PHYLOGENETIC-RELATIONSHIPS OF ANNELIDS, MOLLUSKS, AND ARTHROPODS EVIDENCED FROM MOLECULES AND MORPHOLOGY

Annelids and arthropods have long been considered each other's closest relatives, as evidenced by similarities in their segmented body plans . An alternative view, more recently advocated by investigators who ha ve examined partial 18S ribosomal RNA data, proposes that annelids, mo lluscs, and certain other minor phyla with trochophore larva stages sh are a more recent common ancestor with one another than any do with ar thropods. The two hypotheses are mutually exclusive in explaining spir alian relationships. Cladistic analysis of morphological data does not reveal phylogentic relationships among major spiralian taxa but does suggest monophyly for both the annelids and molluscs. Distance and max imum-likelihood analyses of 18S rRNA gene sequences from major spirali an taxa suggest a sister relationship between annelids and molluscs an d provide a clear resolution within the major groups of the spiralians . The parsimonious tree based on molecular data, however, indicates a sister relationship of the Annelida and Bivalvia, and an earlier diver gence of the Gastropoda than the Annelida-Bivalvia clade. To test furt her hypotheses on the phylogenetic relationships among annelids, mollu scs, and arthropods, and the ingroup relationships within the major sp iralian taxa, we combine the molecular and morphological data sets and subject the combined data matrix to parsimony analysis. The resulting tree suggests that the molluscs and annelids form a monophyletic line age and unites the molluscan taxa to a monophyletic group, Therefore, the result supports the Eutrochozoa hypothesis and the monophyly of mo lluscs, and indicates early acquisition of segmented body plans in art hropods.