PHYLOGENETIC POSITION OF THE HEXACTINELLIDA WITHIN THE PHYLUM PORIFERA BASED ON THE AMINO-ACID-SEQUENCE OF THE PROTEIN-KINASE-C FROM RHABDOCALYPTUS-DAWSONI

Recent analyses of genes encoding proteins typical for multicellularit y, especially adhesion molecules and receptors, favor the conclusion t hat all metazoan phyla, including the phylum Porifera (sponges), are o f monophyletic origin. However, none of these data includes cDNA encod ing a protein from the sponge class Hexactinellida. We have now isolat ed and characterized the cDNA encoding a protein kinase C, belonging t o the C subfamily (cPKC), from the hexactinellid sponge Rhabdocalyptus dawsoni. The two conserved regions, the regulatory part with the pseu dosubstrate site, the two zinc fingers, and the C2 domain, as well as the catalytic domain were used for phylogenetic analyses. Sequence ali gnment and construction of a phylogenetic tree from the catalytic doma ins revealed that the yeast Saccharomyces cerevisiae and the protozoan Trypanosoma brucei are at the base of the tree, while the hexactinell id R. dawsoni branches off first among the metazoan sequences; the oth er two classes of the Porifera, the Calcarea (the sequence from Sycon raphanus was used) and the Demospongiae (sequences from Geodia cydoniu m and Suberites domuncula were used), branch off later. The statistica lly robust tree also shows that the two cPKC sequences from the higher invertebrates Drosophila melanogaster and Lytechinus pictus are most closely related to the calcareous sponge. This finding was also confir med by comparing the regulatory part of the kinase gene. We suggest, t hat (i) within the phylum Porifera, the class Hexactinellida diverged first from a common ancestor to the Calcarea and the Demospongiae, whi ch both appeared later, and (ii) the higher invertebrates are more clo sely related to the calcareous sponges.