Early evolution of the Metazoa and phylogenetic status of diploblasts as inferred from amino acid sequence of elongation factor-1 alpha.

To understand the early evolution of the Metazoa, it is necessary to determine the correct phylogenetic status of diploblastic animals (poriferan, cnidaria, and ctenophora). Despite clasdistic studies of morphological characters and recent molecular phylogenetic studies, it remains uncertain whether diploblasts are monophyletic or paraphyletic, and how these three phyla of diploblasts are phylogenetically related. To obtain insight into these phylogenetic problems, we sequenced almost the entire nucleic acid sequence of elongation factor-1 alpha from a sponge, two cnidarians, a ctenophora, and a turbellarian. We then investigated the phylogenetic status of the diploblasts. We compared the amino acid sequences, nucleotide sequences at the first and second codon positions, and those at the second positions. Phylogenetic trees were inferred by neighbor-joining, maximum likelihood, and maximum parsimony, and they supported the monophyly of the Metazoa. However, the phylogenetic relationships of the diploblast groups were not significantly resolved, although the trees preferred the monopoly of the diploblasts.