Evolutionary Dynamics of American Manatee Species on the Northern Coast of South America: Origins and Maintenance of an Interspecific Hybrid Zone

Interspecific hybridization has been historically neglected in research and conservation practice, but it is a common phenomenon in nature, and several models have been developed to characterize it genetically. Even though Trichechus inunguis (Amazonian manatee) and T. manatus (West Indian manatee) exhibit large morphological, karyotypic, and molecular differences, a hybrid zone was identified on the northern coast of South America, from the Amazon River estuary toward the Guianas coastline. Two major populations or evolutionarily significant units (ESUs) within T. manatus , namely, the Caribbean and Atlantic, were separated and their differentiation was likely promoted or reinforced by the interspecific hybridization zone. We used nuclear and mtDNA sequences to reconstruct manatee speciation, population diversification through time and space, and secondary contact, which resulted in a hybrid zone. In this hybrid zone, the genetic contribution of each parental species was estimated, and different models for generating the current scenario were tested using statistical phylogeographic tools. All the results suggest a long hybridization history, during which a stable and structured hybrid swarm is generated. The coastline hybrid zone is composed of individuals with a lesser genomic contribution from T. inunguis ; this zone works as a genetic sink that restricts gene flow between neighbouring Atlantic (Brazil) and Caribbean (all others) T. manatus populations, which further reinforces the isolation and differentiation of the Brazilian manatees.