MOLECULAR PHYLOGENY OF THE ANNELIDA AND INSIGHTS INTO THE EVOLUTION OF MORPHOLOGICAL CHARACTERS
McHugh, D.
Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, U.S.A.
The Annelida has been traditionally defined as a group of segmented, worms with chitinous chaetae, comprising two sister classes, the clitellates (oligochaetes and leeches) and the large class of mainly marine polychaetes; the Arthropoda, the Mollusca, the Echiura, and the Pogonophora have each been proposed as the sister group to the Annelida. I used molecular sequences from the nuclear coding gene, elongation factor-1a, to analyze the phylogenetic relationships of the annelids (McHugh (1997) (PNAS 94:8006-8009). These molecular analyses, the first to include the appropriate taxa to address annelid relationships, support placement of the Pogonophora, the unsegmented Echiura, and the clitellates within a paraphyletic grade of polychaete annelids. In contrast, a recent morphology-based phylogenetic study of the relationships of polychaete annelids (Rouse and Fauchald (1997) Zool. Scripta 26:139-204) supports a sister relationship between clitellates and polychaetes, and a monophyletic Polychaeta that includes the Pogonophora. I will discuss the implications of the conflicting results from molecular and morphological data. Furthermore, I will argue that homology assessments for some morphological characters may currently be best made in the light of a molecular-based phylogenetic framework.