A molecular phylogenetic framework for the phylum Ctenophora using 18S rRNA genes

This paper presents the first molecular phylogenetic analysis of the phylum Ctenophora, by use of 18S ribosomal RNA sequences from most of the major t axa. The ctenophores form a distinct monophyletic group that, based on this gene phylogeny, is most closely related to the cnidarians. Our results sug gest that the ancestral ctenophore was tentaculate and cydippid-like and th at the presently recognized order Cydippida forms a polyphyletic group. The other ctenophore orders that we studied (Lobata, Beroida, and Platyctenida ) are secondarily derived from cydippid-like ancestors, a conclusion that i s also supported by developmental and morphological data. The very short ev olutionary distances between characterized ctenophore 18S rRNA gene sequenc es suggests that extant ctenophores are derived from a recent common ancest or. This has import-ant consequences for future studies and for an understa nding of the evolution of the metazoans. (C) 2001 Academic Press.