Hybrid zones and avian diversification in Amazonia: a genomic perspective

Hybrid zones are critical areas for understanding the process of speciation. In Amazonia, the upper reaches of major rivers such as the Tapajós and Xingu represent areas where several phenotypically and genetically well-differentiated taxa (classified as species or subspecies) meet away from any noticeable physical barrier. A recent study demonstrated that gene flow occurs between several of those taxa in this area, although the extent and significance of introgression between has not been determined with confidence to date. Here, we further analyze several of those closely related taxon pairs in direct contact using next generation sequencing methods to unveil the local dynamics of gene flow. A first result was that lineages in direct contact across the Tapajós / Xingu headwaters are not each other closest relatives, suggesting that their current contact is a more recent event preceded by historical isolation and differentiation in allopatry. Second, measured levels of gene flow for some taxa indicate strong selection against hybrids, resulting in very narrow hybrid zones. In conclusion, complete reproductive isolation takes a very long time to evolve within Amazonian species complexes and apparently post-zygotic barriers play a more important role in reinforcing genetic differentiation than pre-zygotic barriers. Also, climate change may promote multiple events of isolation and contact between closely related lineages, enhancing genetic differentiation but eventually facilitating hybridization, which may have multiple outcomes.