STRONG GENETIC STRUCTURE OVER THE AMERICAN CONTINENTS AND TRANSOCEANIC DISPERSAL IN THE MANGROVE GENUS RHIZOPHORA (RHIZOPHORACEAE) REVEALED BY BROAD-SCALE NUCLEAR AND CHLOROPLAST DNA ANALYSIS

Premise of the study: The global distribution of mangroves is attributed to interactions between long-distance propagule dispersal and geographical barriers, which are manifest in genetic structuring. Uncovering this genetic structure thus provides a window into the ecological, evolutionary, and phylogeographic history of mangroves. We used cpDNA and nuclear microsatellites to evaluate transbarrier (transoceanic and transisthmian) linkages in the genus Rhizophora in the Atlantic East Pacific (AEP) and South Pacific region. Methods: Leaf samples of 756 individuals of Rhizophora mangle, R. racemosa, R. xharrisonii, and R. samoensis from 36 populations across the AEP supplied material from which we used the cpDNA haplotypes and nine microsatellite markers for population analyses. Key results: Clear genetic differentiation of cpDNA haplotypes was found between the Pacific and Atlantic populations in R. mangle and R. racemosa, supporting the hypothesis of the Central American Isthmus as a barrier to gene flow. Both cpDNA and microsatellite analyses support the hypothesis of recent and frequent transatlantic propagule dispersal for R. mangle. Finally, we provide strong evidence for genetic similarity of Pacific R. mangle and R. samoensis suggesting trans-Pacific dispersal of R. mangle. Conclusion: The American continents are strong geographical barriers to dispersal of Rhizophora, to the point where the Pacific and Atlantic populations are distinct genealogical units, supporting the recommendation to treat the populations as separate conservation and management units. Trans-Pacific propagule dispersal of Rhizophora has occurred; R. mangle and R. samoensis might be the same species and this question should be resolved with further taxonomic study.