Molecular phylogeny of the phylum Gastrotricha: New data brings together molecules and morphology

Gastrotricha is a species-rich phylum of microscopical animals that contains two main orders, Chaetonotida and Macrodasyida. Gastrotrichs are important members of the aquatic environment and significant players in the study of animal evolution. In spite of their ecological and evolutionary importance, their internal relationships are not yet well understood. We have produced new sequences for the 18S rDNA gene to improve both the quality and quantity of taxon sampling for the gastrotrichs. Our phylogeny recovers the monophyly of the two main Gastrotricha clades, in contrast to recent studies with similar sampling, but in agreement with morphology based analyses. However, our topology is not able to resolve the first branches of the macrodasyidans or settle the position of the puzzling Neodasys, a controversial genus classified as a chaetonotidan on morphological grounds but placed within macrodasyidans by molecular studies. This analysis is the most exhaustive molecular phylogeny of the phylum to date, and significantly increases our knowledge of gastrotrich evolution.

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► A new phylogeny of the phylum Gastrotricha using the 18S rRNA gene and wider sampling is presented.
► The results are more congruent with morphology than former comparable molecular works.
► The two main Gastrotricha clades, Macrodasyiida and Chaetonotida, are monophyletic.
► Macrodasyida radiation or the position of the puzzling Neodasys are not supported.
► These results shed new light into the evolution of their habitats and morphology.