SPONGE PHYLOGENY, ANIMAL MONOPHYLY, AND THE ORIGIN OF THE NERVOUS-SYSTEM - 18S RIBOSOMAL-RNA EVIDENCE

We sequenced 18S rRNA genes of a calcareous sponge, Clathrina cerebrum , a demosponge, Axinella polypoides, and a zoanthid cnidarian, Parazoa nthus arinellae. Our phylogenetic analysis supports the monophyly of k ingdom Animalia and confirms that choanoflagellate protozoans are thei r closest relatives. Sponges as a whole are monophyletic, but possibly paraphyletic; demosponges and hexactinellids form a monophyletic grou p of silicious sponges. Our phylogenetic trees support a monophyletic origin of the nervous system in the immediate common ancestor of Cnida ria and Ctenophora. They weakly suggest that animals with a nervous sy stem may be more closely related to calcareous sponges than to silicio us sponges; the nervous system might have originated in an early calca reous sponge. Our trees confirm that Myxozoa and Placozoa are animals that arose by secondary loss of the nervous system, but suggest that M yxozoa may be sisters of, rather than derived from, Bilateria. Kingdom Animalia is divided into four subkingdoms: Radiata (Porifera, Cnidari a, Placozoa, Ctenophora), Myxozoa, Mesozoa, and Bilateria. The 18S rRN A genes of Myxozoa evolved over twice as fast as in Radiata. Compariso n with the fossil record reveals a brief 10-fold (or greater) accelera tion in the rate of rRNA evolution in early Bilateria followed by norm al low rates for about 500 million years.