MOLECULAR SYSTEMATICS AND PALEOBIOGEOGRAPHY OF THE SOUTH-AMERICAN SIGMODONTINE RODENTS

The murid rodent subfamily Sigmodontinae contains 79 genera which are distributed throughout the New World. The time of arrival of the first sigmodontines in South America and the estimated divergence time(s) o f the different lineages of South American sigmodontines have been con troversial due to the lack of a good fossil record and the immense num ber of extant species. The ''early-arrival hypothesis'' states that th e sigmodontines must have arrived in South America no later than the e arly Miocene, at least 20 MYA, in order to account for their vast pres ent-day diversity, whereas the ''late-arrival hypothesis'' includes th e sigmodontines as part of the Plio-Pleistocene Great American Interch ange, which occurred approximately 3.5 MYA, The phylogenetic relations hips among 33 of these genera were reconstructed using mitochondrial D NA (mtDNA) sequence data from the ND3, ND4L, arginine tRNA, and ND4 ge nes, which we show to be evolving at the same rate. A molecular clock was calibrated for these genes using published fossil dates, and the g enetic distances were estimated from the DNA sequences in this study. The molecular clock was used to estimate the dates of the South Americ an sigmodontine origin and the main sigmodontine radiation in order to evaluate the ''early-'' and ''late-arrival'' scenarios. We estimate t he time of the sigmodontine invasion of South America as between simil ar to 5 and 9 MYA, supporting neither of the scenarios but suggesting two possible models in which the invading lineage was either (1) ances tral to the oryzomyines, akodonts, and phyllotines or (2) ancestral to the akodonts and phyllotines and accompanied by the oryzomyines. The sigmodontine invasion of South America provides an example of the adva ntage afforded to a lineage by the fortuitous invasion of a previously unexploited habitat, in this case an entire continent.