Genetic differentiation and habitat connectivity across towhee hybrid zones in Mexico
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Hybridization can either reinforce or erode species boundaries; therefore, hybrid zones offer a natural experimental setting in which to assess the dynamics of reproductive isolation. Secondary contact zones, in particular, present a partial separation of the original divergence mechanisms and the subsequent genomic architecture of reproductive isolation (or lack thereof). The spatial context of secondary contact and its consequent effect on dispersal play vital roles on the contact’s outcome. In a hybrid complex between two towhee species in Mexico, Pipilo maculatus and Pipilo ocai, two major hybrid gradients provide natural replicates for comparison. However, genetic analyses demonstrate significant divergence between geographically separate parental populations of each species and divergence of populations within each hybrid zone. The two hybrid transects (Teziutlán and Transvolcanic) are distinct and evidence suggests allelic introgression both across the species boundary and between the two transects. Habitat corridors for dispersal represent functional connectivity hotspots where the two transects meet. Both habitat connectivity and genetic differentiation between geographically disparate parental populations appear to influence the dynamics of gene flow across the hybrid gradients. In southern sympatric populations (Mt. Orizaba and Oaxaca) where morphological evidence for hybridization is scarce, opposing species’ alleles appear to traverse through the hybrid zones rather than arising from cryptic local hybridization. These results illustrate the importance of environmentally mediated gene flow in the context of secondary contact as an important force influencing evolutionary trajectory.
KeywordsGene flow Habitat connectivity Hybrid zone Introgression Dispersal
We appreciate greatly Blanca Hernández for extending us collecting permits in Mexico. We thank Laura Villaseñor, Sarahy Contreras, Alejandro Gordillo, Gala Cortés, Daniela Tovilla-Sierra, Rubén Ortega, Marco Ortiz, Mauricio Pérez-Vera, Laila Yunes-Jiménez, and César A. Ríos-Muñoz for their invaluable guidance, leadership, and assistance in the field across the Transvolcanic Belt in Mexico. We thank David Braun, Stephen Braun, Nicholas Lanson, Nimrod Funk, and George Oliver for their able assistance in the field in the Teziutlán gradient. Thanks to Darrilyn Albright for labwork assistance. We thank our funding sources for this project: Smithsonian Research Endowment Fund, University of Maryland Program in Behavior, Ecology, Evolution, and Systematics, Smithsonian Ornithology, and the Department of Vertebrate Zoology, National Museum of Natural History (NMNH). S. Kingston received research assistantships from the NMNH Frontiers in Phylogenetics Program and NSF grants DEB0228675 and DEB0733029.
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