The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis.
The Orthonectida is a small, poorly known phylum of parasites of marine invertebrates. Their phylogenetic placement is obscure; they have been considered to be multicellular protozoans, primitive animals at a "mesozoan" grade of organization, or secondarily simplified flatworm-like organisms. The best known species in the phylum, Rhopalura ophiocomae, was collected on San Juan Island, Wash. and a complete 18S rDNA sequence was obtained. Using the models of minimum evolution and parsimony, phylogenetic analyses were undertaken and the results lend support to the following hypotheses about orthonectids: (1) orthonectids are more closely aligned with triploblastic metazoan taxa than with the protist or diploblastic metazoan taxa considered in this analysis; (2) orthonectids are not derived members of the phylum Platyhelminthes; and (3) orthonectids and rhombozoans are not each other's closest relatives, thus casting further doubt on the validity of the phylum Mesozoa previously used to encompass both groups.
Department of Biology, University of New Mexico, Albuquerque 87131, USA.
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Hanelt
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Van Schyndel
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Adema
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Lewis
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Loker
D000818:Animals; D004275:DNA, Ribosomal; D056890:Eukaryota / Q000145:classification / Q000235:genetics; D005260:Female; D006376:Helminths / Q000145:classification / Q000235:genetics / Q000502:physiology; D007448:Invertebrates / Q000145:classification / Q000235:genetics; D008957:Models, Genetic; D010271:Parasites / Q000145:classification* / Q000235:genetics* / Q000502:physiology; D010802:Phylogeny; D010986:Platyhelminths / Q000145:classification / Q000235:genetics; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA
D004275:DNA, Ribosomal