The comparison of β-thymosin homologues among metazoa supports an arthropod-nematode clade

The definition of an Ecdysozoa clade among the protostomians, including all phyla with a regularly molted alpha -chitin-rich cuticle, has been one of the most provocative hypotheses to arise from recent investigations on animal phylogeny, Here we present evidence in favor of an arthropod-nematode clade, from the comparison of beta -thymosin homologues among the Metazoa. Arthropods and nematodes share the absence of the highly conserved beta -thymosin form found in all other documented bilaterian phyla as well as sponges, and the possession of a very unusual, internally triplicated homologue of the beta -thymosin protein, unknown in other phyla. We argue that such discrete molecular character is phylogenetically very powerful and provides strong evidence for the monophyly of an arthropod-nematode clade.