Population-level studies of multiallelic self-incompatibility loci, with particular reference to Brassicaceae

Here we review some of the unsolved problems in understanding plant self-incompatibility and attempt to show how a population genetics approach is integral to many interesting questions, including some of the problems involved in making inferences about which parts of the sequences of the known genes are involved in recognition functions. We describe preliminary results from work on a species in the Brassica family, Arabidopsis (formerly Arabis) lyrata. Our data indicate that this species has sporophytic self-incompatibility, with extensive dominance/recessivity of alleles. We have not yet attempted to estimate allele numbers in samples from natural populations, but there must be many alleles, since unrelated plants are usually cross-compatible. Using primers based on Brassica SLG and SRK sequences, we have amplified several S-domain loci from A. lyrata genomic DNA and estimate that at least seven such loci are present. Using plants from geographically separated populations, we find that sequence diversity differs greatly from locus to locus. One set of sequences, representing one locus or at most a few loci, shows linkage with incompatibility groups segregating within several independent families, consistent with this set of sequences representing alleles of a self-incompatibility gene. However, further work is needed to test whether these sequences represent more than a single locus and whether expression patterns of these sequences are consistent with their having a role in self-incompatibility. (C) 2000 Annals of Botany Company.