Morphological and genetic variation between two sympatric forms of oxymycterus (Rodentia: Sigmodontinae):: An evaluation of hypotheses of differentiation within the genus

Species limits in the genus Oxymycterus have been controversial because of difficulties in detecting discrete morphological and cytogenetic variation. Several authors have considered many of the species described from Brazil to be conspecific with O. rufus, including O. dasytrichus described from Bahia. This tendency represents a null hypothesis for the species-level diversity within the genus. A recently proposed alternative hypothesis based on analyses of allopatric samples recognizes O. rufus and O. dasytrichus as valid species within rufus and dasytrichus species complexes, respectively. Two sympatric forms of Oxymycterus from Vicosa, Minas Gerais State, southeastem Brazil, are phenotypically similar to members of these 2 species-groups. Investigation on the status of these populations and their relationships to the rufus and dasytrichus complexes provided a test of the alternative hypothesis regarding diversity within the genus. Morphometric, morphological, and molecular (cytochrome-b DNA sequences) comparisons of samples representing the rufus, dasytrichus, and judex species-groups provided evidence that the 2 sympatric forms from Vicosa are distinct evolutionary units, one related to the dasytrichus species-group and the other to the rufus species-group. The structures of morphological and molecular variation within each species-group supported an isolation-by-distance model of phenotypic differentiation in the rufus group, and a smooth pattern of phenotypic differentiation along a north-south axis in the dasytrichus group. The relevance of these patterns for species delimitation within these groups and the validity of O. dasytrichus and O. rufus are discussed.