| Pseudocoelomates are a diverse assemblage of 10-12 distinct phyla grouped together because they lack a true body cavity. It has been proposed that pseudocoelomates are polyphyletic and pseudocoeloms are modified eucoeloms. It has also been proposed that acanthocephalans and rotifers are closely related and that ectoprocts are derived from entoprocts. The present investigation re-examines these relationships using DNA sequences of the entire 18S rRNA gene. The Clustal program was used to align five new sequences from representatives of phyla Annelida, Phoronida, Entoprocta, and Rotifera with previously published and unpublished sequences from a range of metazoans. Trees were prepared by the distance matrix and maximum parsimony methods. Five distinct branches of pseudocoelomate animals were detected: priapulans with protostomes; nematodes as a basal triploblastic lineage; the acanthocephalan/rotifer clade and entoproct clade as sister groups to the protostomes; and an isolated nematomorph branch. The analyses support the hypothesis that acanthocephalans and rotifers are related, reject the hypothesis that ectoprocts are derived from entoprocts, and lend support to the polyphyletic origin of pseudocoelomates. |
