Molecular systematics of a holarctic rodent (Microtus : Muridae)

The Bering Land Bridge was the intermittent connection that allowed exchang e of mammals between Asia and North America. Because some mammalian genera are widely distributed on both continents, recovery of phylogenetic histori es of species within these genera may help reconstruct the sequence of inte rcontinental exchanges. We tested phylogenetic and biogeographic hypotheses in the widespread genus Microtus through parsimony and likelihood analysis of mtDNA-sequence data. The extant species of Microtus in North America ar e thought to be derived from multiple invasions from Asia or, alternatively , as a single invasion followed by autochthonous speciation. Mitochondrial cytochrome-b gene sequences were obtained for 78 individuals representing 2 4 species of Microtus. Data supported 1 clade of taiga voles (M. pennsylvan icus, M. montanus, M. townsendii, and M. canicaudus), a clade of Asian spec ies (M. kikuchii, M. fortis, M montebelli, and M. middendorffi), plus the H olarctic M. oeconomus and several other previously identified clades. M. gr egalis also was found to be distant from M. abbreviatus and M. miurus, thus contradicting monophyly of the subgenus Stenocranius. Monophyly of North A merican species was supported, albeit weakly. Basal relationships were not robust, reflecting a single pulse of diversification about 1.3 x 10(6) year s ago. This pulse mirrors the fossil record and may be partially responsibl e for the unstable taxonomic history.