Inbreeding depression is difficult to purge in self-incompatible populations of Leavenworthia alabamica

The extent to which inbreeding depression can be purged is a major determinant of mating system evolution and is important to conservation and crop improvement. Studies of inbreeding depression purging have not been conducted in self-incompatible plants before. An experimental ('ancestral') treatment was first created from self-incompatible plants of Leavenworthia alabamica. Lines derived from this population were maintained by self-pollination for three generations in the attempt to create a 'purged' population with fewer recessive, deleterious mutations of large effect. Fitness components and the frequency of malformed phenotypes were monitored in progeny derived from selfing and outcrossing in the ancestral and purged treatments. Fitness component means and inbreeding depression were largely unchanged by three generations of forced self-pollination, and there was no reduction in the frequency of plants exhibiting malformed phenotypes. Our findings indicate that inbreeding depression in this species is largely a result of mutations of mild effect, consistent with the observation that self-incompatibility is maintained in most populations of L. alabamica, despite the presence of genetic variants with weaker self-incompatibility. Moreover, although population theory suggests that deleterious mutations of large effect should be sheltered from selection in the region of self-incompatibility locus, our results do not support this prediction.