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-in compatibility and attempt to show how a population genetics approach is int egral to many interesting questions, including some of the problems involve d in making inferences about which parts of the sequences of the known gene s are involved in recognition functions. We describe preliminary results fr om work on a species in the Brassica family, Arabidopsis (formerly Arabis) lyrata. Our data indicate that this species has sporophytic self-incompatib ility, with extensive dominance/recessivity of alleles. We have not yet att empted to estimate allele numbers in samples from natural populations, but there must be many alleles, since unrelated plants are usually cross-compat ible. Using primers based on Brassica SLG and SRK sequences, we have amplif ied several S-domain loci from A. lyrata genomic DNA and estimate that at l east seven such loci are present. Using plants from geographically separate d populations, we find that sequence diversity differs greatly from locus t o locus. One set of sequences, representing one locus or at most a few loci , shows linkage with incompatibility groups segregating within several inde pendent families, consistent with this set of sequences representing allele s of a self-incompatibility gene. However, further work is needed to test w hether these sequences represent more than a single locus and whether expre ssion patterns of these sequences are consistent with their having a role i n self-incompatibility. (C) 2000 Annals of Botany Company.