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 anim
al phylogeny, Here we present evidence in favor of an arthropod-nematode cl
ade, from the comparison of beta -thymosin homologues among the Metazoa. Ar
thropods and nematodes share the absence of the highly conserved beta -thym
osin 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 dis
crete molecular character is phylogenetically very powerful and provides st
rong evidence for the monophyly of an arthropod-nematode clade.