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.
Hanelt
B
B
Department of Biology, University of New Mexico, Albuquerque 87131, USA.
Van Schyndel
D
D
Adema
C M
CM
Lewis
L A
LA
Loker
E S
ES
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