Phylogeny and evolution of the Echiniscidae (Echiniscoidea, Tardigrada) – an investigation of the congruence between molecules and morphology

Abstract

The heterotardigrade family Echiniscidae consists of 12 currently recognized genera that are generally represented by few species with the exception of the genera Echiniscus and Pseudechiniscus, which have numerous species. The echiniscids inhabit a wide variety of different moss and lichen substrates. Phylogenetic relationships within the Echiniscidae have previously been investigated using morphological characters. This study includes molecular data from the nuclear 18S and 28S genes and the mitochondrial COI gene, as well as the morphological data to investigate the incongruence between the studies by Kristensen (1987) and Jorgensen (2000) and between morphology and molecules. The arthrotardigrades Batillipes and Florarctus and the echiniscoideans Echiniscoides and Oreella have been included as out-groups and ten genera of echiniscids as the in-group in the phylogenetic analyses. The data sets were analysed individually and combined with Bayesian inference, maximum likelihood and maximum parsimony. In this study, Echiniscidae was always inferred to be monophyletic in analyses of the combined data set (18S, 28S and morphology) excluding COI. The analyses inferred a ‘Pseudechiniscus lineage’ not including Mopsechiniscus. Furthermore, the inferred ‘Pseudechiniscus lineage’ only included part of the Pseudechiniscus species making the genus polyphyletic. The five Echiniscus species, which were included in the analysis, were monophyletic and sister-group to Testechiniscus (COI had Echiniscus paraphyletic). The clade [Hypechiniscus, (Echiniscus + Testechinsicus)] was also well supported. Parechiniscus was inferred to be the sister-group to all other echiniscid taxa, a phylogenetic position that corresponds well with its weakly sclerotized dorsal plates. Echiniscoidea was not inferred to be monophyletic in contradiction to recently phylogenetic analyses focused on Arthrotardigrada.