Phylogenetic Systematics of South American Akodontine Rodents (Muridae: Sigmodontinae)

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Rats and mice of the South American murid rodent radiation (the largely endemic subfamily Sigmodontinae) represent a very large array of organisms that range in morphologic adaptations from subfossorial to aquatic to arborel and in habitat from tropical rainforest of the Amazon Basin to puna grassland in the highest elevations of the Andes. This adaptive radiation comprises some 70 genera, most of which are commonly divided into one of five major lineages. Among these, the akodontines are among the most diverse and oldest, with fossils of modern forms extending back some 6 million years in Argentina. Despite the widely recognized diversification of this lineage, most taxa are both poorly known and defined, and phylogenetic, ecologic, and life history aspects of their biology are either totally unknown or only barely so. This is particularly true for the akodontine group, a radiation of some 15 genera and about 50 species largely confined to grassland habitats from the northern Andes to the Patagonian steppes. The present research will utilize DNA sequences, obtained from several genes within the mithochrondrial genome by the technique of the Polymerase Chain Reaction, to address the following set of general goals: (1) establish an hypothesis of phylogenetic relationships among all genera and most species of the complex; (2) perform a text of the model of species formation resulting from divergence across strong habitat gradients, using selected species inhabiting the steep eastern slope of the Andes as exemplars; and (3) determine whether or not particular cranial conformations that identify highly derived taxa within this assemblage share a close common ancestor or whether this type of cranial structure has evolved repeatedly in separate lineages. The exciting aspect of this research is that, since DNA can be extracted and amplified from dried skin specimens already preserved within natural history collections, now extinct or exceedingly rare taxa can be compared in ways never before available. Five of the genera of concern, for example, are each known form very few specimens, most of which only collected once from very remote areas. This is, therefore, the first study of relationships within the group to examine all extant taxa in a comparative way.