Phylogenetic relationships of otters (Carnivora : Mustelidae) based on mitochondrial cytochrome b sequences

Otters are classified in the Lutrinae and are united by a suite of semi-aqu atic adaptations that distinguish them from the rest of the Mustelidae. Pas t systematic studies have been based primarily on overall similarity of mor phological characters but have not been concordant, with regard either to r elationships within the subfamily or to the relationships of the Lutrinae t o other taxa in the Mustelidae. We evaluate the relationships among nine of the 13 species of otters and their position in the Mustelidae through phyl ogenetic analysis of the complete nucleotide sequence of the mitochondrial cytochrome b gene. Based on the sequence data presented here, our results s uggest that otters are divided into three primary clades that include: (1) the North American river, neotropical and marine otters; (2) the sea, Euras ian, spotted-necked, cape clawless and small-clawed otters; and (3) the gia nt otter. Furthermore, our results indicate that among sampled mustelid tax a, members of the genus Mustela are most closely related to otters. The hie rarchical relationships among clades are not well resolved, particularly th e monophyly of otters, largely because a pattern of short internal branches combined with long terminal branches suggests a rapid evolutionary radiati on. Estimates of divergence time calibrated by the fossil record suggest th at the lineages leading to the North American river, neotropical and marine otters, and the giant otter diverged before the end of the Miocene, much e arlier than predicted from the fossil record.