Diallel analysis of the level of self-incompatibility evaluated by insect-pollination in Brassica rapa L.

The level of self-incompatibility (SI) directly influences the purity of F-1 seed produced by using an SI system. Using insect-pollination test, we performed diallel analysis to determine the levels of SI of 6 inbred lines of Brassica rapa L. differing in their rates of pod set. Each of 3 lines with a high level of SI (Kal-22, Ka1-3 and Ch1-504) appeared to have different modes of inheritance. To clarify these in detail, we performed 4 x 4 diallel analysis using each of these 3 lines together with 3 other lines (Ko1-303, Ha1-400 and Ha1-10). The crosses with Kal-22 showed an incomplete dominance and non-significant correlation between the proportion of dominant alleles of the parents (Vr+Wr) and the parental values (Pr), suggesting that Kal-22 possesses dominant genes controlling the high level of SI. The crosses with Ch1-504 showed significant correlation between Vr+Wr and Pr, suggesting that Ch1-504 possesses recessive genes controlling the high level of SI. The crosses with Ka1-3 showed epistatic effects and specific combining effects. This study is the first to define strictly the genetic diversity controlling a high level of SI. Breeders must be careful in selecting the breeding material, because the mode of inheritance of the level of SI could differ in lines showing a high level of SI.