Placozoa Are Not Derived Cnidarians: Evidence from Molecular Morphology

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Abstract
The phylum Placozoa is represented by a single known species, Trichoplax adhaerens, a tiny marine organism that represents the most simple metazoan bauplan. Because of the latter, placozoans were originally considered the most basal metazoan phylum. A misinterpretation of the life cycle at the turn of the century and some more recent molecular phylogenetic analyses have placed Trichoplax as a derived species within the Cnidaria. The latter hypothesis assumes that the primitive organization of the Placozoa is the result of secondary reduction. Here we compare the molecular morphology of the predicted 16S rDNA structure and the mitochondrial genome between Trichoplax and representatives of all four cnidarian classes. Trichoplax shares a circular mtDNA molecule as a plesiomorphy with all other metazoans except for the derived cnidarian classes Hydrozoa, Scyphozoa, and Cubozoa. The predicted secondary structure of the 16S rRNA molecule differs substantially between Trichoplax and cnidarians, particularly with respect to the number and length of stem and loop regions. The new molecular morphological characters provide compelling evidence that Trichoplax is not a derived (medusozoan) cnidarian. Furthermore, it was found that the mitochondrial genome in Cubozoa consists of four linear molecules instead of a single circular molecule or two linear molecules, suggesting that the cubozoans may represent the most derived cnidarian group.