Insights into the evolutionary and demographic history of the extant endemic rodents of the Galápagos Islands

Evolutionary radiations stemming from colonization of archipelagos provide valuable insights into mechanisms and modes of speciation.  For this reason, the fauna inhabiting the Galapagos Islands has been the focus of numerous emblematic ecological and evolutionary studies.  However, studies focused on rodents have been scarce.  Rice rats radiated in situ into at least seven endemic species: Aegialomys galapagoensis, Nesoryzomys narboroughi, N. swarthi, N. fernandinae, N. indefessus, N. darwini and Megaoryzomys curioi.  Only the first four species remain extant on the archipelago.  These species are considered vulnerable, mainly due to human activities and invasive species.  Despite their interesting evolutionary history, questions surrounding phylogenetic relationships, colonization events, genetic diversity and demography of populations remain unresolved.  We used the D-loop region of mtDNA to infer phylogenetic relationships, colonization events, date divergences, and conduct population genetic analyses of the four extant endemic species inhabiting the Galapagos Islands.  We found that all species were monophyletic. A. galapagoensis is sister to A. xanthaeolus from the continent, and both of them are the sister clade of the genus Nesoryzomys.  Our results also showed that there were two colonization events to the islands.  The first event was the arrival of the ancestor of Nesoryzomys during the Pliocene, when divergences between genera occurred.  The second was Aegialomys during middle Pleistocene, when species diversification began.  Populations on each island show high genetic diversity and most show signals of recent expansion.  However, future studies are needed to accurately assess the conservation status of these populations.  We suggest ongoing monitoring of these vulnerable endemic species, including ecological and population genetic studies.  In addition, future studies using genome-wide molecular markers and additional species from the continent, as well as sampling extinct species from the islands, will improve our knowledge about the origin and relationships of the endemic rodents of the Galapagos Islands.