Genetic variation and dispersal patterns in three varieties of Pinus caribaea (Pinaceae) in the Caribbean Basin

Background - Pinus caribaea Morelet comprises three varieties of tropical pines distributed in the Caribbean Basin: P. caribaea var. hondurensis, var. caribaea, and var. bahamensis. The insular and continental distribution of these varieties, as well as the geological processes in the region, have been important factors for analysing evolutionary processes implicated in the diversification of these lineages. In this study, we evaluate the genetic and geographic structure within and between these three varieties in order to infer the possible origin and dispersal routes of these taxa. Methods - We used six polymorphic nuclear microsatellites (nSSR) in fifteen representative populations of the three pine varieties, sampled throughout their natural range in Central America, Cuba and the Bahamas islands. Results - The varieties contain similar levels of genetic variation (mean H-e = 0.571), with several populations out of Hardy-Weinberg equilibrium, and significant levels of inbreeding (0.097-0.184, P = 0.05). A slight but significant genetic differentiation was found between the varieties (R-ST = 0.088) and populations (R-ST = 0.082), and genetic differentiation increased with geographic distance (r(2) = 0.263). Distance and Bayesian BAPS analyses generated seven groups; two represented by the two island varieties and the remainder by the Central American populations of var. hondurensis. Migration rate estimates between pairs of groups ranged from M = 0.47 to M = 20.16. Estimates were generally higher from the continent to islands, with the highest migration rate estimated from a continental genetic group to the Cuba island group of var. hondurensis (M = 20.16). Conclusions - This study supports the hypothesis of a recent origin of these pine taxa through the migration of an ancestor from Central America, where the historical demography is associated with events of colonization, expansion and contraction of populations. The genetic variation and differentiation suggest that the three varieties are divergent lineages that currently share allelic variants, indicating that their speciation has not yet completed.