* ITZ, Ecology and Evolution, Tierärztliche Hochschule Hannover, Hannover, Germany
Department of Molecular Cellular and Developmental Biology, Yale University, New Haven, Connecticut
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Abstract |
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Key Words: Trichoplax adhaerens Placozoa Cnidaria molecular systematics mtDNA genome structure 16S rDNA molecular morphology
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Introduction |
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Materials and Methods |
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Results and Discussion |
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The amplified 16S rDNA region corresponds to the 3' half of the gene and showed significant length variation among taxa. Sequence analyses revealed a remarkable size difference between the Placozoa and the Cnidaria. The representatives of the four cnidarian classes harbor comparatively short fragmentsCarybdea marsupialis 482 bp, Tripedalia cystophora 480 bp (estimated from gel electrophoresis), Eleutheria dichotoma 518 bp, Aurelia aurita 521 bp, Nematostella vectensis 604 bp-whereas the homologous region in Placozoa reads 933 bp. This observation by itself supports a more distant rather than a close phylogenetic relationship between the Placozoa and Cnidaria. Further evidence for the latter hypothesis derives from structural information of the 16S rRNA molecule (fig. 3). The uncorrected alignment of 16S sequences comprises 903 nucleotide positions. Ambiguous regions, which in most cases surround variable loops introduced by the Trichoplax sequence, were removed (alignment is available upon request). The DNA sequences of the reduced alignment of 357 bp did not harbor sufficient phylogenetic information to unequivocally resolve phylogenetic relationships at the given taxonomic level of phyla and classes (phylograms not shown).
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Both the mitochondrial genome (linear vs. circular) and 16S secondary structure morphology provide compelling evidence that (1) the placozoan T. adhaerens is not a derived (medusozoan) cnidarian and (2) the Cubozoa are the most derived taxon within the Cnidaria with respect to these characters. Furthermore the data suggest a distant rather than any close relationship between the Placozoa and Cnidaria. Complete mtDNA genome sequences of the Porifera (Werner Mueller, work in progress) and Placozoa (The Trichoplax Consortium, work in progress) will finally decide on the phylogenetic placement of T. adhaerens within the diploblasts.
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Acknowledgements |
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Footnotes |
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Literature Cited |
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