INTERNAL AND EXTERNAL RELATIONSHIPS OF THE CNIDARIA - IMPLICATIONS OFPRIMARY AND PREDICTED SECONDARY STRUCTURE OF THE 5'-END OF THE 23S-LIKE RDNA

Since both internal (class-level) and external relationships of the Cn idaria remain unclear on the basis of analyses of 18S and (partial) 16 S rDNA sequence data, we examined the informativeness of the 5'-end of the 23S-like rDNA. Here we describe analyses of both primary and pred icted secondary structure data for this region from the ctenophore Bol inopsis sp., the placozoan Trichoplax adhaerens, the sponge Hymeniacid on heliophila, and representatives of all four cnidarian classes. Prim ary sequence analyses clearly resolved the Cnidaria from other lower M etazoa, supported sister group relationships between the Scyphozoa and Cubozoa and between the Ctenophora and the Placozoa, and confirmed th e basal status of the Anthozoa within the Cnidaria. Additionally, in t he ctenophore, placozoan and sponge, noncanonical base pairing is requ ired to maintain the secondary structure of the B12 region, whereas am ongst the Cnidaria this is not the case. Although the phylogenetic sig nificance of this molecular character is unclear, our analyses do not support the close relationship between Cnidaria and Placozoa suggested by previous studies.