Meeting Abstract
Despite rapid advances in the study of metazoan phylogeny, the significance of several rare interstitial taxa remains unclear. The longest-known such taxon is Diurodrilus, widely considered a member of the phylum Annelida. However, Diurodrilus shares none of the common apomorphies of Annelida, instead presenting many morphological autapomorphies, as well as characters reminiscent of other animal phyla (especially Gastrotricha). A similar conundrum is presented by Lobatocerebrum, bearing some “turbellariform” characters, but also some characters reminiscent of Annelida and Gastrotricha. Finally, the genus Limnognathia has been posited as a member of Gnathifera due to its complex jaws; however, the limited available molecular data do not support this position, and morphological similarities to Diurodrilus have also been discussed. Here, we address these challenges using high-throughput sequencing, mining orthologous genes from RNA-seq data. Results from a phylogenetic analysis of cDNA and genome sequences from 90 taxa consistently position Limnognathia as sister to Syndermata within a monophyletic Gnathifera. Furthermore, the monophyly of Annelida, with both Lobatocerebrum and Diurodrilus as deeply-nested members, is robustly recovered under all but the simplest phylogeny reconstruction methods, consistent with the hypothesis of a progenetic origin of these interstitial taxa. However, both taxa are among the longest-branched annelids in our dataset, and their precise position within Annelida varies among analyses. This study also speaks to the status of Platyzoa, having added several additional deeply branching taxa from Gastrotricha, Platyhelminthes, and Gnathifera. Under unpartitioned ML the taxa of “Platyzoa” form a clade with “Polyzoa”. However, under the site-heterogeneous CAT+GTR+G4 model, Platyzoa is recovered as paraphyletic, with Gnathifera as the earliest-diverging branch; a clade of Lophophorata is also recovered under some conditions. Among the most prominent signals in this dataset is the high substitution rate of both the interstitial and colonial higher taxa (“Platyzoa” and “Polyzoa”); we discuss these rate variations both from the effect of their impact on phylogeny reconstruction, and from the standpoint of their biological provenance.
