SELF-INCOMPATIBILITY IN RYEGRASS .10. NUMBER AND FREQUENCY OF ALLELES IN A NATURAL-POPULATION OF LOLIUM-PERENNE L

A crossing procedure to extract one S- and one Z-allele, at random, from each of a number of plants of Lolium perenne, to examine the number and frequency of incompatibility alleles they contain is described. This procedure has been used on a random sample of plants from the long established population of perennial ryegrass in North Meadow, Cricklade. In all, 17 different S-alleles and 17 different Z-alleles were found in a sample of 38 and 39 plants, respectively. The frequencies of these alleles were very unequal, one of the S-alleles occurring 12 times and two of the Z-alleles nine times each. The E(2) estimator of O'Donnell & Lawrence (1984) gave estimates of 31 alleles at each locus, although these estimates are probably less than the actual number of S- and Z-alleles in this population. Analysis of the theoretical properties of the two-locus polymorphism, suggests that, with this number of alleles, the strength of the frequency-dependent selection that maintains the polymorphism must be weak, that a population will contain only a subset of the total number of genotypes and that the unequal allele frequencies might be caused by either or both of these effects.