ORIGIN OF THE MESOZOA INFERRED FROM 18S RIBOSOMAL-RNA GENE-SEQUENCES

The phylum Mesozoa comprises small, simply organized wormlike parasite s of marine invertebrates and is composed of two classes, the Rhombozo a and the Orthonectida. The origin of Mesozoa is uncertain; they are c lassically considered either as degenerate turbellarians or as primiti ve multicellular animals related to ciliated protists. In order to pre cisely determine the phylogenetic position of this group we sequenced the complete 18S rRNA gene of one rhombozoid, Dicyema sp., and one ort honectid, Rhopalura ophiocomae. The sequence analysis shows that the M esozoa branch early in the animal evolution, closely to nematodes and myxozoans. Our data indicate probably separate origins of rhombozoids and orthonectids, suggesting that their placement in the same phylum n eeds to be revised.