Development and genetic mapping of microsatellite markers from genome survey sequences in Brassica napus

Development and utility of SSR markers based on Brassica sp. whole-genome in triangle of U

Introduction

Simple sequence repeats (SSR), also known as microsatellites, are crucial molecular markers in both animals and plants. Despite extensive previous research on SSRs, the development of microsatellite markers in Brassica crops remains limited and inefficient.

Methods

Krait software was used to identify microsatellites by genome-wide and marker development based on three recently sequenced basic species of Brassica crops in the triangle of U (Brassica rapa, B. nigra and B. oleracea), as well as three allotetraploids (B. juncea, B. napus and B. carinata) using public databases. Subsequently, the primers and the characteristics of microsatellites for most of them were accordingly designed on each chromosome of each of the six Brassica species, and their physical locations were identified,and the cross-transferability of primers have been carried out. In addition, a B-genome specific SSR marker was screened out.

Results

A total of 79341, 92089, 125443, 173964, 173604, and 222160 SSR loci have been identified from the whole genome sequences of Brassica crops within the triangle of U crops, B. rapa (AA), B. nigra (BB), B. oleracea (CC), B. napus (AACC), B. juncea (AABB) and B. carinata (BBCC), respectively. Comparing the number distribution of the three allotetraploid SSR loci in the three subgenomes AA, BB and CC, results indicate that the allotetraploid species have significant reduction in the number of SSR loci in the genome compared with their basic diploid counterparts. Moreover, we compared the basic species with their corresponding varieties, and found that the microsatellite characters between the allotetraploids and their corresponding basic species were very similar or almost identical. Subsequently, each of the 40 SSR primers was employed to investigate the polymorphism potential of B. rapa (85.27%), B. nigra (81.33%) and B. oleracea (73.45%), and B. rapa was found to have a higher cross-transfer rate among the basic species in the triangle of U. Meanwhile, a B-genome specific SSR marker, BniSSR23228 possessing the (AAGGA)3 sequence characteristics was obtained, and it located in chromosome B3 with a total length of 97 bp.

Discussion

In this study, results suggest that the pattern of distribution may be highly conserved during the differentiation of basic Brassica species and their allotetraploid counterparts. Our data indicated that the allotetraploidization process resulted in a significant reduction in SSR loci in the three subgenomes AA, BB and CC. The reasons may be partial gene dominated chromosomal homologous recombination and rearrangement during the evolution of basic diploid species into allotetraploids. This study provides a basis for future genomics and genetic research on the relatedness of Brassica species.

High-Throughput Association Mapping in Brassica napus L.: Methods and Applications

Oil seed rape (Braasica napus L.) is ranked second among oil seed crops cultivated globally for edible oil for human, and seed cake for animal consumption. Recent genetic and genomics advancements highlighted the diversity that exists within B. napus, which is largely discovered using the most promising genetic markers called single nucleotide polymorphism (SNP). Their calling rate is also enhanced to ~100 folds after the continuous advancements in the next generation sequencing (NGS) technologies. As the high throughput of NGS resulted in multi-Giga bases data, the detailed quality control (QC) prior to downstream analyses is a pre-requisite. It mainly involved the removal of false positives, missing proportions, filtering of low-quality SNPs, and adjustments of minor-allele frequency and heterozygosity. After marker-trait association, for conformation of target SNPs, validations of SNPs can be performed using various methods, especially allele-specific PCR assay-based methods have been utilized for SNP genotyping of genes targeting agronomic traits and somaclonal variations occurred during transgenic studies. In the present study, the authors mainly argue on the genotypic progress, and pipelines/methods that are being used for detection, calling, filtering, and validation of SNPs. Also, insight is provided into the application of SNPs in linkage and association mapping, including QTL mapping and genome-wide association studies targeting mainly developmental traits related to the root system and plant architecture, flowering time, silique, and oil quality. Briefly, the present study provides the recent information and recommendations on the SNP genotyping methods and its applications, which can be useful for marker-assisted breeding in B. napus and other crops.

Full-length transcriptome analysis of Zanthoxylum nitidum (Roxb.) DC

Zanthoxylum nitidum (Roxb.) DC. (Z. nitidum) is a type of Chinese Dao-di herb, also called Liangmianzhen, which is widely used to treat arthralgia, rheumatic arthralgia, and stomach pain. However, genomic resources for Z. nitidum are still scarce. This study provides transcriptomic resources for Z. nitidum by applying single-molecule real-time (SMRT) sequencing technology. In total, 456,109 circular consensus sequencing (CCS) reads were generated with a mean length of 2,216 bp from Z. nitidum roots, old stems, young branches, leaves, flowers, and fruits. Of these total reads, 353,932 were full-length nonchimeric (FLNC) reads with an average length of 1,996 bp. A total of 16,163 transcripts with a mean length of 1,171 bp were acquired. Of these transcripts, 14,231 (88%) were successfully annotated using public databases. Across all the 16,163 transcripts, we identified 6,255 long non-coding RNAs (lncRNAs) and 22,780 simple sequence repeats (SSRs). Furthermore, 3,482 transcription factors were identified. Among the SSR loci, 1-3 nucleotide repeats were dominant, occupying 99.36% of the total SSR loci, with mono-, di-, and tri-nucleotide repeats accounting for 61.80%, 19.89%, and 5.02% of the total SSR loci, respectively. A total of 36 out of 100 randomly selected primer pairs were verified to be positive, 20 of which showed polymorphism. These findings enrich the genetic resources available for facilitating future studies and research on relevant topics such as population genetics in Z. nitidum.

Transcriptome Analysis and Identification of a Female-Specific SSR Marker in Pistacia chinensis Based on Illumina Paired-End RNA Sequencing

Pistacia chinensis Bunge (P. chinensis), a dioecious plant species, has been widely found in China. The female P. chinensis plants are more important than male plants in agricultural production, as their seeds can serve as an ideal feedstock for biodiesel. However, the sex of P. chinensis plants is hard to distinguish during the seedling stage due to the scarcity of available transcriptomic and genomic information. In this work, Illumina paired-end RNA sequencing assay was conducted to unravel the transcriptomic profiles of female and male P. chinensis flower buds. In total, 50,925,088 and 51,470,578 clean reads were obtained from the female and male cDNA libraries, respectively. After quality checks and de novo assembly, a total of 83,370 unigenes with a mean length of 1.3 kb were screened. Overall, 64,539 unigenes (77.48%) could be matched in at least one of the NR, NT, Swiss-Prot, COG, KEGG, and GO databases, 71 of which were putatively related to the floral development of P. chinensis. Additionally, 21,662 simple sequence repeat (SSR) motifs were identified in 17,028 unigenes of P. chinensis, and the mononucleotide motif was the most dominant type of repeats (52.59%) in P. chinensis, followed by dinucleotide (22.29%), trinucleotide (20.15%). The most abundant repeats were AG/CT (13.97%), followed by AAC/GTT (6.75%) and AT/TA (6.10%). Based on these SSR, 983 EST-SSR primers were designed, 151 of which were randomly chosen for validation. Of these validated EST-SSR markers, 25 SSR markers were found to be polymorphic between male and female plants. One SSR marker, namelyPCSSR55, displayed excellent specificity in female plants, which could clearly distinguish between male and female P. chinensis. Altogether, our findings not only reveal that the EST-SSR marker is extremely effective in distinguishing between male and female P. chinensis but also provide a solid framework for sex determination of plant seedlings.

Genome-wide identification and characterization of novel non-coding RNA-derived SSRs in wheat

Expression of eukaryotic genes is largely regulated by non-coding RNAs (ncRNA). Sequence variations in the regulatory RNAs may have critical biological consequences including transcriptional and post-transcriptional gene regulation. ncRNA-derived markers thus can be proved useful in molecular breeding, QTL mapping and association studies for trait dissection. In present study, we identified a total of 661 SSRs dwelling in pre-miRNA (15), small nuclear RNA (25) and lncRNA (621). Of these, 46 were validated and 100% amplification success was observed in selected wheat genotypes. A set of 36 ncRNA-SSRs markers was utilized for genetic variability assessment in forty-eight Indian wheat genotypes (which includes bread wheat, durum wheat and relatives). Number of alleles ranged from 1 to 4 with an average of two alleles per SSR locus. Mean PIC, observed heterozygosity and Shannon information index were found to be 0.258, 0.37 and 0.476 which suggests ncRNA-SSRs show higher polymorphism compared to genic SSRs but lower polymorphism compared to genomic SSRs. Thirty-six ncRNA-SSRs showed transferability ranging from 42.1% to 100%. Average genetic dissimilarity among wheat genotypes was found to be 0.29 based on Jaccard's dissimilarity. This is the first report of ncRNA-SSRs in wheat which will be useful for molecular breeding and genetic improvement of wheat.

A method for obtaining flexible broccoli varieties for sustainable agriculture

BACKGROUND

The use of high inputs in agriculture resulted in few varieties (hybrids and pure lines) used in all agricultural systems. Also varieties of vegetables, including broccoli, for organic and low-input agriculture, are almost exclusively hybrids, since there are very few specific breeding programs and varieties for sustainable agriculture systems. A strategy to overcome this issue is the adoption of specific breeding programs for developing heterogeneous varieties (i.e. synthetics, open pollinated varieties, composite cross populations and mixtures). In fact, heterogeneous varieties are able to evolve and adapt to specific agro-climatic conditions. The aim of this study was to develop a method (an Evolutionary Breeding Program, EBP) for obtaining heterogeneous varieties of broccoli and test its efficiency in developing highly diverse varieties, as needed in sustainable agriculture. A synthetic variety originated from a landrace was multiplied in different environments for 3 cycles and morpho-phenological and genetic diversity of the derived populations were assessed.

RESULTS

The presented results are the first and unique indication about the efficiency of a short-time EBP for an allogamous species like broccoli. Few morphological changes were observed among varieties multiplied in different environments with different agro-climatic conditions. This could be probably due to the initial genetic diversity of the landrace from which the populations were selected and also to the great plasticity of the crop. However, SSR data highlighted a genetic differentiation among populations multiplied for two/three years across Europe and in Central Italy, that was not so evident when considering morphological data only.

CONCLUSIONS

Few years of multiplication in different environments resulted in genetically differentiated broccoli populations that nonetheless preserved the original genetic diversity and productivity level and appear to evolve in relationship to different environments: the applied EBP is useful for developing heterogeneous materials for sustainable agriculture.

Genetic dissection of thousand-seed weight and fine mapping of cqSW.A03-2 via linkage and association analysis in rapeseed (Brassica napus L.)

KEY MESSAGE: cqSW.A03-2, one of the six identified quantitative trait loci associated with thousand-seed weight in rapeseed, is mapped to a 61.6-kb region on chromosome A03 and corresponds to the candidate gene BnaA03G37960D. Seed weight is an important factor that determines the seed yield of oilseed rape (Brassica napus L.). To elucidate the genetic mechanism of thousand-seed weight (TSW), quantitative trait locus (QTL) mapping was conducted using a double haploid population derived from the cross between an elite line ZY50 and a pol cytoplasmic male sterility restorer line 7-5. The genetic basis of TSW was dissected into six major QTLs. One major QTL denoted as cqSW.A03-2, which explained 8.46-13.70% of the phenotypic variation, was detected across multiple environments. To uncover the genetic basis of cqSW.A03-2, a set of near-isogenic lines were developed. Based on the test of self-pollinated progenies, cqSW.A03-2 was identified as a single Mendelian factor and the ZY50 allele at cqSW.A03-2 showed a positive effect on TSW. Fine mapping delimited the cqSW.A03-2 locus into a 61.6-kb region, and 18 genes within this region were predicted. Candidate gene association analysis and expression analysis indicated that a histidine kinase gene (BnaA03G37960D) is likely to be the candidate gene for the cqSW.A03-2 locus. Our results may contribute to a better understanding of the molecular mechanism of seed weight regulation and promote the breeding program for yield improvement in rapeseed.

Root Morphological Traits of Seedlings Are Predictors of Seed Yield and Quality in Winter Oilseed Rape Hybrid Cultivars

The root system is responsible for soil resources acquisition. Hence, optimizing crop root characteristics has considerable implications for agricultural production. This study evaluated a panel of twenty-eight European modern cultivars of oilseed rape (Brassica napus L.) cultivated in laboratory and field environments. Root morphology was screened using a high-throughput hydroponic growth system with two divergent nitrogen supplies. The panel showed an important diversity for biomass production and root morphological traits. Differences in root and shoot dry biomasses and lateral root length were mainly explained by the genotype, and differences in primary root length by nitrogen nutrition. The cultivars were tested in a pluriannual field trial. The field variation for yield and seed quality traits attributed to the genotype was more important than the year or the genotype × year interaction effects. The total root length measured at the seedling stage could predict the proportion of nitrogen taken up from the field and reallocated to seed organs, a component of the nitrogen use efficiency. The genetic interrelationship between cultivars, established with simple sequence repeat markers, indicated a very narrow genetic base. Positive correlations were found between the genetic distance measures, root morphological traits during nitrogen depletion and yield components. This study illustrates a root phenotyping screen in the laboratory with a proof of concept evaluation in the field. The results could assist future genetic improvements in oilseed rape for desirable root characteristics to reduce nutrient losses in the environment.

Quantitative Trait Locus Mapping and Identification of Candidate Genes Controlling Flowering Time in Brassica napus L

Flowering time plays a vital role in determining the life-cycle period, yield, and seed quality of rapeseed (Brassica napus L.) in certain environments. Quantitative trait locus (QTL) mapping to identify the genetic architecture of genes controlling flowering time helps accelerate the early maturity breeding process. In this study, simple sequence repeats (SSR) and specific-locus amplified fragment sequencing (SLAF-seq) technologies were adopted to map the QTLs for flowering time in four environments. As a result, three target intervals, FTA09, FTA10, and FTC05 were identified. Among this, FTA09 was considered as a novel interval, FTA10 and FTC05 as stable regions. Based on the parental re-sequencing data, 7,022 single nucleotide polymorphisms (SNPs) and 2,195 insertion-deletions (InDels) between the two parents were identified in these three target regions. A total of 186 genes possessed genetic variations in these intervals, 14 of which were related to flowering time involved in photoperiod, circadian clock, vernalization, and gibberellin pathways. Six InDel markers linked to flowering time were developed in the three target intervals, indicating that the results were credible in this study. These results laid a good foundation for further genetic studies on flowering-time regulation in B. napus L.

Development of microsatellite markers and analysis of genetic diversity of Barbatia virescens in the southern coasts of China

BACKGROUND

The blood clam Barbatia virescens is an ecologically and economically important species in the southern coast of China. Understanding of the genetic structure of B. virescens populations is vital to breeding strategies and conservation programs.

OBJECTIVE

To develop and characterize a set of microsatellites loci primers for B. virescens, and provide helpful information for reasonable utilization and protection of B. virescens natural resources.

METHODS

The microsatellites of B. virescens were detected using a RAD-seq approach based on an Illumina sequencing platform. For the test of microsatellite development, we calculated the number of alleles (Na), observed heterozygosities (Ho), expected heterozygosities (He) and exact tests for deviations from Hardy-Weinberg equilibrium (HWE). Twelve polymorphic loci were used to access the genetic diversity and population structure of four B. virescens populations.

RESULTS

In this study, 50,729 microsatellites of B. virescens were detected. Twenty-two polymorphic microsatellite loci were developed for B. virescens. The number of alleles per locus ranged from 6 to 15, and expected heterozygosities varied from 0. 567 to 0.911. All the PIC values of the 22 loci were greater than 0.5, indicating that these markers were highly informative for further genetic analysis. Twelve loci were selected to analyze genetic diversity and population structure of four B. virescens populations collected from different geographical regions along the southern coast of China. The results showed moderate to high levels of genetic diversity in the four populations (mean Ar = 7.756-8.133; mean Ho = 0.575-0.639; mean He = 0.754-0.775). Pairwise F_ST estimates indicated that there was significant divergence among the four populations.

CONCLUSION

This study not only provides a large scale of sequence information of microsatellites which are valuable for future genetic mapping, trait association and kinship among B. virescens, but also offers useful information for the sustainable management of natural stocks and the development of breeding industry of B. virescens.

A comprehensive and precise set of intervarietal substitution lines to identify candidate genes and quantitative trait loci in oilseed rape (Brassica napus L.)

A set of intervarietal substitution lines were developed in rapeseed by recurrent backcrossing and marker-assisted selection and employed for mapping both qualitative and quantitative traits. Intervarietal substitution lines (ISLs) may be assembled into advanced secondary mapping populations that have remarkable potential for resolving trait loci and mapping candidate genes. To facilitate the identification of important genes in oilseed rape (canola, Brassica napus), we developed 89 ISLs using an elite cultivar 'Zhongyou 821' (ZY821) as the recipient and a re-synthesized line 'No.2127' as the donor. In the whole process of ISLs development, the target chromosome segments were selected based on the genotypes of 300 microsatellite markers evenly distributed across the genome. Eighty-nine ISLs fixed at BC₅F₄ were genotyped by sequencing using double digestion to survey the lengths of target substitution segments from the donor parent and the background segments from the recurrent parent. The total length of the substituted chromosome segments was 3030.27 Mb, representing 3.56 × of the Darmor-bzh reference genome sequence (version 4.1). Gene mapping was conducted for two qualitative traits, flower colour and seed-coat colour, and nine quantitative traits including yield- and quality-related traits, with 19 QTLs identified for the latter. Overlapping substitution segments were identified for flower colour and seed-coat colour loci, as well as for QTLs consistently detected in 2 or 3 years. These results demonstrate the value of these ISLs for locus resolution and subsequent cloning, targeted mutation or editing of genes controlling important traits in oilseed rape.

Genetic flow among olive populations within the Mediterranean basin

Background

The olive tree is a typical crop of the Mediterranean basin where it shows a wide diversity, accounting for more than 2,600 cultivars. The ability to discriminate olive cultivars and determine their genetic variability is pivotal for an optimal exploitation of olive genetic resources.

Methods

We investigated the genetic diversity within 128 olive accessions belonging to four countries in the Mediterranean Basin (Italy, Algeria, Syria, and Malta), with the purpose of better understanding the origin and spread of the olive genotypes across Mediterranean Basin countries. Eleven highly polymorphic simple sequence repeat (SSR) markers were used and proved to be very informative, producing a total of 179 alleles.

Results

Cluster analysis distinguished three main groups according to their geographical origin, with the current sample of Maltese accessions included in the Italian group. Phylogenetic analysis further differentiated Italian and Maltese olive accessions, clarifying the intermediate position of Maltese accessions along the x/y-axes of principal coordinate analysis (PCoA). Model-based and neighbor clustering, PCoA, and migration analysis suggested the existence of two different gene pools (Algerian and Syrian) and that the genetic exchange occurred between the Syrian, Italian and Maltese populations.

Discussion

The close relationship between Syrian and Italian and Maltese olives was consistent with the historical domestication and migration of olive tree from the North Levant to eastern Mediterranean basin. This study lays the foundations for a better understanding of olive genetic diversity in the Mediterranean basin and represents a step toward an optimal conservation and exploitation of olive genetic resources.

Enzymatic activities and pathogenesis-related genes expression in sunflower inbred lines affected by Sclerotinia sclerotiorum culture filtrate

AIMS

Studying biochemical responses and pathogenesis-related gene expression in sunflower-Sclerotinia interaction can shed light on factors participating to disease resistance.

METHODS AND RESULTS

Partially resistant and susceptible lines were exposed to pathogen culture filtrate. The activity of antioxidant enzymes and proline was much more pronounced in partially resistant line. The more resistant to Sclerotinia sclerotiorum, the less (1,4)-β-glucanase activity was observed. PDF 1.2 and PR5-1 exhibited higher transcript abundance in the partially resistant line than in the susceptible line.

CONCLUSIONS

Considering the dual roles of oxalic acid, activation of the antioxidant system in partially resistant line might lead to suppression of oxidative burst which is beneficial for the growth of fungus at later stages of infection. The ability of the partially resistant line in balancing antioxidant enzymes could reserve H₂ O₂ as a substrate for peroxidase that might lead to lignification. The contribution of (1,4)-β-glucanase defence responses against Sclerotinia was observed. The roles of SA and JA marker genes were demonstrated in sunflower defence responses.

SIGNIFICANCE AND IMPACT OF THE STUDY

The time of antioxidant system activation in host is important in order to contribute to defence responses. To date, the changes in the expression of PR1 and PDF 1.2 and contribution of (1,4)-β-glucanase enzyme in sunflower defence responses were not reported in previous studies.

Molecular mapping of QTL alleles of Brassica oleracea affecting days to flowering and photosensitivity in spring Brassica napus

Earliness of flowering and maturity are important traits in spring Brassica napus canola–whether grown under long- or short-day condition. By use of a spring B. napus mapping population carrying the genome content of B. oleracea and testing this population under 10 to 18 h photoperiod and 18 to 20 ⁰C (day) temperature conditions, we identified a major QTL on the chromosome C1 affecting flowering time without being influenced by photoperiod and temperature, and a major QTL on C9 affecting flowering time under a short photoperiod (10 h); in both cases, the QTL alleles reducing the number of days to flowering in B. napus were introgressed from the late flowering species B. oleracea. Additive effect of the C1 QTL allele at 14 to18 h photoperiod was 1.1 to 2.9 days; however, the same QTL allele exerted an additive effect of 6.2 days at 10 h photoperiod. Additive effect of the C9 QTL at 10 h photoperiod was 2.8 days. These two QTL also showed significant interaction in the control of flowering only under a short-day (10 h photoperiod) condition with an effect of 2.3 days. A few additional QTL were also detected on the chromosomes C2 and C8; however, none of these QTL could be detected under all photoperiod and temperature conditions. BLASTn search identified several putative flowering time genes on the chromosomes C1 and C9 and located the physical position of the QTL markers in the Brassica genome; however, only a few of these genes were found within the QTL region. Thus, the molecular markers and the genomic regions identified in this research could potentially be used in breeding for the development of early flowering photoinsensitive B. napus canola cultivars, as well as for identification of candidate genes involved in flowering time variation and photosensitivity.

Development of Highly Informative Genome-Wide Single Sequence Repeat Markers for Breeding Applications in Sesame and Construction of a Web Resource: SisatBase

The sequencing of the full nuclear genome of sesame (Sesamum indicum L.) provides the platform for functional analyses of genome components and their application in breeding programs. Although the importance of microsatellites markers or simple sequence repeats (SSR) in crop genotyping, genetics, and breeding applications is well established, only a little information exist concerning SSRs at the whole genome level in sesame. In addition, SSRs represent a suitable marker type for sesame molecular breeding in developing countries where it is mainly grown. In this study, we identified 138,194 genome-wide SSRs of which 76.5% were physically mapped onto the 13 pseudo-chromosomes. Among these SSRs, up to three primers pairs were supplied for 101,930 SSRs and used to in silico amplify the reference genome together with two newly sequenced sesame accessions. A total of 79,957 SSRs (78%) were polymorphic between the three genomes thereby suggesting their promising use in different genomics-assisted breeding applications. From these polymorphic SSRs, 23 were selected and validated to have high polymorphic potential in 48 sesame accessions from different growing areas of Africa. Furthermore, we have developed an online user-friendly database, SisatBase (http://www.sesame-bioinfo.org/SisatBase/), which provides free access to SSRs data as well as an integrated platform for functional analyses. Altogether, the reference SSR and SisatBase would serve as useful resources for genetic assessment, genomic studies, and breeding advancement in sesame, especially in developing countries.

Mapping resistance responses to Sclerotinia infestation in introgression lines of Brassica juncea carrying genomic segments from wild Brassicaceae B. fruticulosa

Sclerotinia stem rot (Sclerotinia sclerotiorum) is a major disease of Brassica oilseeds. As suitable donors to develop resistant cultivars are not available in crop Brassicas, we introgressed resistance from a wild Brassicaceae species, B. fruticulosa. We produced 206 B. juncea-B. fruticulosa introgression lines (ILs). These were assessed for pollen grain fertility, genome size variations and resistance responses to Sclerotinia following stem inoculations under disease-conducive conditions. Of these, 115 ILs showing normal fertility and genome size were selected for cytogenetic characterization using florescent genomic in situ hybridization (Fl-GISH). B. fruticulosa segment substitutions were indicated in 28 ILs. These were predominantly terminal and located on B-genome chromosomes. A final set of 93 highly fertile and euploid (2n = 36) ILs were repeat-evaluated for their resistance responses during 2014-15. They were also genotyped with 202 transferable and 60 candidate gene SSRs. Association mapping allowed detection of ten significant marker trait associations (MTAs) after Bonferroni correction. These were: CNU-m157-2, RA2G05, CNU-m353-3, CNU-m442-5, ACMP00454-2, ACMP00454-3, EIN2-3-1, M641-1, Na10D09-1 and Na10D11-1. This is the first time such a molecular mapping technique has been deployed with introgression lines carrying genomic segments from B. fruticulosa, and the first to show that they possess high levels of resistance against S. sclerotiorum.

Quantitative Trait Locus Mapping of Salt Tolerance and Identification of Salt-Tolerant Genes in Brassica napus L

Salinity stress is one of typical abiotic stresses that seriously limit crop production. In this study, a genetic linkage map based on 532 molecular markers covering 1341.1 cM was constructed to identify the loci associated with salt tolerance in Brassica napus. Up to 45 quantitative trait loci (QTLs) for 10 indicators were identified in the F2:3 populations. These QTLs can account for 4.80-51.14% of the phenotypic variation. A major QTL, qSPAD5 on LG5 associated with chlorophyll can be detected in three replicates. Two intron polymorphic (IP) markers in this QTL region were developed successfully to narrow down the QTL location to a region of 390 kb. A salt tolerance related gene Bra003640 was primary identified as the candidate gene in this region. The full length of the candidate gene was 1,063 bp containing three exons and two introns in B. napus L. The open reading frame (ORF) is 867 bp and encodes 287 amino acids. Three amino acid differences (34, 54, and 83) in the conserved domain (B-box) were identified. RT-qPCR analysis showed that the gene expression had significant difference between the two parents. The study laid great foundation for salt tolerance related gene mapping and cloning in B. napus L.

Brassica napus DS-3, encoding a DELLA protein, negatively regulates stem elongation through gibberellin signaling pathway

Identification and characterization of a semi-dwarfing gene ds-3 encoding a mutant DELLA protein regulating plant height through gibberellin signaling pathway. Lodging is one of the most important factors causing severe yield loss in oilseed rape. Utilization of semi-dwarf varieties has been proved the most effective way to increase lodging resistance and yield in many crops. To develop semi-dwarf germplasm in oilseed rape, we identified a semi-dwarf mutant ds-3 which showed a reduced response to phytohormones gibberellins (GAs). Genetic analysis indicated the dwarfism was controlled by a single semi-dominant gene, ds-3. The DS-3 gene was mapped to a genomic region on chromosome C07, which is syntenic to the region of a previously identified semi-dwarf gene ds-1 (BnaA06.RGA). In this region, DS-3 (BnaC07.RGA) gene was identified to encode a DELLA protein that functions as a repressor in GA signaling pathway. A substitution of proline to leucine was identified in ds-3 in the conserved VHYNP motif, which is essential for GA-dependent interaction between gibberellin receptor GID1 and DELLA proteins. Segregation analysis in the F₂ population derived from the cross between ds-1 and ds-3 demonstrated that BnaA06.RGA displayed a stronger effect on plant height than BnaC07.RGA, indicating that different RGA genes may play different roles in stem elongation. In addition to BnaA06.RGA and BnaC07.RGA, two more RGA genes (BnaA09.RGA and BnaC09.RGA) were identified in the Brassica napus (B. napus) genome. Reverse-transcription polymerase chain reaction (RT-PCR) and yeast two-hybrid (Y2H) assays suggest that both BnaA09.RGA and BnaC09.RGA are transcribed in leaves and stems and can mediate GA signaling in vivo. These genes represent potential targets for screening ideal semi-dwarfing alleles for oilseed rape breeding.

Genetic Diversity Among Historical Olive (Olea europaea L.) Genotypes from Southern Anatolia Based on SSR Markers

Olive (Olea europaea) is an ancient and important crop in both olive oil production and table use. It is important to identify the genetic diversity of olive genetic resources for cultivar development and evaluation of olive germplasm. In the study, 14 microsatellite markers (UDO4, UDO8, UDO9, UDO11, UDO12, UDO22, UDO24, UDO26, UDO28, DCA9, DCA11, DCA13, DCA15, and DCA18) were used to assess the genetic variation on 76 olive (Olea europaea L.) genotypes from Mardin province together with 6 well-known Turkish and 4 well-known foreign reference cultivars. All microsatellite markers showed polymorphism and the number of alleles varied between 9 and 22, with an average of 14.57. The most informative loci were DCA 11 (22 alleles) and DCA 9 (21 alleles). Dendrogram based on genetic distances was constructed for the 86 olive genotypes/cultivars, which revealed the existence of different clusters. The high genetic similarity was evident between Bakırkire2 and Zinnar5 (0.74) genotypes, while the most genetically divergent genotypes were Gürmeşe5 and Yedikardeşler2 (0.19). It was concluded that there was abundant SSR polymorphism in olive germplasm in southern Anatolia in Turkey and could be important for future breeding activities.

Genetics and molecular mapping of resistance to Plasmodiophora brassicae pathotypes 2, 3, 5, 6, and 8 in rutabaga (Brassica napus var. napobrassica)

Clubroot disease, caused by Plasmodiophora brassicae, is a threat to the production of Brassica crops including oilseed B. napus. In Canada, several pathotypes of this pathogen, such as pathotypes 2, 3, 5, 6, and 8, were identified, and resistance to these pathotypes was found in a rutabaga (B. napus var. napobrassica) genotype. In this paper, we report the genetic basis and molecular mapping of this resistance by use of F₂, backcross (BC₁), and doubled haploid (DH) populations generated from crossing of this rutabaga line to a susceptible spring B. napus canola line. The F₁, F₂, and BC₁ populations were evaluated for resistance to pathotype 3, and the DH population was evaluated for resistance to pathotypes 2, 3, 5, 6, and 8. A 3:1 segregation in F₂ and a 1:1 segregation in BC₁ were found for resistance to pathotype 3, and a 1:1 segregation was found in the DH population for resistance to all pathotypes. Molecular mapping by using the DH population identified a genomic region on chromosome A8 carrying resistance to all five pathotypes. This suggests that a single gene or a cluster of genes, located in this genomic region, is involved in the control of resistance to these pathotypes.

High-throughput development of simple sequence repeat markers for genetic diversity research in Crambe abyssinica

BACKGROUND

The allohexaploid Crambe abyssinica (crambe) is an oilseed crop that has been recognized for its potential value in the chemical industry, particularly in terms of producing high-erucic acid content vegetable oil. However, as an understudied crop, improvement of crambe has been hampered by the lack of genetic and genomic information to enhance its yield, oil quality and resistance against biotic and abiotic stress. Development of molecular markers is therefore of great significance to facilitate genetic improvement of crambe.

RESULTS

In this study, high-throughput sequencing was performed to generate sequences for the transcriptome and genome of a widely planted crambe cultivar, Galactica. A total of 186,778 expressed sequence tag (EST) contigs as 8,130,350 genomic contigs were assembled as well. Altogether, 82,523 pairs of primers were designed in the flanking sequences of the simple sequence repeat (SSR) within these contigs. Virtual PCR analysis showed that a fraction of these primers could be mapped onto the genomes of related species of Brassica, including Brassica rapa, B. oleraceae and B. napus. Genetic diversity analysis using a subset of 166 markers on 30 independent C. abyssinica accessions exhibited that 1) 95 % of the designed SSRs were polymorphic among these accessions; 2) the polymorphism information content (PIC) value of the markers ranged from 0.13 to 0.89; 3) the genetic distances (coefficient NEI72) between accessions varied from 0.06 to 0.36. Cluster analysis subsequent on the accessions demonstrated consistency with crambe breeding history. F-statistics analysis revealed a moderate level of genetic differentiation in C. abyssinica (Gst = 0.3934) and a accordingly low estimated gene flow (Nm = 0.7709).

CONCLUSION

Application of high-throughput sequencing technology has facilitated SSR marker development, which was successfully employed in evaluating genetic diversity of C. abyssinica as demonstrated in our study. Results showed these molecular markers were robust and provided powerful tools for assessing genetic diversity and estimating crambe breeding history. Moreover, the SSR primers and sequence information developed in the study are freely available to the research community.

Characterization and Transferable Utility of Microsatellite Markers in the Wild and Cultivated Arachis Species

Microsatellite or simple sequence repeat (SSR) is one of the most widely distributed molecular markers that have been widely utilized to assess genetic diversity and genetic mapping for important traits in plants. However, the understanding of microsatellite characteristics in Arachis species and the currently available amount of high-quality SSR markers remain limited. In this study, we identified 16,435 genome survey sequences SSRs (GSS-SSRs) and 40,199 expressed sequence tag SSRs (EST-SSRs) in Arachis hypogaea and its wild relative species using the publicly available sequence data. The GSS-SSRs had a density of 159.9–239.8 SSRs/Mb for wild Arachis and 1,015.8 SSR/Mb for cultivated Arachis, whereas the EST-SSRs had the density of 173.5–384.4 SSR/Mb and 250.9 SSRs/Mb for wild and cultivated Arachis, respectively. The trinucleotide SSRs were predominant across Arachis species, except that the dinucleotide accounted for most in A. hypogaea GSSs. From Arachis GSS-SSR and EST-SSR sequences, we developed 2,589 novel SSR markers that showed a high polymorphism in six diverse A. hypogaea accessions. A genetic linkage map that contained 540 novel SSR loci and 105 anchor SSR loci was constructed by case of a recombinant inbred lines F₆ population. A subset of 82 randomly selected SSR markers were used to screen 39 wild and 22 cultivated Arachis accessions, which revealed a high transferability of the novel SSRs across Arachis species. Our results provided informative clues to investigate microsatellite patterns across A. hypogaea and its wild relative species and potentially facilitate the germplasm evaluation and gene mapping in Arachis species.

Molecular Mapping and QTL for Expression Profiles of Flavonoid Genes in Brassica napus

Flavonoids are secondary metabolites that are extensively distributed in the plant kingdom and contribute to seed coat color formation in rapeseed. To decipher the genetic networks underlying flavonoid biosynthesis in rapeseed, we constructed a high-density genetic linkage map with 1089 polymorphic loci (including 464 SSR loci, 97 RAPD loci, 451 SRAP loci, and 75 IBP loci) using recombinant inbred lines (RILs). The map consists of 19 linkage groups and covers 2775 cM of the B. napus genome with an average distance of 2.54 cM between adjacent markers. We then performed expression quantitative trait locus (eQTL) analysis to detect transcript-level variation of 18 flavonoid biosynthesis pathway genes in the seeds of the 94 RILs. In total, 72 eQTLs were detected and found to be distributed among 15 different linkage groups that account for 4.11% to 52.70% of the phenotypic variance atrributed to each eQTL. Using a genetical genomics approach, four eQTL hotspots together harboring 28 eQTLs associated with 18 genes were found on chromosomes A03, A09, and C08 and had high levels of synteny with genome sequences of A. thaliana and Brassica species. Associated with the trans-eQTL hotspots on chromosomes A03, A09, and C08 were 5, 17, and 1 genes encoding transcription factors, suggesting that these genes have essential roles in the flavonoid biosynthesis pathway. Importantly, bZIP25, which is expressed specifically in seeds, MYC1, which controls flavonoid biosynthesis, and the R2R3-type gene MYB51, which is involved in the synthesis of secondary metabolites, were associated with the eQTL hotspots, and these genes might thus be involved in different flavonoid biosynthesis pathways in rapeseed. Hence, further studies of the functions of these genes will provide insight into the regulatory mechanism underlying flavonoid biosynthesis, and lay the foundation for elaborating the molecular mechanism of seed coat color formation in B. napus.

Genotyping-by-sequencing map permits identification of clubroot resistance QTLs and revision of the reference genome assembly in cabbage (Brassica oleracea L.)

Clubroot is a devastating disease caused by Plasmodiophora brassicae and results in severe losses of yield and quality in Brassica crops. Many clubroot resistance genes and markers are available in Brassica rapa but less is known in Brassica oleracea. Here, we applied the genotyping-by-sequencing (GBS) technique to construct a high-resolution genetic map and identify clubroot resistance (CR) genes. A total of 43,821 SNPs were identified using GBS data for two parental lines, one resistant and one susceptible lines to clubroot, and 18,187 of them showed >5× coverage in the GBS data. Among those, 4,103 were credibly genotyped for all 78 F₂ individual plants. These markers were clustered into nine linkage groups spanning 879.9 cM with an average interval of 1.15 cM. Quantitative trait loci (QTLs) survey based on three rounds of clubroot resistance tests using F_2:3 progenies revealed two and single major QTLs for Race 2 and Race 9 of P. brassicae, respectively. The QTLs show similar locations to the previously reported CR loci for Race 4 in B. oleracea but are in different positions from any of the CR loci found in B. rapa. We utilized two reference genome sequences in this study. The high-resolution genetic map developed herein allowed us to reposition 37 and 2 misanchored scaffolds in the 02–12 and TO1000DH genome sequences, respectively. Our data also support additional positioning of two unanchored 3.3 Mb scaffolds into the 02–12 genome sequence.

Development and Characterization of 1,906 EST-SSR Markers from Unigenes in Jute (Corchorus spp.)

Jute, comprising white and dark jute, is the second important natural fiber crop after cotton worldwide. However, the lack of expressed sequence tag-derived simple sequence repeat (EST-SSR) markers has resulted in a large gap in the improvement of jute. Previously, de novo 48,914 unigenes from white jute were assembled. In this study, 1,906 EST-SSRs were identified from these assembled uingenes. Among these markers, di-, tri- and tetra-nucleotide repeat types were the abundant types (12.0%, 56.9% and 21.6% respectively). The AG-rich or GA-rich nucleotide repeats were the predominant. Subsequently, a sample of 116 SSRs, located in genes encoding transcription factors and cellulose synthases, were selected to survey polymorphisms among12 diverse jute accessions. Of these, 83.6% successfully amplified at least one fragment and detected polymorphism among the 12diverse genotypes, indicating that the newly developed SSRs are of good quality. Furthermore, the genetic similarity coefficients of all the 12 accessions were evaluated using 97 polymorphic SSRs. The cluster analysis divided the jute accessions into two main groups with genetic similarity coefficient of 0.61. These EST-SSR markers not only enrich molecular markers of jute genome, but also facilitate genetic and genomic researches in jute.

Linkage and regional association analysis reveal two new tightly-linked major-QTLs for pod number and seed number per pod in rapeseed (Brassica napus L.)

To facilitate the pseudochromosomes assembly and gene cloning in rapeseed, we developed a reference genetic population/map (named BnaZNF2) from two sequenced cultivars, Zhongshuang11 and No.73290, those exhibit significant differences in many traits, particularly yield components. The BnaZNF2 genetic map exhibited perfect collinearity with the physical map of B. napus, indicating its high quality. Comparative mapping revealed several genomic rearrangements between B. napus and B. rapa or B. oleracea. A total of eight and 16 QTLs were identified for pod number and seed number per pod, respectively, and of which three and five QTLs are identical to previously identified ones, whereas the other five and 11 are novel. Two new major QTL respectively for pod number and seed number per pod, qPN.A06-1 and qSN.A06-1 (R(2 )= 22.8% and 32.1%), were colocalised with opposite effects, and only qPN.A06-1 was confirmed and narrowed by regional association analysis to 180 kb including only 33 annotated genes. Conditional QTL analysis and subsequent NILs test indicated that tight linkage, rather than pleiotropy, was the genetic causation of their colocalisation. Our study demonstrates potential of this reference genetic population/map for precise QTL mapping and as a base for positional gene cloning in rapeseed.

Disruption of a CAROTENOID CLEAVAGE DIOXYGENASE 4 gene converts flower colour from white to yellow in Brassica species

In Brassica napus, yellow petals had a much higher content of carotenoids than white petals present in a small number of lines, with violaxanthin identified as the major carotenoid compound in yellow petals of rapeseed lines. Using positional cloning we identified a carotenoid cleavage dioxygenase 4 gene, BnaC3.CCD4, responsible for the formation of flower colour, with preferential expression in petals of white-flowered B. napus lines. Insertion of a CACTA-like transposable element 1 (TE1) into the coding region of BnaC3.CCD4 had disrupted its expression in yellow-flowered rapeseed lines. α-Ionone was identified as the major volatile apocarotenoid released from white petals but not from yellow petals. We speculate that BnaC3.CCD4 may use δ- and/or α-carotene as substrates. Four variations, including two CACTA-like TEs (alleles M1 and M4) and two insertion/deletions (INDELs, alleles M2 and M3), were identified in yellow-flowered Brassica oleracea lines. The two CACTA-like TEs were also identified in the coding region of BcaC3.CCD4 in Brassica carinata. However, the two INDELs were not detected in B. napus and B. carinata. We demonstrate that the insertions of TEs in BolC3.CCD4 predated the formation of the two allotetraploids.

A bi-filtering method for processing single nucleotide polymorphism array data improves the quality of genetic map and accuracy of quantitative trait locus mapping in doubled haploid populations of polyploid Brassica napus

Background

Single nucleotide polymorphism (SNP) markers have a wide range of applications in crop genetics and genomics. Due to their polyploidy nature, many important crops, such as wheat, cotton and rapeseed contain a large amount of repeat and homoeologous sequences in their genomes, which imposes a huge challenge in high-throughput genotyping with sequencing and/or array technologies. Allotetraploid Brassica napus (AACC, 2n = 4x = 38) comprises of two highly homoeologous sub-genomes derived from its progenitor species B. rapa (AA, 2n = 2x = 20) and B. oleracea (CC, 2n = 2x = 18), and is an ideal species to exploit methods for reducing the interference of extensive inter-homoeologue polymorphisms (mHemi-SNPs and Pseudo-simple SNPs) between closely related sub-genomes.

Results

Based on a recent B. napus 6K SNP array, we developed a bi-filtering procedure to identify unauthentic lines in a DH population, and mHemi-SNPs and Pseudo-simple SNPs in an array data matrix. The procedure utilized both monomorphic and polymorphic SNPs in the DH population and could effectively distinguish the mHemi-SNPs and Pseudo-simple SNPs that resulted from superposition of the signals from multiple SNPs. Compared with conventional procedure for array data processing, the bi-filtering method could minimize the pseudo linkage relationship caused by the mHemi-SNPs and Pseudo-simple SNPs, thus improving the quality of SNP genetic map. Furthermore, the improved genetic map could increase the accuracies of mapping of QTLs as demonstrated by the ability to eliminate non-real QTLs in the mapping population.

Conclusions

The bi-filtering analysis of the SNP array data represents a novel approach to effectively assigning the multi-loci SNP genotypes in polyploid B. napus and may find wide applications to SNP analyses in polyploid crops.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1559-4) contains supplementary material, which is available to authorized users.

Genome-Wide Characterization of Simple Sequence Repeat (SSR) Loci in Chinese Jujube and Jujube SSR Primer Transferability

Chinese jujube (Ziziphus jujuba), an economically important species in the Rhamnaceae family, is a popular fruit tree in Asia. Here, we surveyed and characterized simple sequence repeats (SSRs) in the jujube genome. A total of 436,676 SSR loci were identified, with an average distance of 0.93 Kb between the loci. A large proportion of the SSRs included mononucleotide, dinucleotide and trinucleotide repeat motifs, which accounted for 64.87%, 24.40%, and 8.74% of all repeats, respectively. Among the mononucleotide repeats, A/T was the most common, whereas AT/TA was the most common dinucleotide repeat. A total of 30,565 primer pairs were successfully designed and screened using a series of criteria. Moreover, 725 of 1,000 randomly selected primer pairs were effective among 6 cultivars, and 511 of these primer pairs were polymorphic. Sequencing the amplicons of two SSRs across three jujube cultivars revealed variations in the repeats. The transferability of jujube SSR primers proved that 35/64 SSRs could be transferred across family boundary. Using jujube SSR primers, clustering analysis results from 15 species were highly consistent with the Angiosperm Phylogeny Group (APGIII) System. The genome-wide characterization of SSRs in Chinese jujube is very valuable for whole-genome characterization and marker-assisted selection in jujube breeding. In addition, the transferability of jujube SSR primers could provide a solid foundation for their further utilization.

The Natural Variation of Seed Weight Is Mainly Controlled by Maternal Genotype in Rapeseed (Brassica napus L.)

Seed weight is a very important and complex trait in rapeseed (Brassica napus L.). The seed weight of rapeseed shows great variation in its natural germplasm resources; however, the morphological, cytological and genetic causes of this variation have remained unclear. In the present study, nine highly pure inbred rapeseed lines with large seed weight variation and different genetic backgrounds were selected for morphological, cytological and genetic studies on seed weight. The results showed the following: (1) Seed weight showed an extremely significant correlation and coordinated variation with seed size (including seed diameter, seed surface area and seed volume), but it showed no significant correlation with bulk density, which suggests that seed weight is determined by size rather than bulk density. (2) Seed weight showed a higher correlation with the cell numbers of seed coats and cotyledons than the cell sizes of seed coats and cotyledons, which suggests that cell number is more tightly correlated with final seed weight. (3) Seed weight was mainly controlled by the maternal genotype, with little or no xenia and cytoplasmic effects. This is the first report on the morphological and cytological causes of seed weight natural variation in rapeseed. We concluded that the natural variation of seed weight is mainly controlled by maternal genotype. This finding lays a foundation for genetic and breeding studies of seed weight in rapeseed and opens a new field of research on the regulation of seed traits in plants.

High-Density SNP Map Construction and QTL Identification for the Apetalous Character in Brassica napus L

The apetalous genotype is a morphological ideotype for increasing seed yield and should be of considerable agricultural use; however, only a few studies have focused on the genetic control of this trait in Brassica napus. In the present study, a recombinant inbred line, the AH population, containing 189 individuals was derived from a cross between an apetalous line 'APL01' and a normally petalled variety 'Holly'. The Brassica 60 K Infinium BeadChip Array harboring 52,157 single nucleotide polymorphism (SNP) markers was used to genotype the AH individuals. A high-density genetic linkage map was constructed based on 2,755 bins involving 11,458 SNPs and 57 simple sequence repeats, and was used to identify loci associated with petalous degree (PDgr). The linkage map covered 2,027.53 cM, with an average marker interval of 0.72 cM. The AH map had good collinearity with the B. napus reference genome, indicating its high quality and accuracy. After phenotypic analyses across five different experiments, a total of 19 identified quantitative trait loci (QTLs) distributed across chromosomes A3, A5, A6, A9 and C8 were obtained, and these QTLs were further integrated into nine consensus QTLs by a meta-analysis. Interestingly, the major QTL qPD.C8-2 was consistently detected in all five experiments, and qPD.A9-2 and qPD.C8-3 were stably expressed in four experiments. Comparative mapping between the AH map and the B. napus reference genome suggested that there were 328 genes underlying the confidence intervals of the three steady QTLs. Based on the Gene Ontology assignments of 52 genes to the regulation of floral development in published studies, 146 genes were considered as potential candidate genes for PDgr. The current study carried out a QTL analysis for PDgr using a high-density SNP map in B. napus, providing novel targets for improving seed yield. These results advanced our understanding of the genetic control of PDgr regulation in B. napus.

Assessing and broadening genetic diversity of a rapeseed germplasm collection

Assessing the level of genetic diversity within a germplasm collection contributes to evaluating the potential for its utilization as a gene pool to improve the performance of cultivars. In this study, 45 high-quality simple sequence repeat (SSR) markers were screened and used to estimate the genetic base of a world-wide collection of 248 rapeseed (Brassica napus) inbred lines. For the whole collection, the genetic diversity of A genome was higher than that of C genome. The genetic diversity of C genome for the semi-winter type was the lowest among the different germplasm types. Because B. oleracea is usually used to broaden the genetic diversity of C genome in rapeseed, we evaluated the potential of 25 wild B. oleracea lines. More allelic variations and a higher genetic diversity were observed in B. oleracea than in rapeseed. One B. oleracea line and one oilseed B. rapa line were used to generate a resynthesized Brassica napus line, which was then crossed with six semi-winter rapeseed cultivars to produce 7 F1 hybrids. Not only the allele introgression but also mutations were observed in the hybrids, resulting in significant improvement of the genetic base.

Analysis of microsatellites in the vulnerable orchid Gastrodia flavilabella: the development of microsatellite markers, and cross-species amplification in Gastrodia

BACKGROUND

Gastrodia flabilabella is a mycoheterotrophic orchid that obtains carbohydrates and nutrients from its symbiotic mycorrhizal fungi. The species is an endemic and vulnerable species enlisted in the "A Preliminary Red List of Taiwanese Vascular Plants" according to the IUCN Red List Categories and Criteria Version 3.1. G. flabilabella dwells the underground of broadleaf and coniferous forest with richness litter. Based on herbarium records, this species is distributed in central Taiwan. Twenty eight microsatellite loci were developed in G. flabilabella and were tested for cross-species amplification in additional taxa of G. confusoides, G. elata, and G. javanica. We estimated the genetic variation that is valuable for conservation management and the development of the molecular identification system for G. elata, a traditional Chinese medicine herb.

RESULTS

Microsatellite primer sets were developed from G. flabilabella using the modified AFLP and magnetic bead enrichment method. In total, 257 microsatellite loci were obtained from a magnetic bead enrichment SSR library. Of the 28 microsatellite loci, 16 were polymorphic, in which the number of alleles ranged from 2 to 15, with the observed heterozygosity ranging from 0.02 to 1.00. In total, 15, 13, and 7 of the loci were found to be interspecifically amplifiable to G. confusoides, G. elata, and G. javanica, respectively.

CONCLUSIONS

Amplifiable and transferable microsatellite loci are potentially useful for future studies in investigating intraspecific genetic variation, reconstructing phylogeographic patterns among closely related species, and establishing the standard operating system of molecular identification in Gastrodia.

A high-density genetic map identifies a novel major QTL for boron efficiency in oilseed rape (Brassica napus L.)

Low boron (B) seriously limits the growth of oilseed rape (Brassica napus L.), a high B demand species that is sensitive to low B conditions. Significant genotypic variations in response to B deficiency have been observed among B. napus cultivars. To reveal the genetic basis for B efficiency in B. napus, quantitative trait loci (QTLs) for the plant growth traits, B uptake traits and the B efficiency coefficient (BEC) were analyzed using a doubled haploid (DH) population derived from a cross between a B-efficient parent, Qingyou 10, and a B-inefficient parent, Westar 10. A high-density genetic map was constructed based on single nucleotide polymorphisms (SNPs) assayed using Brassica 60 K Infinium BeadChip Array, simple sequence repeats (SSRs) and amplified fragment length polymorphisms (AFLPs). The linkage map covered a total length of 2139.5 cM, with 19 linkage groups (LGs) and an average distance of 1.6 cM between adjacent markers. Based on hydroponic evaluation of six B efficiency traits measured in three separate repeated trials, a total of 52 QTLs were identified, accounting for 6.14-46.27% of the phenotypic variation. A major QTL for BEC, qBEC-A3a, was co-located on A3 with other QTLs for plant growth and B uptake traits under low B stress. Using a subset of substitution lines, qBEC-A3a was validated and narrowed down to the interval between CNU384 and BnGMS436. The results of this study provide a novel major locus located on A3 for B efficiency in B. napus that will be suitable for fine mapping and marker-assisted selection breeding for B efficiency in B. napus.

Genetic diversity among some canola cultivars as revealed by RAPD, SSR and AFLP analyses

To assess the genetic diversity among four canola cultivars (namely, Serw-3, Serw-4, Misser L-16 and Semu 249), random amplified polymorphic DNA (RAPD), simple sequence repeat polymorphism (SSR) and amplified fragment length polymorphism (AFLP) analyses were performed. The data indicated that all of the three molecular markers gave different levels of polymorphism. A total of 118, 31 and 338 markers that show 61, 67.7 and 81 % polymorphism percentages were resulted from the RAPD, SSR and AFLP analyses, respectively. Based on the data obtained the three markers can be used to differentiate between the four canola cultivars. The genotype-specific markers were determined, 18 out of the 72 polymorphic RAPD markers generated were found to be genotype-specific (25 %). The highest number of RAPD specific markers was scored for Semu 249 (15 markers), while Serw-4 scored two markers. On the other hand, Serw-3 scored one marker. The cultivar Semu 249 scored the highest number of unique AFLP markers, giving 57 unique markers, followed by Misser L-16 which was characterized by 40 unique AFLP markers, then Serw-3 giving 31 unique markers. While Serw-4 was characterized by the lowest number producing 14 unique positive markers. The dendrogram built on the basis of combined data from RAPD, SSR and AFLP analysis represents the genetic distances among the four canola cultivars. Understanding the genetic variability among the current canola cultivars opens up a possibility for developing a molecular genetic map that will lead to the application of marker-assisted selection tools in genetic improvement of canola.

A combined linkage and regional association mapping validation and fine mapping of two major pleiotropic QTLs for seed weight and silique length in rapeseed (Brassica napus L.)

BACKGROUND

Seed weight (SW) and silique length (SL) are important determinants of the yield potential in rapeseed (Brassica napus L.). However, the genetic basis of both traits is poorly understood. The main objectives of this study were to dissect the genetic basis of SW and SL in rapeseed through the preliminary mapping of quantitative trait locus (QTL) by linkage analysis and fine mapping of the target major QTL by regional association analysis.

RESULTS

Preliminary linkage mapping identified thirteen and nine consensus QTLs for SW and SL, respectively. These QTLs explained 0.7-67.1% and 2.1-54.4% of the phenotypic variance for SW and SL, respectively. Of these QTLs, three pairs of SW and SL QTLs were co-localized and integrated into three unique QTLs. In addition, the significance level and genetic effect of the three co-localized QTLs for both SW and SL showed great variation before and after the conditional analysis. Moreover, the allelic effects of the three QTLs for SW were highly consistent with those for SL. Two of the three co-localized QTLs, uq.A09-1 (mean R(2) = 20.1% and 19.0% for SW and SL, respectively) and uq.A09-3 (mean R(2) = 13.5% and 13.2% for SW and SL, respectively), were detected in all four environments and showed the opposite additive-effect direction. These QTLs were validated and fine mapped (their confidence intervals were narrowed down from 5.3 cM to 1 cM for uq.A09-1 and 13.2 cM to 2.5 cM for uq.A09-3) by regional association analysis with a panel of 576 inbred lines, which has a relatively rapid linkage disequilibrium decay (0.3 Mb) in the target QTL region.

CONCLUSIONS

A few QTLs with major effects and several QTLs with moderate effects might contribute to the natural variation of SW and SL in rapeseed. The meta-, conditional and allelic effect analyses suggested that pleiotropy, rather than tight linkage, was the genetic basis of the three pairs of co-localized of SW and SL QTLs. Regional association analysis was an effective and highly efficient strategy for the direct fine mapping of target major QTL identified by preliminary linkage mapping.

Development of simple sequence repeat (SSR) markers of sesame (Sesamum indicum) from a genome survey

Sesame (Sesamum indicum), an important oil crop, is widely grown in tropical and subtropical regions. It provides part of the daily edible oil allowance for almost half of the world's population. A limited number of co-dominant markers has been developed and applied in sesame genetic diversity and germplasm identity studies. Here we report for the first time a whole genome survey used to develop simple sequence repeat (SSR) markers and to detect the genetic diversity of sesame germplasm. From the initial assembled sesame genome, 23,438 SSRs (≥5 repeats) were identified. The most common repeat motif was dinucleotide with a frequency of 84.24%, followed by 13.53% trinucleotide, 1.65% tetranucleotide, 0.3% pentanucleotide and 0.28% hexanucleotide motifs. From 1500 designed and synthesised primer pairs, 218 polymorphic SSRs were developed and used to screen 31 sesame accessions that from 12 countries. STRUCTURE and phylogenetic analyses indicated that all sesame accessions could be divided into two groups: one mainly from China and another from other countries. Cluster analysis classified Chinese major sesame varieties into three groups. These novel SSR markers are a useful tool for genetic linkage map construction, genetic diversity detection, and marker-assisted selective sesame breeding.

Transcriptome sequencing of two parental lines of cabbage (Brassica oleracea L. var. capitata L.) and construction of an EST-based genetic map

Background

Expressed sequence tag (EST)-based markers are preferred because they reflect transcribed portions of the genome. We report the development of simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers derived from transcriptome sequences in cabbage, and their utility for map construction.

Results

Transcriptome sequences were obtained from two cabbage parental lines, C1184 and C1234, which are susceptible and resistant to black rot disease, respectively, using the 454 platform. A total of 92,255 and 127,522 reads were generated and clustered into 34,688 and 40,947 unigenes, respectively. We identified 2,405 SSR motifs from the unigenes of the black rot-resistant parent C1234. Trinucleotide motifs were the most abundant (66.15%) among the repeat motifs. In addition, 1,167 SNPs were detected between the two parental lines. A total of 937 EST-based SSR and 97 SNP-based dCAPS markers were designed and used for detection of polymorphism between parents. Using an F₂ population, we built a genetic map comprising 265 loci, and consisting of 98 EST-based SSRs, 21 SNP-based dCAPS, 55 IBP markers derived from B. rapa genome sequence and 91 public SSRs, distributed on nine linkage groups spanning a total of 1,331.88 cM with an average distance of 5.03 cM between adjacent loci. The parental lines used in this study are elite breeding lines with little genetic diversity; therefore, the markers that mapped in our genetic map will have broad spectrum utility.

Conclusions

This genetic map provides additional genetic information to the existing B. oleracea map. Moreover, the new set of EST-based SSR and dCAPS markers developed herein is a valuable resource for genetic studies and will facilitate cabbage breeding. Additionally, this study demonstrates the usefulness of NGS transcriptomes for the development of genetic maps even with little genetic diversity in the mapping population.

Genome survey sequencing provides clues into glucosinolate biosynthesis and flowering pathway evolution in allotetrapolyploid Brassica juncea

BACKGROUND

Brassica juncea is an economically important vegetable crop in China, oil crop in India, condiment crop in Europe and selected for canola quality recently in Canada and Australia. B. juncea (2n = 36, AABB) is an allotetraploid derived from interspecific hybridization between B. rapa (2n = 20, AA) and B. nigra (2n = 16, BB), followed by spontaneous chromosome doubling.

RESULTS

Comparative genome analysis by genome survey sequence (GSS) of allopolyploid B. juncea with B. rapa was carried out based on high-throughput sequencing approaches. Over 28.35 Gb of GSS data were used for comparative analysis of B. juncea and B. rapa, producing 45.93% reads mapping to the B. rapa genome with a high ratio of single-end reads. Mapping data suggested more structure variation (SV) in the B. juncea genome than in B. rapa. We detected 2,921,310 single nucleotide polymorphisms (SNPs) with high heterozygosity and 113,368 SVs, including 1-3 bp Indels, between B. juncea and B. rapa. Non-synonymous polymorphisms in glucosinolate biosynthesis genes may account for differences in glucosinolate biosynthesis and glucosinolate components between B. juncea and B. rapa. Furthermore, we identified distinctive vernalization-dependent and photoperiod-dependent flowering pathways coexisting in allopolyploid B. juncea, suggesting contribution of these pathways to adaptation for survival during polyploidization.

CONCLUSIONS

Taken together, we proposed that polyploidization has allowed for accelerated evolution of the glucosinolate biosynthesis and flowering pathways in B. juncea that likely permit the phenotypic variation observed in the crop.

Association mapping of six yield‑related traits in rapeseed (Brassica napus L.)

Yield is one of the most important traits for rapeseed (Brassica napus L.) breeding, but its genetic basis remains largely ambiguous. Association mapping has provided a robust approach to understand the genetic basis of complex agronomic traits in crops. In this study, a panel of 192 inbred lines of B. napus from all over the world was genotyped using 451 single-locus microsatellite markers and 740 amplified fragment length polymorphism markers. Six yield-related traits of these inbred lines were investigated in three consecutive years with three replications, and genome-wide association studies were conducted for these six traits. Using the model controlling both population structure and relative kinship (Q + K), a total of 43 associations (P < 0.001) were detected using the means of the six yield-related traits across 3 years, with two to fourteen markers associated with individual traits. Among these, 18 markers were repeatedly detected in at least 2 years, and 12 markers were located within or close to QTLs identified in previous studies. Six markers commonly associated with correlated traits. Conditional association analysis indicated that five of the associations between markers and correlated traits are caused by one QTL with pleiotropic effects, and the remaining association is caused by linked but independent QTLs. The combination of favorable alleles of multiple associated markers significantly enhances trait performance, illustrating a great potential of utilization of the associations in rapeseed breeding programs.

Identification of QTLs associated with oil content in a high-oil Brassica napus cultivar and construction of a high-density consensus map for QTLs comparison in B. napus

Increasing seed oil content is one of the most important goals in breeding of rapeseed (B. napus L.). To dissect the genetic basis of oil content in B. napus, a large and new double haploid (DH) population containing 348 lines was obtained from a cross between 'KenC-8' and 'N53-2', two varieties with >10% difference in seed oil content, and this population was named the KN DH population. A genetic linkage map consisting of 403 markers was constructed, which covered a total length of 1783.9 cM with an average marker interval of 4.4 cM. The KN DH population was phenotyped in eight natural environments and subjected to quantitative trait loci (QTL) analysis for oil content. A total of 63 identified QTLs explaining 2.64-17.88% of the phenotypic variation were identified, and these QTLs were further integrated into 24 consensus QTLs located on 11 chromosomes using meta-analysis. A high-density consensus map with 1335 marker loci was constructed by combining the KN DH map with seven other published maps based on the common markers. Of the 24 consensus QTLs in the KN DH population, 14 were new QTLs including five new QTLs in A genome and nine in C genome. The analysis revealed that a larger population with significant differences in oil content gave a higher power detecting new QTLs for oil content, and the construction of the consensus map provided a new clue for comparing the QTLs detected in different populations. These findings enriched our knowledge of QTLs for oil content and should be a potential in marker-assisted breeding of B. napus.

Genome-wide microsatellite characterization and marker development in the sequenced Brassica crop species

Although much research has been conducted, the pattern of microsatellite distribution has remained ambiguous, and the development/utilization of microsatellite markers has still been limited/inefficient in Brassica, due to the lack of genome sequences. In view of this, we conducted genome-wide microsatellite characterization and marker development in three recently sequenced Brassica crops: Brassica rapa, Brassica oleracea and Brassica napus. The analysed microsatellite characteristics of these Brassica species were highly similar or almost identical, which suggests that the pattern of microsatellite distribution is likely conservative in Brassica. The genomic distribution of microsatellites was highly non-uniform and positively or negatively correlated with genes or transposable elements, respectively. Of the total of 115 869, 185 662 and 356 522 simple sequence repeat (SSR) markers developed with high frequencies (408.2, 343.8 and 356.2 per Mb or one every 2.45, 2.91 and 2.81 kb, respectively), most represented new SSR markers, the majority had determined physical positions, and a large number were genic or putative single-locus SSR markers. We also constructed a comprehensive database for the newly developed SSR markers, which was integrated with public Brassica SSR markers and annotated genome components. The genome-wide SSR markers developed in this study provide a useful tool to extend the annotated genome resources of sequenced Brassica species to genetic study/breeding in different Brassica species.

BAC-end sequences analysis provides first insights into coffee (Coffea canephora P.) genome composition and evolution

Coffee is one of the world's most important agricultural commodities. Coffee belongs to the Rubiaceae family in the euasterid I clade of dicotyledonous plants, to which the Solanaceae family also belongs. Two bacterial artificial chromosome (BAC) libraries of a homozygous doubled haploid plant of Coffea canephora were constructed using two enzymes, HindIII and BstYI. A total of 134,827 high quality BAC-end sequences (BESs) were generated from the 73,728 clones of the two libraries, and 131,412 BESs were conserved for further analysis after elimination of chloroplast and mitochondrial sequences. This corresponded to almost 13 % of the estimated size of the C. canephora genome. 6.7 % of BESs contained simple sequence repeats, the most abundant (47.8 %) being mononucleotide motifs. These sequences allow the development of numerous useful marker sites. Potential transposable elements (TEs) represented 11.9 % of the full length BESs. A difference was observed between the BstYI and HindIII libraries (14.9 vs. 8.8 %). Analysis of BESs against known coding sequences of TEs indicated that 11.9 % of the genome corresponded to known repeat sequences, like for other flowering plants. The number of genes in the coffee genome was estimated at 41,973 which is probably overestimated. Comparative genome mapping revealed that microsynteny was higher between coffee and grapevine than between coffee and tomato or Arabidopsis. BESs constitute valuable resources for the first genome wide survey of coffee and provide new insights into the composition and evolution of the coffee genome.

Identification of QTLs for resistance to sclerotinia stem rot and BnaC.IGMT5.a as a candidate gene of the major resistant QTL SRC6 in Brassica napus

Stem rot caused by Sclerotinia sclerotiorum in many important dicotyledonous crops, including oilseed rape (Brassica napus), is one of the most devastating fungal diseases and imposes huge yield loss each year worldwide. Currently, breeding for Sclerotinia resistance in B. napus, as in other crops, can only rely on germplasms with quantitative resistance genes. Thus, the identification of quantitative trait locus (QTL) for S. sclerotiorum resistance/tolerance in this crop holds immediate promise for the genetic improvement of the disease resistance. In this study, ten QTLs for stem resistance (SR) at the mature plant stage and three QTLs for leaf resistance (LR) at the seedling stage in multiple environments were mapped on nine linkage groups (LGs) of a whole genome map for B. napus constructed with SSR markers. Two major QTLs, LRA9 on LG A9 and SRC6 on LG C6, were repeatedly detected across all environments and explained 8.54-15.86% and 29.01%-32.61% of the phenotypic variations, respectively. Genotypes containing resistant SRC6 or LRA9 allele showed a significant reduction in disease lesion after pathogen infection. Comparative mapping with Arabidopsis and data mining from previous gene profiling experiments identified that the Arabidopsis homologous gene of IGMT5 (At1g76790) was related to the SRC6 locus. Four copies of the IGMT5 gene in B. napus were isolated through homologous cloning, among which, only BnaC.IGMT5.a showed a polymorphism between parental lines and can be associated with the SRC6. Furthermore, two parental lines exhibited a differential expression pattern of the BnaC.IGMT5.a gene in responding to pathogen inoculation. Thus, our data suggested that BnaC.IGMT5.a was very likely a candidate gene of this major resistance QTL.

Detection and genotyping of restriction fragment associated polymorphisms in polyploid crops with a pseudo-reference sequence: a case study in allotetraploid Brassica napus

Background

The presence of homoeologous sequences and absence of a reference genome sequence make discovery and genotyping of single nucleotide polymorphisms (SNPs) more challenging in polyploid crops.

Results

To address this challenge, we constructed reduced representation libraries (RRLs) for two Brassica napus inbred lines and their 91 doubled haploid (DH) progenies using a modified ddRADseq technique. A bioinformatics pipeline termed RFAPtools was developed to discover and genotype SNPs and presence/absence variations (PAVs). Using this pipeline, a pseudo-reference sequence (PRF) containing 180,991 sequence tags was constructed. By aligning sequence reads to the pseudo-reference sequence, allelic SNPs as well as PAVs were identified and genotyped with RFAPtools. Two parallel linkage maps, one SNP bin map containing 8,780 SNP loci and one PAV linkage map containing 12,423 dominant loci, were constructed. By aligning marker sequences to B. rapa sequence scaffolds, whose genome is available, we assigned 44 unassembled sequence scaffolds comprising 8.15 Mb onto the B. rapa chromosomes, and also identified 14 instances of misassembly and eight instances of mis-ordering sequence scaffolds.

Conclusions

These results indicate that the modified ddRADseq approach is a cost-effective and simple method to genotype tens of thousands SNPs and PAV markers in a polyploidy plant species. The results also demonstrated that RFAPtools developed in this study are powerful to mine allelic SNPs from homoeologous sequences in polyploids, therefore they are generally applicable in either diploid or polyploid species with or without a reference genome sequence.

A consensus map of rapeseed (Brassica napus L.) based on diversity array technology markers: applications in genetic dissection of qualitative and quantitative traits

Background

Dense consensus genetic maps based on high-throughput genotyping platforms are valuable for making genetic gains in Brassica napus through quantitative trait locus identification, efficient predictive molecular breeding, and map-based gene cloning. This report describes the construction of the first B. napus consensus map consisting of a 1,359 anchored array based genotyping platform; Diversity Arrays Technology (DArT), and non-DArT markers from six populations originating from Australia, Canada, China and Europe. We aligned the B. napus DArT sequences with genomic scaffolds from Brassica rapa and Brassica oleracea, and identified DArT loci that showed linkage with qualitative and quantitative loci associated with agronomic traits.

Results

The integrated consensus map covered a total of 1,987.2 cM and represented all 19 chromosomes of the A and C genomes, with an average map density of one marker per 1.46 cM, corresponding to approximately 0.88 Mbp of the haploid genome. Through in silico physical mapping 2,457 out of 3,072 (80%) DArT clones were assigned to the genomic scaffolds of B. rapa (A genome) and B. oleracea (C genome). These were used to orientate the genetic consensus map with the chromosomal sequences. The DArT markers showed linkage with previously identified non-DArT markers associated with qualitative and quantitative trait loci for plant architecture, phenological components, seed and oil quality attributes, boron efficiency, sucrose transport, male sterility, and race-specific resistance to blackleg disease.

Conclusions

The DArT markers provide increased marker density across the B. napus genome. Most of the DArT markers represented on the current array were sequenced and aligned with the B. rapa and B. oleracea genomes, providing insight into the Brassica A and C genomes. This information can be utilised for comparative genomics and genomic evolution studies. In summary, this consensus map can be used to (i) integrate new generation markers such as SNP arrays and next generation sequencing data; (ii) anchor physical maps to facilitate assembly of B. napus genome sequences; and (iii) identify candidate genes underlying natural genetic variation for traits of interest.

Development of a core set of single-locus SSR markers for allotetraploid rapeseed (Brassica napus L.)

Brassica napus (AACC) is a recent allotetraploid species evolved through hybridization between two diploids, B. rapa (AA) and B. oleracea (CC). Due to extensive genome duplication and homoeology within and between the A and C genomes of B. napus, most SSR markers display multiple fragments or loci, which limit their application in genetics and breeding studies of this economically important crop. In this study, we collected 3,890 SSR markers from previous studies and also developed 5,968 SSR markers from genomic sequences of B. rapa, B. oleracea and B. napus. Of these, 2,701 markers that produced single amplicons were putative single-locus markers in the B. napus genome. Finally, a set of 230 high-quality single-locus SSR markers were established and assigned to the 19 linkage groups of B. napus using a segregating population with 154 DH individuals. A subset of 78 selected single-locus SSR markers was proved to be highly stable and could successfully discriminate each of the 45 inbred lines and hybrids. In addition, most of the 230 SSR markers showed the single-locus nature in at least one of the Brassica species of the U's triangle besides B. napus. These results indicated that this set of single-locus SSR markers has a wide range of coverage with excellent stability and would be useful for gene tagging, sequence scaffold assignment, comparative mapping, diversity analysis, variety identification and association mapping in Brassica species.

Integrated consensus genetic and physical maps of flax (Linum usitatissimum L.)

Three linkage maps of flax (Linum usitatissimum L.) were constructed from populations CDC Bethune/Macbeth, E1747/Viking and SP2047/UGG5-5 containing between 385 and 469 mapped markers each. The first consensus map of flax was constructed incorporating 770 markers based on 371 shared markers including 114 that were shared by all three populations and 257 shared between any two populations. The 15 linkage group map corresponds to the haploid number of chromosomes of this species. The marker order of the consensus map was largely collinear in all three individual maps but a few local inversions and marker rearrangements spanning short intervals were observed. Segregation distortion was present in all linkage groups which contained 1-52 markers displaying non-Mendelian segregation. The total length of the consensus genetic map is 1,551 cM with a mean marker density of 2.0 cM. A total of 670 markers were anchored to 204 of the 416 fingerprinted contigs of the physical map corresponding to ~274 Mb or 74 % of the estimated flax genome size of 370 Mb. This high resolution consensus map will be a resource for comparative genomics, genome organization, evolution studies and anchoring of the whole genome shotgun sequence.

Genetic linkage maps for Asian and American lotus constructed using novel SSR markers derived from the genome of sequenced cultivar

Background

The genus Nelumbo Adans. comprises two living species, N. nucifera Gaertan. (Asian lotus) and N. lutea Pers. (American lotus). A genetic linkage map is an essential resource for plant genetic studies and crop improvement but has not been generated for Nelumbo. We aimed to develop genomic simple sequence repeat (SSR) markers from the genome sequence and construct two genetic maps for Nelumbo to assist genome assembly and integration of a genetic map with the genome sequence.

Results

A total of 86,089 SSR motifs were identified from the genome sequences. Di- and tri-nucleotide repeat motifs were the most abundant, and accounted for 60.73% and 31.66% of all SSRs, respectively. AG/GA repeats constituted 51.17% of dinucleotide repeat motifs, followed by AT/TA (44.29%). Of 500 SSR primers tested, 386 (77.20%) produced scorable alleles with an average of 2.59 per primer, and 185 (37.00%) showed polymorphism among two parental genotypes, N. nucifera ‘Chinese Antique’ and N. lutea ‘AL1’, and six progenies of their F₁ population. The normally segregating markers, which comprised 268 newly developed SSRs, 37 previously published SSRs and 53 sequence-related amplified polymorphism markers, were used for genetic map construction. The map for Asian lotus was 365.67 cM with 47 markers distributed in seven linkage groups. The map for American lotus was 524.51 cM, and contained 177 markers distributed in 11 genetic linkage groups. The number of markers per linkage group ranged from three to 34 with an average genetic distance of 3.97 cM between adjacent markers. Moreover, 171 SSR markers contained in linkage groups were anchored to 97 genomic DNA sequence contigs of ‘Chinese Antique’. The 97 contigs were merged into 60 scaffolds.

Conclusion

Genetic mapping of SSR markers derived from sequenced contigs in Nelumbo enabled the associated contigs to be anchored in the linkage map and facilitated assembly of the genome sequences of ‘Chinese Antique’. The present study reports the first construction of genetic linkage maps for Nelumbo, which can serve as reference linkage maps to accelerate characterization germplasm, genetic mapping for traits of economic interest, and molecular breeding with marker-assisted selection.