The genetic diversity and heterotic groups of 169 Chinese semi-winter rapeseed (Brassica napus) cultivars and inbred lines

Until now, no distinct heterotic groups have been established in many rapeseed-growing countries. Numerous modern breeding lines with desirable seed quality have been developed but the loss of genetic diversity would be problematic. Simple sequence repeat markers covering all 19 B. napus linkage groups were used to evaluate the genetic diversity and to classify the heterotic groups of 169 Chinese semi-winter cultivars or inbred lines. The results indicate moderate genetic differentiation among the 169 accessions, which distributed in 10 major groups according to their pedigrees or origins. The Chinese accessions could be divided into two genotypes: northern restorers and southern maintainers for Polima cytoplasm male sterility. The maintainers consisted of nine clades originating from 10 major rapeseed-growing provinces in the upper, middle and lower reaches of the Yangtze River. The mid-parent heterosis on plant height and yield ability of the intragroup crosses was inferior to that of the intergroup hybrids. When the intragroup parental combinations with short genetic distances (GD) were included in all regressing data, GD is significantly correlated to heterosis level. Three major heterotic patterns were identified: (1) northern restorer × southern maintainer, (2) maintainer from the upper reaches of the Yangtze River × maintainer from the lower reaches of the Yangtze River and (3) maintainer from the middle Yangtze River × maintainer from the lower reaches of the Yangtze River. For the first time, the results showed the outline of heterotic groups in Chinese rapeseed under the conditions of the Huang-Huai River region.