Number of incompatibility alleles in clover and other species

Abstract

Atwood's (1942, 1944) data on Trifolium repens, those of Williams & Williams (1947) on T. pratense and those of Williams (1951) on T. hybridum have been re-analysed to provide maximum likelihood estimates of the number of incompatibility alleles in the populations or breeders' stocks from which the samples investigated were obtained. These new estimates suggest that populations of T. repens contain about 100 alleles and those of T. pratense contain up to twice this number. The single estimate from T. hybridum, however, suggests that the species in North America possesses only 17 S-alleles. The estimates from clover populations are compared with those from the nine most thoroughly investigated 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 likely explanation of the large number of S-alleles that natural populations of these species appear to contain is that they are substructured into a large number of semi-isolated neighbourhoods. © 1996 The Genetical Society of Great Britain.