Sponge paraphyly and the origin of Metazoa

In order to allow critical evaluation of the interrelationships between the three sponge classes, and to resolve the question of mono- or paraphyly of sponges (Porifera), we used the polymerase chain reaction (PCR) to amplify almost the entire nucleic acid sequence of the 18S rDNA from several hexac tinellid, demosponge and calcareous sponge species. The amplification produ cts were cloned, sequenced and then aligned with previously reported sequen ces from other sponges and nonsponge metazoans and variously distant outgro ups, and trees were constructed using both neighbour-joining and maximum pa rsimony methods. Our results suggest that sponges are paraphyletic, the Cal carea being more related to monophyletic Eumetazoa than to the siliceous sp onges (Demospongiae, Hexactinellida). These results have important implicat ions for our understanding of metazoan origins, because they suggest that t he common ancestor of Metazoa was a sponge. They also have consequences for basal metazoan classification, implying that the phylum Porifera should be abandoned. Our results support the upgrading of the calcareous sponge clas s to the phylum level.