Plant genetics: Seeing selection in S allele sequences

The self-incompatibility (S) genes of flowering plants are, with the fungal incompatibility and mammalian major histocompatibility (MHC) loci, among the most highly polymorphic loci known. In Oenothera organensis, for example, 35 alleles have been detected [1] by laborious compatibility tests between plants of this endemic species, the total population size of which may not exceed 5000 [2]. Different populations of the same species have usually been found to have highly overlapping sets of alleles [3], though only four of twenty-nine alleles were common to Turkish and Japanese populations of Brassica campestris [4]. The self-recognition aspect of plant incompatibility — pollen carrying alleles in common with a potential recipient plant is recognized as ‘self’ and prevented from fertilizing ovules — interests cell biologists [5,6], while its impressive polymorphism has attracted the interest of population geneticists.