COLLINS, AG; MEDINA, M; COLLINS, JA; SCHIERWATER, B; TiHo-Hannover; Joint Genome Institute; TiHo-Hannover; TiHo-Hannover: 28S rDNA Data Further Resolve the Phylogeny of Cnidaria

Cnidaria has been the subject of several analyses of molecular data that attempt to resolve evolutionary relationships among and within its component groups. These studies have renewed interest in cnidarian character evolution and may be partly responsible for spurring the numerous investigations of cnidarian relationships based on morphological and life history features. This bounty of analyses aimed at achieving the single best phylogenetic hypothesis has produced an unprecedented number of alternative ideas about the evolutionary history of Cnidaria. Great uncertainty would appear to exist when only a short time ago we enjoyed unprecedented optimism concerning the tractability of phylogenetic questions. However, independently proposed phylogenetic trees based on diverse data, when looked at as a set of hypotheses, show strong congruence of results. We further clarify the emerging view of cnidarian phylogeny by using nearly complete 28S rDNA sequences (roughly 3,300 bp). Diverse data now indicate that Hydrozoa and Cubozoa are monophyletic, while Scyphozoa is not. The sessile members of Stauromedusae are likely to be the sister group of all other medusozoans. Thus, the ancestral medusozoan was probably solitary, biradial in its early ontogeny, with four intramesogleal muscles of ectodermal origin associated with peristomial pits surrounding the mouth. From this state, hydrozoan and cubozoan polyps are probably simplified. The ancestral hydrozoan probably produced medusae with a velum via lateral budding from polyps and development involving an entocodon. Gastric filaments, hollow sensory structures derived ontogenetically from tentacles, a circular coronal muscle, and septa of the medusa probably were lost in the ancestry of Hydrozoa, perhaps indicating (though not confirming) independent origins of the medusa.