FREQUENCY AND DISTRIBUTION OF SELF-INCOMPATIBILITY ALLELES IN RAPHANUS RAPHANISTRUM

Abstract

ESTIMATES of the number of S alleles in species of flowering plants with gametophytic self-incompatibility run to several hundreds. PAXMAN (1963), for example, estimated 192 different S alleles in one population of Trifolium pratense and 215 in a second population, with lower 5% fiducial limits of 149 and 1 10 respectively. In contrast, a study to determine the number of loci controlling self-incompatibility in Raphunus raphanistrum (SAMPSON 1964) disclosed a frequent recurrence of certain S alleles in the initial research material. R. raphanistrum, a wild radish, has a sporophytic type of incompatibility determined by S alleles at one locus. The allele distribution found in the initial material from two isolated populations in Nova Scotia, Canada and from a third population in Poland are given here, together with the results from sampling a New Brunswick population and a second Polish population for nine S alleles. Having determined the frequency of nine alleles in the wild, the question arose as to what theoretical frequency to expect. The answer would be simple for species with the gametophytic type of self-incompatibility, where, if N is the number of S alleles, the equilibrium frequency for all alleles (deterministic model) is I,". However, in species with the sporophytic type of incompatibility three types of S allele interaction occur (recessive, dominant, independent) and the pollen phenotype appears to be independent of the stigma phenotype, giving nine possible types of alleles each with a different frequency at equilibrium. Four of these types were present among the sampled S alleles of R. raphanistrum: pollen reces-sive-stigma dominant (S I , s,, s,,) ; pollen dominant-stigma recessive (S9, s,,) ;