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

A crossing procedure to extract one S- and one Z-allele, at random, fr om each of a number of plants of Lolium perenne, to examine the number and frequency of incompatibility alleles they contain is described. T his procedure has been used on a random sample of plants from the long established population of perennial ryegrass in North Meadow, Crickla de. In all, 17 different S-alleles and 17 different Z-alleles were fou nd in a sample of 38 and 39 plants, respectively. The frequencies of t hese alleles were very unequal, one of the S-alleles occurring 12 time s and two of the Z-alleles nine times each. The E(2) estimator of O'Do nnell & Lawrence (1984) gave estimates of 31 alleles at each locus, al though these estimates are probably less than the actual number of S- and Z-alleles in this population. Analysis of the theoretical properti es of the two-locus polymorphism, suggests that, with this number of a lleles, the strength of the frequency-dependent selection that maintai ns 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.