Factors influencing the genetic structure of phacelia dubia, a species with a seed bank and large fluctuations in population size

The potential factors that may influence the genetic structure of the gynodioecious annual Phacelia dubia were assessed using electrophoretic and ecological information at a locality in which the population size changed by about three orders of magnitude. Fluctuations in population size appeared to have little influence on the allelic composition, heterozygosity and mixed mating system of the population. Despite wide fluctuations in total population size, the estimated effective population size during (Ne = 20) and after the bottleneck (N., = 28) was little changed. Also, a significant spatial substructuring, evidenced by a cline in MDFI and by the F-statistics, was observed before and after the bottleneck. The recovery of the population and the preservation of genetic diversity was attributable in part to the seed bank in the soil. Spatial gene flow via seeds was small compared with pollen flow and both were restricted. Nevertheless, substructuring contributes to a small portion of the total inbreeding. Also, the rate of apparent selfing of male stériles did not provide evidence of biparental inbreeding. Most of the inbreeding, however, was within subpopulations and autogamy appears to be the major contributor. It was concluded that the mating system is the leading factor determining the genetic structure and that the seed bank ensures genetic constancy in time in the face of large fluctuations in population size.