The SSR-derived data were used to calculate genetic similarities (GS) among 40 Chinese endemic wheat accessions. In these Chinese landraces, the GS values ranged from 0.540 to 0.941, with the mean of 0.717 (Table 4). The mean GS values within the SWW, YH, TW and XR wheat groups were 0.815, 0.841, 0.747 and 0.750, respectively. It indicated that the mean GS values of the SWW and YH wheat groups were relatively higher than those of the TW and XR groups.
The genetic relationships (Fig.3) among 40 Chinese endemic wheat accessions were estimated by a UPGMA cluster analysis of the genetic diversity (1-GS) matrix. The results showed that all 40 accessions could be discriminated by 24 SSR markers. All 9 XR accessions were less related with other accessions, and divergent from the other accessions. Three subgroups were evident in the remaining 31 accessions. Among them, the subgroup Ib more closely related to Ic. All 8 accessions belonging to the SWW wheat group were included in the subgroup Ic. The subgroup lb included all 14 YH wheat accessions and 3 TW wheat accessions (i.e. AS907, AS908 and AS1026). These things indicated that the XR wheat group was genetically distinct from other three Chinese endemic wheat groups, while the SWW group was genetically related to the YH group. The TW wheat group is more diverse than the SWW and YH groups, with some accessions more related to the YH group.
In wheat, SSR markers show a much higher level of polymorphism and informativeness than any other molecular marker (Roder et al. 1995; Plaschke et al. 1995; Prasad let al. 2000). The similar results were observed in this study. Among 40 Chinese endemic wheat accessions, the genetic similarity (GS) based on SSR markers ranged from 0.540 to 0.941, with the mean of 0.717, while the GS based on STS markers ranged from 0.645 to 0.989, with the mean of 0.822. In earlier study, the mean GS based on RFLP markers among Chinese endemic wheat accessions was 0.969 (Ward et al. 1998). It indicated that SSR markers had the relative superiority in detecting DNA polymorphism than STS and RFLP markers.
The average number of alleles per SSR locus and genetic similarity (GS) in present study was compared with those published earlier in wheat. The average number of alleles per locus in this study was relatively lower than those earlier reported, while the mean genetic similarity in this study was relatively higher than those earlier reported. Only 3.5 alleles per locus were detected among Chinese endemic wheats, while in three earlier studies on wheat the average number of alleles per locus were 4.6, 6.2 and 7.4 alleles per locus reported by Roder et al. (1995), Plaschke et al. (1995) and Prasad et al. (2000), respectively. Among 40 Chinese endemic wheat accessions, the mean GS based on SSR markers was 0.717. In two. earlier studies, the mean GS were 0.31 (Plaschke et al. 1995) and 0.23 (Prasad et al. 2000), respectively. It suggested that the 40 Chinese endemic wheat accessions were more closely related. It is agreement with the results obtained from isoenzyme (Yang et al. 1992), HMW-glutenins and gliadins (Wei et al. 2000, 2002), and RFLP markers (Dvorak et al. 1998; Ward et al, 1998).
From morphological (Yen et al. 1988), HMW-glutenins and gliadins (Wei et al. 2000, 2002), and RFLP (Dvorak et al. 1998; Ward et al. 1998) analyses, it is proposed that Chinese Spring is a strain of Sichuan landrace Chengdu-guangtou, a famous landrace of the Chengdu Plain, which is one of the Sichuan White Wheat. In this study, the cluster analysis based on SSR and STS-PCR markers showed that Chinese Spring was more closely related with Chengdu-guangtou than any other Chinese endemic wheat accession, providing further support to the proposal that Chinese Spring is a strain of Chengdu-guangtou.
From RFLP analysis, Ward et al. (1998) found that the XR wheat group was genetically distinct from the other three Chinese endemic wheat groups and the TW group was genetically similar to, but more diverse than, the SWW and YH groups. The similar results were obtained in this study. The genetic relationships within and between Chinese endemic wheat groups were estimated by a UPGMA cluster analysis of (1-GS) matrix (Fig. 1 and Fig.3). The results indicated that the XR wheat group was genetically distinct from other three Chinese endemic wheat groups, while SWW wheat group was genetically related to the YH group. The TW wheat group is more diverse than the SWW and YH groups, with some accessions more related to the YH group.
Acknowledgments
The authors are thankful to the Ministry of Science and Technology, and the Ministry of Education, China, the Science and Technology Committee, and Education Committee of Sichuan Province, China for their financial supports.