Genetic structure and mating system of Stemonoporus oblongifolius (Dipterocarpaceae) in Sri Lanka

The genetic structure and mating system of an endemic Sri Lankan dipterocarp, Stemonoporus oblongifolius , was examined using allozyme analyses. Considering the small geographic distribution of the species, which is confined to the Peak Wilderness Sanctuary range, the species and population level genetic diversity are exceptionally high (H e = 0.342 and 0.282, respectively). Although the populations sampled are separated by a maximum distance of only 10 kilometers, a fair amount of genetic structuring was detected (G ST = 0.163). The mating system could contribute to the maintenance of a high level of genetic diversity, but in itself is insufficient to explain the high values for this study. The multilocus outcrossing rate, quantified for one population, was high (84%), with evidence for apomixis in one tree. Other factors potentially contributing to the high observed allozyme diversity in this species would be large effective population sizes, introgression of genes from sympatric species, and retention of ancient allozyme polymorphism. Gene flow among populations, estimated to be high, would have to be through pollen dispersal, since the large, resinous seeds typically fall and germinate beneath the maternal tree and are not known to be dispersed by any animal.