Phylogeny of Thaumastodermatidae (Gastrotricha:Macrodasyida) Inferred from Nuclear and Mitochondrial Sequence Data

Background: Phylogenetic relationships within Gastrotricha are poorly known. Attempts to shed light on this subject usingmorphological traits have led to hypotheses lacking satisfactory statistical support; it seemed therefore that a differentapproach was needed.Methodology/Principal Findings: In this paper we attempt to elucidate the relationships within the taxonomically vastfamily Thaumastodermatidae (Macrodasyida) using molecular sequence data. The study includes representatives of all theextant genera of the family and for the first time uses a multi-gene approach to infer evolutionary liaisons withinGastrotricha. The final data set comprises sequences of three genes (18S, 28S rDNA and COI mtDNA) from 41 species,including 29 thaumastodermatids, 11 non-thaumastodermatid macrodasyidans and a single chaetonotidan. Molecular datawas analyzed as a combined set of 3 genes and as individual genes, using Bayesian and maximum likelihood approaches.Two different outgroups were used: Xenotrichula intermedia (Chaetonotida) and members of the putative basalDactylopodola (Macrodasyida). Thaumastodermatidae and all other sampled macrodasyidan families were foundmonophyletic except for Cephalodasyidae. Within Thaumastodermatidae Diplodasyinae and Thaumastodermatinae aremonophyletic and so are most genera. Oregodasys turns out to be the most basal group within Thaumastodermatinae inanalyses of the concatenated data set as well as in analyses of the nuclear genes. Thaumastoderma appears as the sistertaxon to the remaining species. Surprisingly, Tetranchyroderma is non-monophyletic in our analyses as one group of speciesclusters with Ptychostomella while another appears as the sister group of Pseudostomella.Conclusions/Significance: Results in general agree with the current classification; however, a revision of the more derivedthaumastodermatid taxa seems necessary. We also found that the ostensible COI sequences from several species do notconform to the general invertebrate or any other published mitochondrial genetic code; they may be mitochondriallyderived nuclear genes (numts), or one or more modifications of the mitochondrial genetic code within Gastrotricha.