Identidade e alocação taxonômica de duas formas simpátridas de Oxymycterus (Rodentia: Sigmodontinae) do sudeste do Brasil: uma avaliação de hipóteses sobre a diversidade do gênero

Abstract

Oxymycterus is a diverse neotropical rodent genus distributed at the middle latitudes of South America. The nomenclature of the genus remains obscure due to the difficulties in the morphological definition of species and to the controversies on the application of a biological species concept to alopatric samples. Therefore, several authors have considered the species described from Brazil as synonyms of O. rufus (Fischer, 181 4), including O. dasytrichus described from Bahia, a tendency that represent a null hypothesis on the species diversity of the genus in southeastern Brazil. The discovery of two sympatric forms of Oxymycterus in Viçosa, Minas Gerais, phenotypically similar to members of these groups, prompted \ investigations about the status of these populations as evolutionary units and their relationship with the rufus and dasytrichus species groups, which provided a framework to test the hypotheses of validity of these names. Two approaches were used to allow taxonomic inferences: 1) a morphometric · analysis of 22 geographic samples of the dasytrichus, rufus and judex groups, complemented by the analyses of qualitative cranial, phallic and externai characters; and 2) a phylogenetic analysis of cytochrome-b mitochondrial gene nucleotide sequences. The morphological and molecular evidence suggest the following conclusions: 1) the two sympatric forms from Viçosa comprise two genetically and evolutionary independent species; 2) these species were allocated to the dasytrichus and rufas species groups, thus supporting the validity of these names as distinct taxonomic units; 3) the dasytrichus group was characterized by northernsouthern geographic gradient of increase in cranial dimensions, including the darker form from Viçosa; 4) the rufus group was revealed to include only one species, represented by samples from Argentina and from one of the sympatric forms of Viçosa, suggesting a discontinuous geographic distribution for this species group; 5) the morphological and molecular divergences of one sympatric form from Viçosa m relation to the argentine samples suggest an isolation by distance model of differentiation due to a recent geographic structuring of O. Rufus.