Adaptive introgression from distant Caribbean islands contributed to the diversification of a microendemic radiation of trophic specialist pupfishes
Authors: Richards Emilie J. , Martin Christopher H. 
Published: March 2017

Rapid diversification often involves complex histories of gene flow that leave variable and conflicting signatures of evolutionary relatedness across the genome. Identifying the extent and source of variation in these evolutionary relationships can provide insight into the evolutionary mechanisms involved in rapid radiations. Here we compare the discordant evolutionary relationships associated with species phenotypes across 42 whole genomes from a sympatric adaptive radiation ofCyprinodonpupfishes endemic to San Salvador Island, Bahamas and several outgroup pupfish species in order to understand the rarity of these trophic specialists within the larger radiation ofCyprinodon.82% of the genome depicts close evolutionary relationships among the San Salvador Island species reflecting their geographic proximity, but the vast majority of the fixed variants between the specialist species lie in regions with discordant topologies. These regions include signatures of selective sweeps and adaptive introgression from neighboring islands into each of the specialist species. Hard selective sweeps of genetic variation on San Salvador contributed 10-fold more to divergence between specialist species within the radiation than adaptive introgression of Caribbean genetic variation; however, some of these introgressed regions from distant islands were associated with the primary axis of oral jaw divergence within the radiation. For example, standing variation in a proto-oncogene (ski) known to have effects on jaw size introgressed into one San Salvador specialist from an island 300 km away. The complex emerging picture of the origins of adaptive radiation on San Salvador indicates that multiple sources of genetic variation contributed to the adaptive phenotypes of novel trophic specialists on the island. Our findings suggest that a suite of factors, including rare adaptive introgression, may also be required to trigger adaptive radiation in the presence of ecological opportunity.

Author summary

Groups of closely related species can rapidly evolve to occupy diverse ecological roles, but the ecological and genetic conditions that trigger this diversification are still highly debated. We examine patterns of molecular evolution across the genomes of a rapid radiation of pupfishes that includes two trophic specialists. Despite apparently widespread ecological opportunities and gene flow across the Caribbean, this radiation is endemic to a single Bahamian Island. Using the whole genomes of 42 pupfish we find evidence of extensive and previously unexpected variation in evolutionary relatedness among Caribbean pupfish. Two sources of genetic variation have contributed to the adaptive diversification of complex phenotypes in this system: selective sweeps of genetic variation from across the Caribbean that was brought into San Salvador through hybridization and genetic variation found on San Salvador. While genetic variation from San Salvador appears to be relatively more common in the divergence observed among specialists, hybridization probably played an important role in the evolution of the complex phenotypes as well. Our findings that multiple sources of genetic variation contribute to the San Salvador radiation suggest that a complex suite of factors, including hybridization with other species, may be required to trigger adaptive radiation in the presence of ecological opportunity.