Self-incompatibility in ryegrass. X. Number and frequency of alleles in a natural population of Lolium perenne L.

Abstract

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 E2 estimator of O’Donnell and 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. © 1994 The Genetical Society of Great Britain.