Molecular Phylogeny Of The Bear Species (Ursidae)

The bears, which form the family Ursidae, are the best known heavyweights of the land Carnivores. This family includes seven species and has been suggested to consist of either two genera or seven genera. Up to now, the taxomic classifications and phylogenetic relationships within the Ursidae remain issues of controversies and uncertainties. The main problem is that the family Ursidae represents a rapid evolutionary radiation and speciation events.In this paper, we sequenced two single-copy nuclear gene (exon 1 sequence from the gene encoding interphotoreceptor retinoid binding protein (IRBP) and intron 1 from the transthyretin (TTR) gene ) in all extant bear species and giant panda for addressing the relationships among closely related bear species from the perspective of nuclear genes at DNA sequence level.The nuclear gene trees engendered in our study bear little resemblance to previous findings derived from mt sequence data. Phylogenetic analysis suggested that spectacled bear was clearly shown to form a distant lineage from the other six bear species. The topological concordance of the data sets supported a close relationship between the lineage to brown bear/ polar bear cluster and two black bears. In the ML analysis of combined data, the lineage (brown bear/polar bear) was more closely related to American black bear than was Asiatic black bear, however, there was low bootstrap support for this relationship. Both TTR intron and combined two-gene trees clearly indicated sister-taxa relationship between sun bear and sloth bear. We suggested that the spectacled bear be placed in its own genus Tremarctos, and other six bear species be classified into genus Ursus. Our present analyses also revealed that introns as a molecular marker might have the potential to be more appropriate for resolving phylogenetic relationships among closely related species, as recently derived bear species in our case.