Mitochondrial genomes to the rescue – Diurodrilidae in the myzostomid trap

• Determination of the mitochondrial genome and nuclear rRNA cluster of Diurodrilus subterraneus (Diurodrilidae).
• Nuclear rRNA data placed Diurodrilidae in Annelida.
• Mitochondrial genome data support placement of Diurodrilidae in Annelida.
• Diurodrilidae are not closely related to Platyzoa.
• The origin of interstitial Diurodrilidae occured most likely by progenetic evolution.

Diurodrilidae is a taxon of Lophotrochozoa comprising about six, exclusively interstitial species, which are up to 500 μm long and dorsoventrally flattened. Traditionally, Diurodrilidae had been regarded as an annelid family. However, recently Diurodrilidae had been excluded from Annelida and been placed in closer relationship to platyzoan taxa based on both morphological and nuclear rRNA data. Since both, Diurodrilidae and platyzoan taxa, exhibit long branches in the molecular analyses, the close relationship might be due to a long branch attraction artifact. The annelid taxon Myzostomida had been trapped in a similar long branch attraction artifact with platyzoan taxa using nuclear rRNA data, but determination of the nearly complete mitochondrial genome of myzostomids revealed their annelid affinity. Therefore, we determined the nearly complete mitochondrial genome of Diurodrilus subterraneus as well as new nuclear rRNA data for D. subterraneus and some platyzoan taxa. All our analyses of nuclear rRNA and mitochondrial sequence and gene order data presented herein clearly place Diurodrilidae within Annelida and with strong nodal support values in some analyses. Therefore, the previously suggested exclusion of Diurodrilidae from Annelida and its close relationship with platyzoan taxa can be attributed to a long branch artifact. Morphological data do not unambiguously support a platyzoan affinity of Diurodrilidae, but instead would also be in line with a progenetic origin of Diurodrilidae within Annelida.

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