Nudibranchs of the Ross Sea, Antarctica: Phylogeny, diversity, and divergence.

摘要

The Southern Ocean (SO) surrounding Antarctica is extremely cold and geographically isolated. The phylogenetic affinities of only a few SO taxa have been examined in detail; in these, a high degree of endemism and radiation within the SO has been established using molecular phylogenetic methods. In order to address these Antarctic paradigms, we used Bayesian inference to construct phylogenetic trees of nudibranch molluscs based on mitochondrial cytochrome-c oxidase I (COI) and 18S ribosomal DNA. We gathered sequences from temperate (COI n=37; 18S n=31) and polar (COI n=21; 18S n=22) species and then combined them with sequences retrieved from GenBank (COI n=141; 18S n=91) in order to construct phylogenies using all available sequences. We found broad taxonomic diversity within the Nudibranchia of the Ross Sea and recovered reciprocally monophyletic clades of Anthobranchia and Cladobranchia as reported in previous molecular work. Estimates of divergence times of SO lineages from temperate taxa were calculated in three ways, with Bayesian branch lengths and using two molecular clock models implemented using BEAST v1.4.8, a program that jointly infers divergence times as well as phylogenetic relationships among taxa. The COI and 18S tree topologies both show 15 lineages (all >23% divergent at COI from the nearest sequence in the tree) of Antarctic nudibranchs. Seven of these 15 SO lineages contain a single taxon whose closest relative in the phylogeny is a temperate species, while the other eight are contained in three separate clades of SO lineages. This suggests radiation within the SO over the last 25-60 Mya, coinciding with glacial disturbance of the benthos and the initiation of the Antarctic Circumpolar Current.