PHYLOGENETIC POSITION OF THE TARDIGRADA BASED ON THE 18S-RIBOSOMAL-RNA GENE-SEQUENCES

The phylogenetic position of the Tardigrada remains uncertain. This is due to the limited information available, and the uncertainty of whet her some characters are homologous or analogous with other taxa. Based on some morphological characters, current discussion centres on wheth er the taxon branches from the annelid-arthropod lineage, or lies with in the arthropod complex. The molecular data presented here from an an alysis of the 18S rRNA gene sequences are used to test the validity of these two hypotheses. Phylogenetic inference by the maximum parsimony and distance (neighbour-joining) methods suggests that the Tardigrada is a sister group of the major protostome eucoelomate assemblage that emerged before the arthropods, annelids, molluscs, and sipunculids ev olved. The tardigrade clade also appears as an independent lineage sep arate from the nematode clade, thus supporting the current idea that t ardigrades do not have a close aschelminth relationship. The molecular data also imply that several morphological features, considered signi ficant in determining the phylogenetic relationships of tardigrades, a re not synapomorphic characters. (C) 1996 The Linnean Society of Londo n