Sigmodontine rodents diversified in South America prior to the complete rise of the Panamanian Isthmus

The extant distribution of sigmodontine rodents encompasses most of the New World, and the majority of the species in this subfamily inhabit South America. Nevertheless, the basal lineages of the Sigmodontinae are distributed in North and Central America, and the fossil record indicates a North American origin. This evidence has produced contentious theories concerning the evolution of these rodents. The dispute usually stems from a disagreement about the way in which sigmodontines reached South America, which was an isolated landmass during most of the Cenozoic. Fundamentally, the debate is associated with the role of Panamanian Isthmus formation and the Great American Biotic Interchange (GABI) in the diversification of the clade. An early hypothesis implies that sigmodontines arrived in South America before the complete rise of the Panamanian Isthmus, whereas a late hypothesis directly correlates the diversification of the lineage with this event. To address this question, we have sequenced nuclear and mitochondrial sequences, as well as the first Sigmodontinae mitochondrial genomes (Akodon montensis and Wiedomys cerradensis) and performed a Bayesian dating analysis. Our results showed that the most recent common ancestor of the subfamily lived at approximately 15 Ma. Although the diversification of sigmodontines was not associated with the complete rise of the Panamanian Isthmus, we cannot exclude the hypothesis that this event played a relevant role in the evolution of the lineage during the Miocene.