Novelty in phylogeny of Gastrotricha: Evidence from 18S rRNA gene

Gastrotricha form a phylum which is crucial for defining the origin of pseu docoelomates, in that they share a number of characters with Rotifera and N ematoda but also with acoelomates, and even the evolutionary relationships within the phylum are anything but defined. For this reason the first exten sive molecular data on Gastrotricha from the 18S rRNA sequences of both ord ers have been obtained and analyzed. Sequence analyses show that the phylum Gastrotricha is strictly monophyletic along an evolutionary line quite dis tinct from that of both Rotifera and Nematoda. A new view of the evolutiona ry history of the phylum Gastrotricha is put forward, in which Chaetonotida , and not Macrodasyida, are the most primitive forms of the group, contrary to the commonly held view. A polyphyletic origin of aschelminthes is suppo rted, and the misleading term pseudocoelomates should be discarded. (C) 199 9 Academic Press.