MicroRNAs and phylogenomics resolve the relationships of Tardigrada, and suggest the velvet worms are the sister group of Arthropoda

Morphological data traditionally group Tardigrada (water bears), Onychophora (velvet worms), and Arthropoda (e.g. spiders, insects and their allies) in a monophyletic group of invertebrates with walking appendages: the Panarthropoda. Molecular data, however, do not generally support the inclusion of tardigrades within the Panarthropoda, placing them closer to Nematoda (roundworms) instead. Here we present results from the analyses of two independent genomic data sets (expressed sequence tags – ESTs, and microRNAs), congruently resolving the phylogenetic relationships of Tardigrada. Our EST analyses significantly support a monophyletic Panarthropoda including Tardigrada and suggest a sister group relationship between Arthropoda and Onychophora. Using careful experimental manipulations – comparisons of model fit, signal dissection, and taxonomic sampling– we were able to show that support for a Nematoda plus Tardigrada group derives from the phylogenetic artefact of long branch attraction – LBA. Our small RNA libraries – including the first data for tardigrades and onychophorans – fully support our EST results: no microRNAs were found to link Tardigrada and Nematoda, while all panarthropods were found to share one microRNA (miR-276). In addition, Onychophora and Arthropoda were found to share a second microRNA (miR-305). To corroborate our results we have applied a methodology similar to that used for the EST dataset to a ribosomal RNA (rRNA) dataset, reaching again the same conclusions. Our study confirming the monophyly of the legged invertebrates, shows that past support for tardigrades plus nematodes was due to LBA, and suggest that the velvet worms are the most likely sister group of Arthropoda.