Direct and indirect measures of gene flow in three tropical dry forest tree species in southwestern Puerto Rico
Dunphy, Brian Keegan
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The ability to exchange genes across the fragmented dry forest life zone of Puerto Rico’s south coast was examined in three species of tropical trees. Two of the species, Bursera simaruba and Hymenaea courbaril, are native to the island, while the third, Albizia lebbek, was introduced from tropical Asia in historic times. Indirect gene flow estimates, which are relatively easy to obtain, were compared against direct gene flow estimates, which although requiring more effort, entail fewer assumptions. One of the critical assumptions of the indirect approach, the presence of migration-drift equilibrium, was violated by all three species. Possible causes of the absence of equilibrium are discussed. Mating system analyses indicated that the three species were essentially completely outcrossed. Results from the direct gene flow study showed that for all three species, a substantial portion of a tree’s seed production is the result of gene-flow pollen, with between 42% and 100% of seeds being sired by foreign pollen. Gene flow was highest in A. lebbek, and lowest in H. courbaril. This contradicts a priori expectations of relative gene flow rates based on pollinator flight capacities, where the bat-pollinated H. courbaril was expected to have the highest gene flow rates. Asynchronous flowering and a lower population density are likely explanations for the relatively low gene flow estimates in H. courbaril. No relationship was seen between gene flow rates and either distance to nearest neighbor or stand size. In B. simaruba, however, germination rates of seeds increased with increasing stand size, with an almost total absence of germination in stands of three trees or less. A study of pollen movement within a stand of 21 adult H. courbaril trees showed that pollen usually did not come from nearest neighbors, and that relatively few trees contributed the majority of the pollen leading to successful pollinations. Genetic diversity values were higher in B. simaruba and A. lebbek than in other species with similar life history characteristics, while they were lower in H. courbaril. Possible causes of these patterns are discussed.