Inferring potential barriers to gene flow in tropical populations of Avicennia germinans

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Genetic variation within- and divergence among-populations is essential for conservation and management efforts. In plants that are dispersed mainly by ocean currents, physical features of the landscape may influence rates of gene flow among populations by preventing or facilitating dispersal of buoyant propagules. The complex landscape and the pattern of superficial currents acting along the wide latitudinal range of the coasts of Mexico offer an ideal opportunity to evaluate the relevance of oceanographic factors to the genetic diversity of a mangrove species. Here, we infer the role of some oceanic features as potential barriers to gene flow among populations of Avicennia germinans. We used eight nuclear microsatellite markers to estimate recent migration rates and geographic barriers among populations of A. germinans along Mexico’s Atlantic and Pacific coasts. Along the Pacific, we identified potential barriers to gene flow related to the circulation pattern at the mouth of the Gulf of California and Gulf of Tehuantepec in the Pacific. On the Atlantic, population genetic variation coincides with the trends of major ocean currents around the Yucatan Peninsula. Our results suggest that ocean currents and a physical barrier (Baja California Peninsula) have maintained and generated genetic discontinuities along the coasts of Mexico. We find compelling evidence that suggests that northern and southern populations evolve as independent units. Genetic drift has had an important role in determining distribution of genetic diversity in local populations of A. germinans. We strongly recommend the conservation of northern and southern mangrove communities of Mexico; we also suggest use individuals from adjacent populations as genetic source for restoration programs.