Contrasting Colonization Patterns of Black Mangrove (avicennia Germinans (L.) L.) Gene Pools along the Mexican Coasts

Aim: Historical and geological events can impact the genetic structure of species, producing signatures that vary among taxa and among gene pools within taxa. Such signatures can also be affected by local geography and tolerance to environmental conditions. However, disentangling the different drivers of population structure is often difficult. In an attempt to do so, we surveyed two independent gene pools of the same species that followed similar paths of post-glacial colonization across contrasting landscapes and environmental conditions. We aimed to determine how these differences have affected the post-glacial population dynamics of each gene pool. Location: The Pacific and Atlantic coasts of Mexico. Taxon: Black mangrove (Avicennia germinans; Avicenniaceae). Methods: Using microsatellite variation, we estimated the divergence time of black mangrove populations through approximated Bayesian computation and implemented a comparative approach to evaluate different demographic hypotheses within and between coasts. Results: The Pacific and Atlantic gene pools diverged long after the rise of the Central American Isthmus (Mid-Pleistocene), although occasional transisthmian gene exchanges were also inferred. Both coasts showed the characteristic isolation by distance (IBD) pattern expected for expanding gene pools. However, populations from the Atlantic coast were more genetically diverse and admixed than those from the Pacific basin. Both our migration models and the climate data gathered suggested a more ancient establishment and/or more stable conditions for black mangrove on the Atlantic coast. Main conclusions: The Atlantic basin likely bore more favourable climate conditions than the Pacific, allowing for the survival of A. germinans during the Last Glacial Maximum in situ. Populations from the northern Pacific coast became established after the Holocene warming, leading to contrasting genetic patterns between the two gene pools. Nevertheless, the action of environmental factors in determining the contemporary distribution of genetic variation in A. germinans cannot be discarded.