zizanialatifolia Griseb. in general, 131 millions 90 base pair (bp) paired-end reads were generated which covered 13.2 and 21.9 folds of the Matsumae and RZ35 genomes, respectively. Relative to Matsumae, a total of 41,724 homozygous single nucleotide polymorphisms (SNPs) and 17,839 homozygous insertions/deletions (indels) were identified in RZ35, of which 3,797 SNPs were nonsynonymous mutations. Furthermore, rampant mobilization of transposable elements (TEs) was found in the RZ35 genome. The results of pathogen inoculation revealed that RZ35 exhibited enhanced resistance to blast relative to Matsumae. Notably, one nonsynonymous mutation was found in the known blast resistance gene Pid3/Pi25 and real-time quantitative (q) RT-PCR analysis revealed constitutive up-regulation of its expression, suggesting both altered function and expression of Pid3/Pi25 may be responsible for the enhanced resistance to rice blast by RZ35.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that introgressive hybridization by Zizania has provoked genomewide, extensive genomic changes in the rice genome, and some of which have resulted in important phenotypic novelties. These findings suggest that introgressive hybridization by alien pollens of even a sexually incompatible species may represent a potent means to generate novel genetic diversities, and which may have played relevant roles in plant evolution and can be manipulated for crop improvements.

Development of InDel markers for Brassica rapa based on whole-genome re-sequencing

Genome-wide detection of short insertion/deletion length polymorphisms (InDels, <5 bp) in Brassica rapa (named the A genome) was performed by comparing whole-genome re-sequencing data from two B. rapa accessions, L144 and Z16, to the reference genome sequence of Chiifu-401-42. In total, we identified 108,558 InDel polymorphisms between Chiifu-401-42 and L144, 26,795 InDels between Z16 and Chiifu-401-42, and 26,693 InDels between L144 and Z16. From these, 639 InDel polymorphisms of 3-5 bp in length between L144 and Z16 were selected for experimental validation; 491 (77%) yielded single PCR fragments and showed polymorphisms, 7 (1%) did not amplify a product, and 141 (22%) showed no polymorphism. For further validation of these intra-specific InDel polymorphisms, 503 candidates, randomly selected from the 639 InDels, were screened across seven accessions representing different B. rapa cultivar groups. Of these assayed markers, 387 (77%) were polymorphic, 111 (22%) were not polymorphic and 5 (1%) did not amplify a PCR product. Furthermore, we randomly selected 518 InDel markers to validate their polymorphism in B. napus (the AC genome) and B. juncea (the AB genome), of which more than 90% amplified a PCR product; 132 (25%) showed polymorphism between the two B. napus accessions and 41 (8%) between the two B. juncea accessions. This set of novel PCR-based InDel markers will be a valuable resource for genetic studies and breeding programs in B. rapa.

A nuclear phylogenetic analysis: SNPs, indels and SSRs deliver new insights into the relationships in the 'true citrus fruit trees' group (Citrinae, Rutaceae) and the origin of cultivated species

BACKGROUND AND AIMS

Despite differences in morphology, the genera representing 'true citrus fruit trees' are sexually compatible, and their phylogenetic relationships remain unclear. Most of the important commercial 'species' of Citrus are believed to be of interspecific origin. By studying polymorphisms of 27 nuclear genes, the average molecular differentiation between species was estimated and some phylogenetic relationships between 'true citrus fruit trees' were clarified.

METHODS

Sanger sequencing of PCR-amplified fragments from 18 genes involved in metabolite biosynthesis pathways and nine putative genes for salt tolerance was performed for 45 genotypes of Citrus and relatives of Citrus to mine single nucleotide polymorphisms (SNPs) and indel polymorphisms. Fifty nuclear simple sequence repeats (SSRs) were also analysed.

KEY RESULTS

A total of 16 238 kb of DNA was sequenced for each genotype, and 1097 single nucleotide polymorphisms (SNPs) and 50 indels were identified. These polymorphisms were more valuable than SSRs for inter-taxon differentiation. Nuclear phylogenetic analysis revealed that Citrus reticulata and Fortunella form a cluster that is differentiated from the clade that includes three other basic taxa of cultivated citrus (C. maxima, C. medica and C. micrantha). These results confirm the taxonomic subdivision between the subgenera Metacitrus and Archicitrus. A few genes displayed positive selection patterns within or between species, but most of them displayed neutral patterns. The phylogenetic inheritance patterns of the analysed genes were inferred for commercial Citrus spp.

CONCLUSIONS

Numerous molecular polymorphisms (SNPs and indels), which are potentially useful for the analysis of interspecific genetic structures, have been identified. The nuclear phylogenetic network for Citrus and its sexually compatible relatives was consistent with the geographical origins of these genera. The positive selection observed for a few genes will help further works to analyse the molecular basis of the variability of the associated traits. This study presents new insights into the origin of C. sinensis.

Fine mapping of complex traits in non-model species: using next generation sequencing and advanced intercross lines in Japanese quail

Background

As for other non-model species, genetic analyses in quail will benefit greatly from a higher marker density, now attainable thanks to the evolution of sequencing and genotyping technologies. Our objective was to obtain the first genome wide panel of Japanese quail SNP (Single Nucleotide Polymorphism) and to use it for the fine mapping of a QTL for a fear-related behaviour, namely tonic immobility, previously localized on Coturnix japonica chromosome 1. To this aim, two reduced representations of the genome were analysed through high-throughput 454 sequencing: AFLP (Amplified Fragment Length Polymorphism) fragments as representatives of genomic DNA, and EST (Expressed Sequence Tag) as representatives of the transcriptome.

Results

The sequencing runs produced 399,189 and 1,106,762 sequence reads from cDNA and genomic fragments, respectively. They covered over 434 Mb of sequence in total and allowed us to detect 17,433 putative SNP. Among them, 384 were used to genotype two Advanced Intercross Lines (AIL) obtained from three quail lines differing for duration of tonic immobility. Despite the absence of genotyping for founder individuals in the analysis, the previously identified candidate region on chromosome 1 was refined and led to the identification of a candidate gene.

Conclusions

These data confirm the efficiency of transcript and AFLP-sequencing for SNP discovery in a non-model species, and its application to the fine mapping of a complex trait. Our results reveal a significant association of duration of tonic immobility with a genomic region comprising the DMD (dystrophin) gene. Further characterization of this candidate gene is needed to decipher its putative role in tonic immobility in Coturnix.

Exploiting comparative mapping among Brassica species to accelerate the physical delimitation of a genic male-sterile locus (BnRf) in Brassica napus

The recessive genic male sterility (RGMS) line 9012AB has been used as an important pollination control system for rapeseed hybrid production in China. Here, we report our study on physical mapping of one male-sterile locus (BnRf) in 9012AB by exploiting the comparative genomics among Brassica species. The genetic maps around BnRf from previous reports were integrated and enriched with markers from the Brassica A7 chromosome. Subsequent collinearity analysis of these markers contributed to the identification of a novel ancestral karyotype block F that possibly encompasses BnRf. Fourteen insertion/deletion markers were further developed from this conserved block and genotyped in three large backcross populations, leading to the construction of high-resolution local genetic maps where the BnRf locus was restricted to a less than 0.1-cM region. Moreover, it was observed that the target region in Brassica napus shares a high collinearity relationship with a region from the Brassica rapa A7 chromosome. A BnRf-cosegregated marker (AT3G23870) was then used to screen a B. napus bacterial artificial chromosome (BAC) library. From the resulting 16 positive BAC clones, one (JBnB089D05) was identified to most possibly contain the BnRf (c) allele. With the assistance of the genome sequence from the Brassica rapa homolog, the 13.8-kb DNA fragment covering both closest flanking markers from the BAC clone was isolated. Gene annotation based on the comparison of microcollinear regions among Brassica napus, B. rapa and Arabidopsis showed that five potential open reading frames reside in this fragment. These results provide a foundation for the characterization of the BnRf locus and allow a better understanding of the chromosome evolution around BnRf.

High-throughput polymorphism detection and genotyping in Brassica napus using next-generation RAD sequencing

Background

The complex genome of rapeseed (Brassica napus) is not well understood despite the economic importance of the species. Good knowledge of sequence variation is needed for genetics approaches and breeding purposes. We used a diversity set of B. napus representing eight different germplasm types to sequence genome-wide distributed restriction-site associated DNA (RAD) fragments for polymorphism detection and genotyping.

Results

More than 113,000 RAD clusters with more than 20,000 single nucleotide polymorphisms (SNPs) and 125 insertions/deletions were detected and characterized. About one third of the RAD clusters and polymorphisms mapped to the Brassica rapa reference sequence. An even distribution of RAD clusters and polymorphisms was observed across the B. rapa chromosomes, which suggests that there might be an equal distribution over the Brassica oleracea chromosomes, too. The representation of Gene Ontology (GO) terms for unigenes with RAD clusters and polymorphisms revealed no signature of selection with respect to the distribution of polymorphisms within genes belonging to a specific GO category.

Conclusions

Considering the decreasing costs for next-generation sequencing, the results of our study suggest that RAD sequencing is not only a simple and cost-effective method for high-density polymorphism detection but also an alternative to SNP genotyping from transcriptome sequencing or SNP arrays, even for species with complex genomes such as B. napus.

DNA-Based Genetic Markers for Rapid Cycling Brassica Rapa (Fast Plants Type) Designed for the Teaching Laboratory

We have developed DNA-based genetic markers for rapid cycling Brassica rapa (RCBr), also known as Fast Plants. Although markers for B. rapa already exist, ours were intentionally designed for use in a teaching laboratory environment. The qualities we selected for were robust amplification in PCR, polymorphism in RCBr strains, and alleles that can be easily resolved in simple agarose slab gels. We have developed two single nucleotide polymorphism (SNP) based markers and 14 variable number tandem repeat (VNTR)-type markers spread over four chromosomes. The DNA sequences of these markers represent variation in a wide range of genomic features. Among the VNTR-type markers, there are examples of variation in a non-genic region, variation within an intron, and variation in the coding sequence of a gene. Among the SNP-based markers there are examples of polymorphism in intronic DNA and synonymous substitution in a coding sequence. Thus these markers can serve laboratory exercises in both transmission genetics and molecular biology.

Isolation and functional characterisation of the genes encoding Δ(8)-sphingolipid desaturase from Brassica rapa

Δ(8)-Sphingolipid desaturase is the key enzyme that catalyses desaturation at the C8 position of the long-chain base of sphingolipids in higher plants. There have been no previous studies on the genes encoding Δ(8)-sphingolipid desaturases in Brassica rapa. In this study, four genes encoding Δ(8)-sphingolipid desaturases from B. rapa were isolated and characterised. Phylogenetic analyses indicated that these genes could be divided into two groups: BrD8A, BrD8C and BrD8D in group I, and BrD8B in group II. The two groups of genes diverged before the separation of Arabidopsis and Brassica. Though the four genes shared a high sequence similarity, and their coding desaturases all located in endoplasmic reticulum, they exhibited distinct expression patterns. Heterologous expression in Saccharomyces cerevisiae revealed that BrD8A/B/C/D were functionally diverse Δ(8)-sphingolipid desaturases that catalyse different ratios of the two products 8(Z)- and 8(E)-C18-phytosphingenine. The aluminium tolerance of transgenic yeasts expressing BrD8A/B/C/D was enhanced compared with that of control cells. Expression of BrD8A in Arabidopsis changed the ratio of 8(Z):8(E)-C18-phytosphingenine in transgenic plants. The information reported here provides new insights into the biochemical functional diversity and evolutionary relationship of Δ(8)-sphingolipid desaturase in plants and lays a foundation for further investigation of the mechanism of 8(Z)- and 8(E)-C18-phytosphingenine biosynthesis.

Comparative use of InDel and SSR markers in deciphering the interspecific structure of cultivated citrus genetic diversity: a perspective for genetic association studies

Genetic stratification associated with domestication history is a key parameter for estimating the pertinence of genetic association study within a gene pool. Previous molecular and phenotypic studies have shown that most of the diversity of cultivated citrus results from recombination between three main species: C. medica (citron), C. reticulata (mandarin) and C. maxima (pummelo). However, the precise contribution of each of these basic species to the genomes of secondary cultivated species, such as C. sinensis (sweet orange), C. limon (lemon), C. aurantium (sour orange), C. paradisi (grapefruit) and recent hybrids is unknown. Our study focused on: (1) the development of insertion-deletion (InDel) markers and their comparison with SSR markers for use in genetic diversity and phylogenetic studies; (2) the analysis of the contributions of basic taxa to the genomes of secondary species and modern cultivars and (3) the description of the organisation of the Citrus gene pool, to evaluate how genetic association studies should be done at the cultivated Citrus gene pool level. InDel markers appear to be better phylogenetic markers for tracing the contributions of the three ancestral species, whereas SSR markers are more useful for intraspecific diversity analysis. Most of the genetic organisation of the Citrus gene pool is related to the differentiation between C. reticulata, C. maxima and C. medica. High and generalised LD was observed, probably due to the initial differentiation between the basic species and a limited number of interspecific recombinations. This structure precludes association genetic studies at the genus level without developing additional recombinant populations from interspecific hybrids. Association genetic studies should also be affordable at intraspecific level in a less structured pool such as C. reticulata.

A sequence-based genetic linkage map as a reference for Brassica rapa pseudochromosome assembly

Background

Brassica rapa is an economically important crop and a model plant for studies concerning polyploidization and the evolution of extreme morphology. The multinational B. rapa Genome Sequencing Project (BrGSP) was launched in 2003. In 2008, next generation sequencing technology was used to sequence the B. rapa genome. Several maps concerning B. rapa pseudochromosome assembly have been published but their coverage of the genome is incomplete, anchoring approximately 73.6% of the scaffolds on to chromosomes. Therefore, a new genetic map to aid pseudochromosome assembly is required.

Results

This study concerns the construction of a reference genetic linkage map for Brassica rapa, forming the backbone for anchoring sequence scaffolds of the B. rapa genome resulting from recent sequencing efforts. One hundred and nineteen doubled haploid (DH) lines derived from microspore cultures of an F1 cross between a Chinese cabbage (B. rapa ssp. pekinensis) DH line (Z16) and a rapid cycling inbred line (L144) were used to construct the linkage map. PCR-based insertion/deletion (InDel) markers were developed by re-sequencing the two parental lines. The map comprises a total of 507 markers including 415 InDels and 92 SSRs. Alignment and orientation using SSR markers in common with existing B. rapa linkage maps allowed ten linkage groups to be identified, designated A01-A10. The total length of the linkage map was 1234.2 cM, with an average distance of 2.43 cM between adjacent marker loci. The lengths of linkage groups ranged from 71.5 cM to 188.5 cM for A08 and A09, respectively. Using the developed linkage map, 152 scaffolds were anchored on to the chromosomes, encompassing more than 82.9% of the B. rapa genome. Taken together with the previously available linkage maps, 183 scaffolds were anchored on to the chromosomes and the total coverage of the genome was 88.9%.

Conclusions

The development of this linkage map is vital for the integration of genome sequences and genetic information, and provides a useful resource for the international Brassica research community.