Out of Antarctica?--new insights into the phylogeny and biogeography of the Pleurobranchomorpha (Mollusca, Gastropoda).

The aim of the current study was to gain new insights into the phylogeny, biogeography and evolution of the opisthobranch clade Pleurobranchomorpha. We focused on testing the hypothesis of an Antarctic origin of this clade. The combination of four gene markers (18S rDNA, 28S rDNA, 16S rDNA and CO1) was used to infer a phylogenetic hypothesis of the Pleurobranchomorpha employing Maximum likelihood and Bayesian inference methods. Four methodologically distinct approaches were applied to reconstruct the historical biogeography and dating of the tree was performed via relaxed molecular clock analysis. Phylogenetic analyses supported the monophyly of the Pleurobranchomorpha and their sister group relationship to the Nudibranchia. Monophyly of the main subgroups Pleurobranchaeinae and Pleurobranchinae could not be revealed. Reconstruction of the ancestral area of the Pleurobranchomorpha yielded different possibilities in the diverse analyses. However, the Pleurobranchinae most probably derived from an Antarctic origin. Estimation of divergence times revealed a long credible interval for the Pleurobranchomorpha, whereas the Pleurobranchinae diverged in Early Oligocene and underwent rapid radiation during Oligocene and Early Miocene. Divergence of the Pleurobranchinae into the Antarctic Tomthompsonia and the remaining species in Early Oligocene coincides with two major geological events; namely the onset of glaciation in Antarctica and the opening of the Drake Passage with following formation of the Antarctic Circumpolar Current (ACC). These sudden and dramatic changes in climate probably led to subsequent migration of the last common ancestor of the remaining Pleurobranchinae into warmer regions, while the ACC may have accounted for larval dispersal to the Eastern Atlantic.