The sister group relationship of Aeolosomatidae and Potamodrilidae (Annelida: “Polychaeta”) - a molecular phylogenetic approach based on 18S rDNA and cytochrome oxidase I

Aeolosomatidae and Potamodrilidae are small meiofauna annelids of apparently simple organization and uncertain phylogenetic position. Potamodrilidae was regarded either as a subtaxon of Aeolosomatidae, united with them as Aphanoneura, or entirely unrelated to Aeolosomatidae. Moreover, the groups have been placed in various positions within Annelida: as sister group of Clitellata, as a highly derived clitellate taxon, or excluded from Clitellata and not closely related to them due to great morphological differences. Although molecular studies give strong support for the exclusion of these two taxa from Clitellata their questionable sister group relationship to each other has not been addressed specifically. In the present study sequences of the nuclear 18S rDNA and the mitochondrial Cytochrome Oxidase I gene were used for addressing this question. In addition to the available nuclear 18S rDNA sequences, partial sequences of Cytochrome Oxidase I of Rheomorpha neiswestnovae (Lastochkin, 1935) and Potamodrilus fluviatilis Lastochkin, 1935 along with other polychaete taxa were determined. Combined analyses of these two genes were conducted using Maximum Parsimony and Bayesian analysis. A sister group relationship of Aeolosomatidae and Potamodrilidae is significantly supported in all. As in previous studies a relationship to Clitellata is not supported but the phylogenetic position of both Aeolosomatidae and Clitellata within the polychaetes remains enigmatic.