FIRST MOLECULAR EVIDENCE FOR THE EXISTENCE OF A TARDIGRADA PLUS ARTHROPODA CLADE

The complete 18S rDNA gene sequence of Macrobiotus group hufelandi (Ta rdigrada) was obtained and aligned with 18S rDNA and rRNA gene sequenc es of 24 metazoans (mainly protostomes). Discrete character (maximum-p arsimony) and distance (neighbor-joining) methods were used to infer t heir phylogeny. The evolution of bootstrap proportions with sequence l ength (pattern of resolved nodes, PRN) was studied to test the resolut ion of the nodes in neighbor-joining trees. The results show that arth ropods are monophyletic. Tardigrades represent the sister group of art hropods (in parsimony analyses) or they are related with crustaceans ( distance analysis and PRN). Arthropoda are divided into two main evolu tionary lines, the Hexapoda + Crustacea line (weakly supported), and t he Myriapoda + Chelicerata line. The Hexapoda + Crustacea line include s Pentastomida, but the internal resolution is far from clear. The Ins ecta (Ectognatha) are monophyletic, but no evidence for the monophyly of Hexapoda is found. The Chelicerata are a monophyletic group and the Myriapoda cluster close to Arachnida. Overall, the results obtained r epresent the first molecular evidence for a Tardigrada + Arthropoda cl ade. In addition, the congruence between molecular phylogenies of the Arthropoda from other authors and this obtained here indicates the nee d to review those obtained solely on morphological characters.