Assessment Of The Gnetic Diversity Of Sesame (Sesamum Indicum L.) Germplasm Resources

Sesame (Sesamum indicum L.-Pedaliaceae), an important oil-yielding plant, is one of the most ancient crops in the world. It is grown in tropical and temperate areas on 6.5 million hectares worldwide, producing more than three million tons of seed. Sesame is also one of the four major oil crops in China where it has been cultivated for about 2200 years and it is also an important exporting goods to earn foreign currency. Despite its nutritional value and historic importance, the research on sesame has been scarce. For example, no international Consultative Group on International Agricultural Research (CGIAR) agencies mandated to study sesame. Information on the genetic diversity in sesame is limited as well. More recently, a high level of variability of morphological characters within different sesame collections was reported. Genetic variability in sesame has also been studied by molecular techniques. The aim of the present study was to clarify genetic relationships among 192 Chinese and alien sesames germplasm resources using SRAP and EST-SSR markers and to analyze the main agronomic characters, and cluster the 186 resources using 21 morphological characters. The results were as follows:1. Morphological level analysisThe variation of agronomic traits of the 186 sesame germplasm resources was analyzed by using 8 main agronomic characters, which related highly with the yield. In China, the southern region was higher in sesame variation than the others. Flowering period, growth period, capsule length and plant height had more variation. As yield factors, capsules per plant and 1000-seed weight had remarkably positive correlation with the yield per plant and flowering period showed positive correlation.We got 4 factors, using the factor analysis, which proportions of cumulative variability were beyond 80%. For all the sesame germplasm resources, the first factor was yield factor and followed by growth factor, length factor and flowering period factor. But the result revealed that the leading factors of variation in different regions were diverse.The cluster analysis using 21 morphological traits showed that all the 186 resources were divided into 7 groups, suggesting that sesame had a rich diversity. Most of the varieties that came from different provinces, regions and countries were assembled tighter. That means the grouping of genotypes in clusters was not related to their geographic origin.2. Molecular level analysisFirstly we established and optimized the sesame SRAP and SSR reaction systems, which provided a new technological platform for the genetic diversity research of sesame. A total of 270 SRAP markers were recorded using 31 primer combinations, 62.08% of those were polymorphic. Each primer was able to detect 5.45 loci; While 25 pairs of SSR amplified 136 loci, 56.28% of those were polymorphic. Each primer was able to detect 3.04 loci. The result also disclosed that SRAP was more predominant than SSR in detecting the genetic diversity among varieties.Mantel test revealed that the relation of genetic similarity matrix between SRAP and SSR was significant (r=0.7864, t=5.2571, P<0.01) for 192 species, so the UPGMA dendrogram was based on the integrated data of SRAP and SSR. The 192 accessions were grouped in three robust clusters, but the clustering result has not revealed any association between genotype and geographical origin. This was similar to the result of the morphological cluster. This lack of association between geographical distribution and classification based on molecular markers in sesame might be explained by the exchange of sesame germplasm among widely separated locations.The genetic diversity of the alien was higher than Chinese, and in China the southern accessions was higher than the other regions. The improved sesame variety had the lowest level of genetic diversity, and the reason might be artificial selection for its confined genetic background.3. The comparison between morphological and molecular clusters of sesame germplasmThere were significant differences between the two clusters. The reason for why they had no correlation were as the following: (1) there were strong artificial selection and considerable gene flow among different regions; (2) the various environment and cultivation affected sesame morphological traits and growth period remarkably; (3) most of the varieties came from farmhouse, existing lots of heterozygous genotypes; and (4) morphological traits and growth period might have some changes for all resources grown in the same farm.In spite of the great difference between the two clusters, they still had the resemblance: (1) very little association between genotype and geographical origin; (2) both could divided all the resources and reflected sesame's rich diversity; and (3) the domestic resources assembled tighter, the alien dispersive, the archaic and wild assembled solely.