Molecular phylogeny and divergence time estimates for major rodent groups:Evidence from multiple genes

The order Rodentia contains half of all extant mammal species, and from an evolutionary standpoint, there are persistent controversies surrounding the monophyly of the order, divergence dates for major lineages, and relations hips among families. Exons of growth hormone receptor (GHR) and breast canc er susceptibility (BRCA1) genes were sequenced for a wide diversity of rode nts and other mammals and combined with sequences of the mitochondrial 12S rRNA gene and previously published sequences of von Willebrand factor (vWF) . Rodents exhibit rates of amino acid replacement twice those observed for nonrodents, and this rapid rate of evolution influences estimates of diverg ence dates. Based on GHR sequences, monophyly is supported, with the estima ted divergence between hystricognaths and most sciurognaths dating to about 75 MYA. Most estimated dates of divergence are consistent with the fossil record, including a date of 23 MYA for Mus-Rattus divergence. These dates a re considerably later than those derived from some other molecular studies. Among combined and separate analyses of the various gene sequences, modera te to strong support was found for several clades. GHR appears to have grea ter resolving power than do 12S or vWF. Despite its complete unresponsivene ss to growth hormone, Cavia (and other hystricognaths) exhibits a conservat ive rate of change in the intracellular domain of GHR.