Phylogeny of Eutardigrada: new molecular data and their morphological support lead to the identification of new evolutionary lineages.

An extensive study of the phylogeny of Eutardigrada, the largest class of Tardigrada, has been performed analyzing one hundred and forty sequences (eighty of which newly obtained) representative of one hundred and twenty-nine specimens belonging to all families (except Necopinatidae) of this class. The molecular (18S and 28S rRNA) results were compared with new and previous morphological data, allowing us to find new phylogenetic relationships, to identify new phylogenetic lineages, to erect new taxa for some lineages, and to find several morphological synapomorphies supporting the identified clusters. The class Eutardigrada has been confirmed and, within it, the orders Apochela and Parachela, the superfamilies Macrobiotoidea, Hypsibioidea, Isohypsibioidea, and Eohypsibioidea, and all the families and subfamilies considered, although with emended diagnoses in several cases. In addition, new taxa have been erected: the new subfamily Pilatobiinae (Hypsibiidae) with the new genus Pilatobius, as well as an upgrading of Diphascon and Adropion to genus level, previously considered subgenera of Diphascon. Our results demonstrate that while molecular analysis is an important tool for understanding phylogeny, an integrative and comparative approach using both molecular and morphological data is necessary to better elucidate evolutionary relationships.

MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that 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) into a monophyletic group of invertebrates with walking appendages known as the Panarthropoda. However, molecular data generally do not support the inclusion of tardigrades within the Panarthropoda, but instead place them closer to Nematoda (roundworms). Here we present results from the analyses of two independent genomic datasets, expressed sequence tags (ESTs) and microRNAs (miRNAs), which congruently resolve the phylogenetic relationships of Tardigrada. Our EST analyses, based on 49,023 amino acid sites from 255 proteins, 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 pruning--we show that support for a Tardigrada + Nematoda group derives from the phylogenetic artifact of long-branch attraction. Our small RNA libraries fully support our EST results; no miRNAs were found to link Tardigrada and Nematoda, whereas all panarthropods were found to share one unique miRNA (miR-276). In addition, Onychophora and Arthropoda were found to share a second miRNA (miR-305). Our study confirms the monophyly of the legged ecdysozoans, shows that past support for a Tardigrada + Nematoda group was due to long-branch attraction, and suggests that the velvet worms are the sister group to the arthropods.