Molecular variation at the self-incompatibility locus in natural populations of the genera Antirrhinum and Misopates

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Abstract
The self-incompatibility system of flowering plants is a classic example of extreme allelic polymorphism maintained by frequency-dependent selection. We used primers designed from three published Antirrhinum hispanicum S-allele sequences in PCR reactions with genomic DNA of plants sampled from natural populations of Antirrhinum and Misopates species. Not surprisingly, given the polymorphism of S-alleles, only a minority of individuals yielded PCR products of the expected size. These yielded 35 genomic sequences, of nine different sequence types of which eight are highly similar to the A. hispanicum S-allele sequences, and one to a very similar unpublished Antirrhinum S-like RNase sequence. The sequence types are well separated from the S-RNase sequences from Solanaceae and Rosaceae, and also from most known ‘S-like’ RNase sequences (which encode proteins not involved in self-incompatibility). An association with incompatibility types has so far been established for only one of the putative S-alleles, but we describe evidence that the other sequences are also S-alleles. Variability in these sequences follows the pattern of conserved and hypervariable regions seen in other S-RNases, but no regions have higher replacement than silent diversity, unlike the results in some other species.