A comprehensive molecular phylogeny of tardigrades-adding genes and taxa to a poorly resolved phylum-level phylogeny

Abstract

Tardigrades constitute a phylum of miniaturized metazoans with ca. 1030 living species, a fossil record that probably dates back to the Cambrian, and physiological properties that allow them to live in almost any environment known to host life on Earth—they can also survive in space. Despite broad consensus regarding their membership of the superclade Ecdysozoa, the exact position of the phylum remains contested (some analyses suggest onychophorans and arthropods as their closest relatives, while others suggest a relationship to nematodes and nematomorphs) and the internal relationships of the phylum are still poorly understood. In the present study, we present a hypothesis of tardigrade relationships by examining more taxa and more markers than any previously published phylogeny of the group. We generated novel data for three markers (18S rRNA, 28S rRNA, COI) for 42 individuals of 16 carefully identified species, comprising 12 genera and five families from the classes Heterotardigrada and Eutardigrada, and analysed them in conjunction with nearly all data available from GenBank. Our results show certain disagreement with current taxonomy both at higher ranks (families, orders, classes) and at low (generic) taxonomic levels. When studying the sensitivity to outgroup choice, the class Eutardigrada was monophyletic under only one combination of outgroups; all other combinations placed the eutardigrade order Apochela as sister to the class Heterotardigrada. Phylogenetic relationships within the other eutardigrade order, Parachela, were stable to outgroup choice. Eutardigrade superfamilies recently proposed by Sands and collaborators in the order Parachela were tested with the introduction of new sequences from additional genera, and the possible morphological synapomorphies supporting those superfamilies are discussed.