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 electrophoreti c and ecological information at a locality in which the population siz e changed by about three orders of magnitude. Fluctuations in populati on 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 popula tion size during (N-e=20) and after the bottleneck (N-e=28) was little changed. Also, a significant spatial substructuring, evidenced by a d ine in MDH and by the F-statistics, was observed before and after the bottleneck. The recovery of the population and the preservation of gen etic diversity was attributable in part to the seed bank in the soil. Spatial gene flow via seeds was small compared with pollen flow and bo th were restricted. Nevertheless, substructuring contributes to a smal l portion of the total inbreeding. Also, the rate of apparent selfing of male steriles did not provide evidence of biparental inbreeding. Mo st 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 tha t the seed bank ensures genetic constancy in time in the face of large fluctuations in population size.