Phylogenetic Relationships of the Oriental-Australian Rear-Fanged Water Snakes (Colubridae: Homalopsinae) Based on Mitochondrial DNA Sequences

The Homalopsinae (Oriental-Australian rear-fanged water snakes) is a small (34 species, 10 genera) colubrid subfamily notable for its ecological and morphological diversity. Despite considerable interest in the ecology and evolution of this group, phylogenetic relationships within the subfamily are poorly resolved. We present the results of a molecular phylogenetic study of the homalopsines based on partial sequence of three mitochondrial genes (12S and 16S ribosomal RNA and cytochrome b) from 14 ingroup species, five Old and New World natricines and the Old World colubrid, Dinodon semicarinatus. Maximum likelihood analysis in combination with bootstrapping and Bayesian Markov chain Monte Carlo methods for assessing phylogenetic confidence revealed that the single most likely topology contained a number of well-supported nodes. Homalopsine monophyly was strongly supported with respect to the outgroups included in our study. Cantoria violacea, a morphologically distinctive marine crustacean eater, formed the sister group to the rest of the homalopsines. Enhydris, the most species-rich genus in the subfamily, was polyphyletic with respect to other homalopsines although five morphologically and ecologically similar species formed a well-supported clade. The marine crustacean eaters Fordonia leucobalia and Gerarda prevostiana, also formed a novel clade. We discuss the evolutionary and ecological implications of this phylogeny for the Homalopsinae.

Fig. 1.

Phylogeny of the Homalopsinae. (A) Shown is the single most likely tree (−ln(l) = 7551.34) based on sequence from three mitochondrial genes: 12S, 16S, and cytb under a GTR model of sequence evolution with a shape parameter of 0.6723 for gamma-distributed rate heterogeneity and a proportion of invariant sites equal to 0.5334. Numbers above the nodes indicate Bayesian posterior probabilities calculated from the last 900,000 generations (sampling every 100 trees) of a 1,000,000 generation Markov Chain. Numbers below nodes indicate the bootstrap proportion calculated from 200 bootstrap replicates. Values below 50 are not reported. Enhydris is shown to be polyphyletic with respect to other homalopsines. Bayesian posteriors provide support for an Enhydris complex as well as for the topology within the Enhydris clade and strong support for two novel clades: (Gerarda + Fordonia) and Cerberus + (Erpeton + Homalopsis). Cantoria violacea receives moderate support as the sister group to the rest of the homalopsines. (B) Phylogram indicating maximum likelihood branch lengths. Shown is the maximum likelihood topology with branch lengths equal to the expected number of substitutions per site