Interspecific cross-incompatibility system in the genusAvena

A major mechanism of abortive interspecific crosses inAvena appears to be well explained by a hypothesis of polar-nuclei activation (Nishiyama and Yabuno, 1978, 1979). Under the hypothesis, activating value (AV) or response value (RV) of the six following species was estimated, 0.45 being assigned toA. clauda (2x), 0.8 toA. canariensis (2x) andA. prostrata (2x), 0.95 toA. damacena (2x), 1.4 toA. murphyi (4x) and 2.7 toA. byzantina (6x). AV and RV represent the intensity of the mutual reaction between the male and female gametes, respectively, in fertilization. Next the degree of activation of the two polar nuclei by one male nucleus which produce the first endosperm nucleus was shown by the activation index (AI) computed as AV/2RV×100(%). The AI showed a wide range of 7% to 338% in 37 interspecific crosses, the six species being crossed with each other or with some tester species. The pattern of relationship between the crossability, including plumpness, germination and others of hybrid seeds, and the AI% was found to completely agree with that expected from the standard cross-compatibility system inAvena (Fig. 1A, B). That is, compatible crosses occurred within a range of about 20% to 80% AI but incompatible crosses occurred beyond these limits.