NUMBER OF INCOMPATIBILITY ALLELES IN CLOVER AND OTHER SPECIES

Atwood's (1942, 1944) data on Trifolium repens, those of Williams & Wi lliams (1947) on T. pratense and those of Williams (1951) on T. hybrid um have been re-analysed to provide maximum likelihood estimates of th e number of incompatibility alleles in the populations or breeders' st ocks from which the samples investigated were obtained. These new esti mates suggest that populations of T. repens contain about 100 alleles and those of T. pratense contain up to twice this number. The single e stimate from T. hybridum, however, suggests that the species in North America possesses only 17 S-alleles. The estimates from clover populat ions are compared with those from the nine most thoroughly investigate d species of other self-incompatible flowering plants and confirm the long-held belief that populations of T. repens and T. pratense contain more S-alleles than those of the latter. It is argued that the most l ikely explanation of the large number of S-alleles that natural popula tions of these species appear to contain is that they are substructure d into a large number of semi-isolated neighbourhoods.