The effect of mating system differences on nucleotide diversity at the phosphoglucose isomerase locus in the plant genus Leavenworthia

To test the theoretical prediction that highly inbreeding populations shoul d have low neutral genetic diversity relative to closely related outcrossin g populations, we sequenced portions of the cytosolic phosphoglucose isomer ase (PgiC) gene in the plant genus Leavenworthia, which includes both self- incompatible and inbreeding taxa. On the basis of sequences of intron 12 of this gene, the expected low diversity was seen in both populations of the selfers Leavenworthia uniflora and L. torulosa and in three highly inbreedi ng populations of L. crassa, while high diversity was found in self-incompa tible L. stylosa, and moderate diversity in L. crassa populations with part ial or complete self-incompatibility. In L. stylosa, the nucleotide diversi ty was strongly structured into three haplotypic classes, differing by seve ral insertion/deletion sequences, with linkage disequilibrium between seque nces of the three types in intron 12, but not in the adjacent regions. Diff erences between the three kinds of haplotypes are larger than between seque nces of this gene region from different species. The haplotype divergence s uggests the presence of a balanced polymorphism at this locus, possibly pre dating the split between L. stylosa and its two inbreeding sister taxa, L. uniflora and L. torulosa. It is therefore difficult to distinguish between different potential causes of the much lower sequence di diversity at this locus in inbreeding than outcrossing populations. Selective sweeps during t he evolution of these populations are possible, or background selection, or merely loss of a balanced polymorphism maintained by overdominance in the populations that evolved high selfing rates.