The effect of breeding system on the level and pattern of molecular variation in plant populations

Levels of neutral genetic diversity within and between populations were compared between outcrossing (self-incompatible) and inbreeding populations in the annual plant genus Leavenworthia. Two taxonomically independent comparisons are possible, since self-incompatibility has been lost twice in the group of species studied. Diversity data from six loci were compared to discriminate forces that affect a small genomic region such as balancing selection from those affect all loci similarly such as background selection or hitchhiking. Within inbred populations of L. uniflora and torulosa, no DNA sequence variants were seen among the alleles sampled from any population at any of the six loci studied. High diversity was seen in alleles from populations of the outcrosser L. stylosa, and in self-incompatible L. crassa populations. While no between-population diversity was detected in the two L. uniflora populations studied, diversity between populations was seen in both L. crassa and L. stylosa populations. However, between-population values were consistently higher at all the six loci studied in the highly inbreeding L. crassa populations than those in the outcrossing taxa. Possible reasons for these diversity patterns are discussed. Evidence for selection affecting the diversity patterns at a single locus was found from a single intron region of Nir1 locus in L. crassa species, and from a single intron region of PgiC in L. stylosa species separately. Despite the effect of selection on increasing the diversity level at the single locus, the overall effect of inbreeding appears to be a greater than twofold reduction in diversity. Therefore, I argue that some processes such as selection for advantageous mutations, or against deleterious mutations, or bottlenecks occurring predominantly in the inbreeders, appears necessary to account for the findings. If selection for advantageous mutations is responsible, it must be some form of local adaptive selection, rather than substitution of alleles that are advantageous throughout the species. This is consistent with the finding of high between-population diversity in the inbreeding taxa. Both bottleneck effects and background selection could have played important roles in the reduced diversity levels in the selfing populations.