A new karyological variant of Oecomys (Rodentia: Sigmodontinae) and its phylogenetic relationship based on molecular data.

A new karyological variant of the Sigmodontinae genus Oecomys was detected in specimens from Brazilian Pantanal. Karyologic analyses showed 2n = 72 and FN = 90, which differs from all known chromosomal complements of the genus. This new karyomorph showed distinctive morphological attributes and could not be attributed to any described Oecomys species. Molecular analysis using cytochrome-b sequence data suggested the monophyly of Oecomys. Maximum-likelihood analyses placed the Oecomys sp. haplotype clade as the sister branch to Oecomys bicolor, whereas maximum-parsimony analyses did not resolve its relationship with the other Oecomys species. Kimura's two parameters distance estimates between this and other Oecomys species are high, and active gene flow has been found not to coexist with these divergence estimates.