Genetic population structure in the Mediterranean Serapias vomeracea, a nonrewarding orchid group. Interplay of pollination strategy and stochastic forces?

The spatial genetic structure of the widespread orchid Serapias vomeracea ssp. vomeracea and of the two rare S. vomeracea ssp. laxiflora and ssp. orientalis was examined using nuclear microsatellites. The three taxa showed low mean allele number (3.2), frequent significant deviations from the HW equilibrium (13 out of 20 populations) and intermediate to high gene diversity. F-statistics indicated either low or highly unpredictable levels of gene flow, in line with a not significant isolation by distance. Demographic tests suggested that most of populations experienced a size decline and show a small effective population size. The heterozygote excess detected in many populations could be the consequence of non-random mating due to the nonrewarding pollination strategy of Serapias. Hence, the divergence of lineages within the genus could be driven by a combination of stochastic forces such as drift and bottlenecks, as well as locally selective forces.