Journal of Mammalian Evolution, Vol. 6, No. 2, 1999 Phylogenetic Relationships and the Radiation of Sigmodontine Rodents in South America: Evidence from Cytochrome b Margaret F. Smith 1,2 and James L. Patton 1 Phylogenetic relationships among South American sigmodontine rodents were examined based on the complete sequence for the mitochondrial cytochrome b gene [1140 base pairs (bp)] for 66 species and between 759 and 1140 bp for an additional 19 species. Thirty-eight South American genera were represented, coming from eight of nine tribes. Outgroups included the North American murid rodents Peromyscus, Reithrodontomys, Scotinomys, and Neotoma, the Old World murine rodents Mus and Rattus, and the geomyoid genera Thomomys, Geomyx, Dipodomys, and Perognathus as the most distant outgroup. The South American sigmodontines were supported as a monophyletic lineage. Within this radiation several clear-cut suprageneric groupings were identified. Many of the currently recognized tribal groupings of genera were found fairly consistently, although not always with high levels of bootstrap support. The various tribes could not be linked hierarchically with any confidence. In addition, several genera stand out as unique entities, without any apparent close relatives. The overall pattern suggests a rapid radiation of the sigmodontines in South America, followed by differentiation at the tribal and generic levels. KEY WORDS: murid rodents; Sigmodontinae; mtDNA sequences; cytochrome b; phylogeny; biogeography. INTRODUCTION New World murid rodents have fascinated and challenged neontologists and paleontolo- gists due to the diversity of forms, the difficulty in assessing relationships, and the tan- talizing potential of the group for illuminating a chapter in the biogeographic history of North and South America. The predominantly South American sigmodontines will be treated here as a separate subfamily, the Sigmodontinae (sensu Reig, 1980), distinct from a predominantly North American subfamily Neotominae, the neotomine-peromyscines (Reig, 1980), and a Central American subfamily Tylomyinae (Reig, 1984). Steppan (1995) discusses this taxonomic arrangement in detail. South American sigmodontine rodents encompass an impressive array of forms, both in numbers and in adaptive diversity. Mem- 1 Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California, Berkeley, Cal- ifornia 94720. 2 To whom correspondence should be addressed. e-mail: pegsmith@uclink4.berkeley.edu 89 1064-7554/99/0600-0089$16.00/0 © 1999 Plenum Publishing Corporation