Genetic diversity and gene flow in a Caribbean tree Pterocarpus officinalis Jacq. : A study based on chloroplast and nuclear microsatellites

We analysed the molecular diversity of Pterocarpus officinalis, a tree species distributed in Caribbean islands, South and Central America to quantify the genetic variation within island, to assess the pattern of differentiation and infer levels of gene flow; with the overall goal of defining a strategy of conservation. Two hundred two individuals of 9 populations were analysed using three chloroplast and six nuclear microsatellite markers. The observed heterozygosity varied markedly among the populations for nuclear (HOnuc = 0.20-0.50) and chloroplast microsatellites (Hcp = 0.22-0.68). The continental population from French Guyana showed a higher value of HOnuc than island populations, and the differences were significant in some cases. The fixation index FIS ranged from - 0.043 to 0.368; a significant heterozygote deficit was detected in 7 populations. The heterozygosity excess method suggested that two populations in Guadeloupe have undergone a recent bottleneck. Global and pairwise FST were high for both nuclear (FSTnuc = 0.29) and chloroplast microsatellites (FSTcp = 0.58). The neighbour-joining tree based on both markers, presented a differentiation pattern that can be explained by the seed dispersal by flotation and marine stream. The comparison of Bayesian approach and the method based on allelic frequency demonstrate a very limited number of migrants between populations. (Résumé d'auteur