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Fusarium oxysporum f. sp. sesami is an extremely destructive pathogen, causing sesame Fusarium wilt disease worldwide. To clarify the pathogenicity and the genetic characters of F. oxysporum f. sp. sesami, we systematically investigated 69 F. oxysporum isolates collected from major sesame-growing areas in China. Among these isolates, 54 isolates were pathogenic and 15 were nonpathogenic according to pathogenicity testing on sesame seedlings. For the pathogenic isolates, three F. oxysporum f. sp. sesami pathogenicity groups were defined based on the three differential sesame hosts for the first time. A translation elongation factor 1α gene tree was constructed to determine the genetic diversity of the F. oxysporum isolates but could not separate F. oxysporum f. sp. sesami isolates from the nonpathogenic isolates and other F. oxysporum formae speciales. Ten secreted-in-xylem (SIX) genes (one family of effectors) were identified in F. oxysporum f. sp. sesami isolates by a search with the genome data, and were subsequently screened in the 69 F. oxysporum isolates. Compared with the SIX gene profiles in other F. oxysporum formae speciales, the presence and sequence variations of the SIX gene homologs directly correlated with the specific pathogenicity of F. oxysporum f. sp. sesami toward sesame. Furthermore, eight of these F. oxysporum f. sp. sesami SIX genes were significantly expressed in sesame plants as infection of the F. oxysporum f. sp. sesami isolate. These findings have important significance for understanding the pathogenic basis of F. oxysporum f. sp. sesami isolates, and will contribute to improve the diagnostics to effectively control Fusarium wilt disease in sesame.

Seed coat color is an important seed quality trait in sesame. However, the genetic mechanism of seed coat color variation remains elusive in sesame. We conducted a QTL mapping of the seed coat color trait in sesame using an F2 mapping population. With the aid of the newly constructed superdense genetic linkage map comprised of 22,375 bins distributed in 13 linkage groups (LGs), 17 QTLs of the three indices (i.e., L, a, and b values) of seed coat color were detected in seven intervals on four LGs, with a phenotype variance explanation rate of 4.46–41.53%. A new QTL qSCa6.1 on LG 6 and a QTL hotspot containing at least four QTLs on LG 9 were further identified. Variants screening of the target intervals showed that there were 84 genes which possessed the variants that were high-impact and co-segregating with the seed coat color trait. Meanwhile, we performed the transcriptome comparison of the developing seeds of a white- and a black-seeded variety, and found that the differentially expressed genes were significantly enriched in 37 pathways, including three pigment biosynthesis related pathways. Integration of variants screening and transcriptome comparison results suggested that 28 candidate genes probably participated in the regulation of the seed coat color in sesame; of which, 10 genes had been proved or suggested to be involved in pigments biosynthesis or accumulation during seed formation. The findings gave the basis for the mechanism of seed coat color regulation in sesame, and exhibited the effects of the integrated approach of genome resequencing and transcriptome analysis on the genetics analysis of the complex traits.

There are about 20,000 sesame germplasm accessions in the world. South Korea, India, China, and some African countries are the main countries reserving sesame germplasm. However, sesame is the sole cultivated Sesamum species for seed production. The relatively narrow genetic base limits the progress of the world's sesame breeding. Deficiency of the elite accessions also hinders the variety improvement. In the past decades, mutagenesis techniques have been applied for creation of new sesame germplasm and for breeding varieties. In this section, we summarize the main progress in mutation induction methods and mutant creation in sesame. Some key mutants with determinacy, dwarf, and other elite traits are enumerated in detail. The importance of mutagenesis approach in sesame is also discussed.

The chromosome morphology, chromosome structure, karyotype, and the chromosome set relationship in Sesamum are presented in this chapter. Besides, the modified chromosome preparation and application of BAC-FISH techniques are briefly introduced at the same time. The 13 chromosomes of S. indicum have been identified and marked using 27 BACs. A high-density cytogenetic map for sesame is constructed with 210 BACs using successive double-colored BAC-FISH. A tetraploid species S. radiatum was initially determined using the cytogenetic data. The main progress in cytogenetical research in Sesamum species provides the theoretical basis for further genomic research in Sesmaum.

S. indicum is the sole cultivated species in the genus Sesamum. The relatively low genetic diversity in the cultivated species limits the development of new varieties through breeding. In this chapter, the main breeding methods in sesame including space mutation, heterosis application, hybridization, system breeding, and interspecific hybridization are introduced. Main achievements of the new varieties breeding in China, USA, and African countries are shown in this chapter. The main breeding objectives of the world sesame are discussed accordingly.

The Sesame Genome Working Group sequenced the genomes of the six wild Sesamum species, i.e., S. alatum (2n = 26, var. 3651), S. latifollum (2n = 32, var. KEN1), S. angolense (2n = 32, var. K16), S. calycinum (2n = 32, var. KEN8), S. angustifolium (2n = 32, var. G01), and S. radiatum (2n = 64, var. G02), which cover all three types of the basic chromosome number of the genus Sesamum. We describe the complicated sequencing and assembly strategies for the wild species and the first interspecific single nucleotide polymorphism (SNP) genetic map for Sesamum. The chromosome-scaled genome assemblies of the six wild Sesamum species are deliberated in this chapter.

Sesame (Sesamum indicum L.; Pedaliaceae) is a commercially valuable oilseed crop with high oil content. Its small genome size favours the genomic analysis of key biological processes, such as oil synthesis and metabolism. However, the 13 chromosome pairs of sesame have not been characterised because of technological limitations and their small size. We constructed a BAC library comprising 57,600 BAC clones for sesame. The estimated genome coverage of the sesame BAC library was 13.8×. The successive double colour fluorescence in situ hybridisation (FISH) with bacterial artificial chromosomes (BACs) for sesame was established in this study. Subsequently, the 13 sesame chromosome pairs were individually differentiated using 17 specific BACs for the first time. The schematic of the sesame chromosome set was drawn according to the chromosome relative length and relative position of the BAC signal. The cytogenetic characteristics of sesame chromosomes were also explored. The results provide the technical background required for further cytogenetic map construction, genome assembly and localisation of the DNA sequence in sesame.

Sesame is an important oilseed crop with high oil content and oil quality. Abundant unsaturated fatty acids, proteins, and antioxidants in sesame seeds attract the worldwide consumption of sesame products. Sesame is highly tolerant of drought and poor soil condition, even though it is readily affected by diseases and waterlogging stress, thereby leading to reduced seed yield and quality. For sesame, increasing the high and stable yield is requisite and urgent. Meanwhile, it is necessary to increase the mechanization level of its harvest for the world’s sesame production. Sesame, S. indicum, is the sole cultivated species in Sesamum genus. The relatively low genetic diversity limits sesame breeding for new and substantial improved varieties. In this section, we present a review of the key agronomic traits and the breeding methods currently used in the species. We also pinpoint the achievement of the Sesame Genome Project (SGP) and the potential for the genomics-assisted breeding in sesame.

Sesame Fusarium wilt (FW) is a worldwide and hardly controlled fungus disease in sesame. Till now, the molecular resistance mechanism of sesame to FW remains unknown. The two resistant and susceptible sesame varieties, that is ‘Yuzhi 11’ and Rongxian black sesame (RXBS), were inoculated with Fusarium oxysporum f. sp. sesami (Fos) pathogens for 0, 6, 24 or 24 and 48h, respectively. Then, transcriptome samples were sequenced and surveyed systematically using RNA-seq technology. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that ‘phenylpropanoid biosynthesis’ pathway may play more important role in sesame resistance after Fos inoculation. Furthermore, qRT-PCR results demonstrated that the phenylpropanoid pathway with an emphasis on the synthesis of lignin contributes to the sesame disease resistance. Moreover, activity of phenylalanine ammonia lyase (PAL) and peroxidase (POD) proved the differences of gene expression patterns in the two sesame phenotypes. Overall, we speculated that the difference of FW symptoms between the resistant and susceptible plants might depend on whether the plants could activate the pathway timely and efficiently or not. The findings would help to clarify the molecular mechanism of sesame resistance to FW.

Cultivated sesame (Sesamum indicum L.) is an important oil crop because of its high oil content and quality. In order to discover the single nucleotide polymorphisms (SNPs) and insertion/deletions (InDels) in RNA-Seq, we collected a total of 33.47 Gbp of data from three sesame transcriptome datasets. A reference transcriptome covering 267,508 unigenes was constructed. Among the 37,646 transcripts with complete open reading frames, a total of 7,450 SNPs and 362 InDels were found with frequencies of one SNP per 6.66 kb and one InDel per 137 kb, respectively. Most of the SNPs were transition-type with the nucleotide transitions C–T or A–G. A total of 21 InDel types with lengths ranging from 1 to 38 bp were identified, and the short InDels (1–2 bp) were most abundant at a ratio of over 80 %. Furthermore, 4,959 (66.56 %) SNPs were detected in protein-coding regions: 2,899 (58.46 %) were synonymous and 2,060 (41.54 %) were nonsynonymous. All SNPs and InDels detected in this study were bi-allelic. Of the randomly selected 40 SNPs and 40 InDels, 92.5 % of the SNPs and 95.0 % of the InDels exhibited polymorphism according to the PCR-based and Sanger-sequenced results. Furthermore, the efficiencies of the newly developed polymorphic SNP and InDel markers were evaluated among 36 commercial sesame cultivars. More than 90.0 % of the markers displayed the expected polymorphic amplifications. The polymorphism information content values ranged from 0.05 to 0.58 with an average of 0.38. Moreover, all genotypes of the 36 commercial cultivars tested were definitively distinguished by 21 SNPs and 16 InDels. These newly identified molecular markers may provide a foundation for cultivar identification, genetic diversity analysis, qualitative and quantitative trait mapping and marker-assisted selection breeding in sesame.

Sesame (Sesamum indicum L.) is an important oilseed crop and has an indeterminate growth habit. Here we resequenced the genomes of the parents and 120 progeny of an F2 population derived from crossing Yuzhi 11 (indeterminate, Dt) and Yuzhi DS899 (determinate, dt1) and constructed an ultra-dense SNP map for sesame comprised of 3,041 bins including 30,193 SNPs in 13 linkage groups (LGs) with an average marker density of 0.10 cM. Results indicated that the same recessive gene controls the determinacy trait in dt1 and a second determinate line, dt2 (08TP092). The QDt1 locus for the determinacy trait was located in the 18.0 cM–19.2 cM interval of LG8. The target SNP, SiDt27-1 and the determinacy gene, DS899s00170.023 (named here as SiDt), were identified in Scaffold 00170 of the Yuzhi 11 reference genome, based on genetic mapping and genomic association analysis. Unlike the G397A SNP change in the dt1 genotype, the SiDt allele in dt2 line was lost from the genome. This example of map-based gene cloning in sesame provides proof-of-concept of the utility of ultra-dense SNP maps for accurate genome research in sesame.

Sesame (Sesamum indicum L.) is an ancient and important oilseed crop. However, few sesame reference genes have been selected for quantitative real-time PCR until now. Screening and validating reference genes is a requisite for gene expression normalization in sesame functional genomics research. In this study, ten candidate reference genes, i.e., SiACT, SiUBQ6, SiTUB, Si18S rRNA, SiEF1α, SiCYP, SiHistone, SiDNAJ, SiAPT and SiGAPDH, were chosen and examined systematically in 32 sesame samples. Three qRT-PCR analysis methods, i.e., geNorm, NormFinder and BestKeeper, were evaluated systematically. Results indicated that all ten candidate reference genes could be used as reference genes in sesame. SiUBQ6 and SiAPT were the optimal reference genes for sesame plant development; SiTUB was suitable for sesame vegetative tissue development, SiDNAJ for pathogen treatment, SiHistone for abiotic stress, SiUBQ6 for bud development and SiACT for seed germination. As for hormone treatment and seed development, SiHistone, SiCYP, SiDNAJ or SiUBQ6, as well as SiACT, SiDNAJ, SiTUB or SiAPT, could be used as reference gene, respectively. To illustrate the suitability of these reference genes, we analyzed the expression variation of three functional sesame genes of SiSS, SiLEA and SiGH in different organs using the optimal qRT-PCR system for the first time. The stability levels of optimal and worst reference genes screened for seed development, anther sterility and plant development were validated in the qRT-PCR normalization. Our results provided a reference gene application guideline for sesame gene expression characterization using qRT-PCR system. Electronic supplementary material The online version of this article (doi:10.1007/s00425-012-1805-9) contains supplementary material, which is available to authorized users.

Sesame (Sesamum indicum L.) is one of the oldest oilseed crops. In order to investigate the evolutionary characters according to the Sesame Genome Project, apart from sequencing its nuclear genome, we sequenced the complete chloroplast genome of S. indicum cv. Yuzhi 11 (white seeded) using Illumina and 454 sequencing. Comparisons of chloroplast genomes between S. indicum and the 18 other higher plants were then analyzed. The chloroplast genome of cv. Yuzhi 11 contains 153,338 bp and a total of 114 unique genes (KC569603). The number of chloroplast genes in sesame is the same as that in Nicotiana tabacum, Vitis vinifera and Platanus occidentalis. The variation in the length of the large single-copy (LSC) regions and inverted repeats (IR) in sesame compared to 18 other higher plant species was the main contributor to size variation in the cp genome in these species. The 77 functional chloroplast genes, except for ycf1 and ycf2, were highly conserved. The deletion of the cp ycf1 gene sequence in cp genomes may be due either to its transfer to the nuclear genome, as has occurred in sesame, or direct deletion, as has occurred in Panax ginseng and Cucumis sativus. The sesame ycf2 gene is only 5,721 bp in length and has lost about 1,179 bp. Nucleotides 1–585 of ycf2 when queried in BLAST had hits in the sesame draft genome. Five repeats (R10, R12, R13, R14 and R17) were unique to the sesame chloroplast genome. We also found that IR contraction/expansion in the cp genome alters its rate of evolution. Chloroplast genes and repeats display the signature of convergent evolution in sesame and other species. These findings provide a foundation for further investigation of cp genome evolution in Sesamum and other higher plants.

The Sesame Genome Working Group (SGWG) has been formed to sequence and assemble the sesame (Sesamum indicum L.) genome. The status of this project and our planned analyses are described.

Background Sesame (Sesamum indicum L.) is one of the most important oil crops; however, a lack of useful molecular markers hinders current genetic research. We performed transcriptome sequencing of samples from different sesame growth and developmental stages, and mining of genic-SSR markers to identify valuable markers for sesame molecular genetics research. Results In this study, 75 bp and 100 bp paired-end RNA-seq was used to sequence 24 cDNA libraries, and 42,566 uni-transcripts were assembled from more than 260 million filtered reads. The total length of uni-transcript sequences was 47.99 Mb, and 7,324 SSRs (SSRs ≥15 bp) and 4,440 SSRs (SSRs ≥18 bp) were identified. On average, there was one genic-SSR per 6.55 kb (SSRs ≥15 bp) or 10.81 kb (SSRs ≥18 bp). Among perfect SSRs (≥18 bp), di-nucleotide motifs (48.01%) were the most abundant, followed by tri- (20.96%), hexa- (25.37%), penta- (2.97%), tetra- (2.12%), and mono-nucleotides (0.57%). The top four motif repeats were (AG/CT)n [1,268 (34.51%)], (CA/TG)n [281 (7.65%)], (AT/AT)n [215 (5.85%)], and (GAA/TTC)n [131 (3.57%)]. A total of 2,164 SSR primer pairs were identified in the 4,440 SSR-containing sequences (≥18 bp), and 300 SSR primer pairs were randomly chosen for validation. These SSR markers were amplified and validated in 25 sesame accessions (24 cultivated accessions, one wild species). 276 (92.0%) primer pairs yielded PCR amplification products in 24 cultivars. Thirty two primer pairs (11.59%) exhibited polymorphisms. Moreover, 203 primer pairs (67.67%) yielded PCR amplicons in the wild accession and 167 (60.51%) were polymorphic between species. A UPGMA dendrogram based on genetic similarity coefficients showed that the correlation between genotype and geographical source was low and that the genetic basis of sesame in China is narrow, as previously reported. The 32 polymorphic primer pairs were validated using an F2 mapping population; 18 primer pairs exhibited polymorphisms between the parents, and 14 genic-SSRs could be integrated into 9 main linkage groups. Conclusions 2,164 genic-SSR markers have been developed in sesame using transcriptome sequencing. 276 of 300 validated primer pairs successfully yielded PCR amplicons in 24 cultivated sesame accessions. These markers increase current SSR marker resources and will greatly benefit genetic diversity, qualitative and quantitative trait mapping and marker-assisted selection studies in sesame.

Seed coat color is an important agronomic trait in sesame, as it is associated with seed biochemical properties, antioxidant content and activity and even disease resistance of sesame. Here, using a high-density linkage map, we analyzed genetic segregation and quantitative trait loci (QTL) for sesame seed coat color in six generations (P1, P2, F1, BC1, BC2 and F2). Results showed that two major genes with additive-dominant-epistatic effects and polygenes with additive-dominant-epistatic effects were responsible for controlling the seed coat color trait. Average heritability of the major genes in the BC1, BC2 and F2 populations was 89.30%, 24.00%, and 91.11% respectively, while the heritability of polygenes was low in the BC1 (5.43%), in BC2 (0.00%) and in F2 (0.89%) populations. A high-density map was constructed using 724 polymorphic markers. 653 SSR, AFLP and RSAMPL loci were anchored in 14 linkage groups (LG) spanning a total of 1,216.00 cM. The average length of each LG was 86.86 cM and the marker density was 1.86 cM per marker interval. Four QTLs for seed coat color, QTL1-1, QTL11-1, QTL11-2 and QTL13-1, whose heritability ranged from 59.33%-69.89%, were detected in F3 populations using CIM and MCIM methods. Alleles at all QTLs from the black-seeded parent tended to increase the seed coat color. Results from QTLs mapping and classical genetic analysis among the P1, P2, F1, BC1, BC2 and F2 populations were comparatively consistent. This first QTL analysis and high-density genetic linkage map for sesame provided a good foundation for further research on sesame genetics and molecular marker-assisted selection (MAS).