Trichoplax adhaerens at the Root of Multicellular Animal Tree

Trichoplax adhaerens at the Root of Multicellular Animal Tree

The following materials are uncritically posted from wiki and are not new discoveries. I have been looking into Trichoplax adhaerens after seeing a lot of analysis in the Hox-TALE paper rely on this animal.

Trichoplax adhaerens is the only extant representative of phylum Placozoa, which is a basal group of multicellular animals (metazoa). Trichoplax are very flat organisms around a millimeter in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated “cover cells” flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia.[2] Between these layers is the fiber syncytium, a liquid-filled cavity strutted open by star-like fibers.


The Trichoplax genome contains about 98 million base pairs and 11,514 predicted protein-coding genes.[5]

All nuclei of placozoan cells contain six pairs[verification needed] of chromosomes that are only about two to three micrometres in size. Three pairs are metacentric, meaning that the centromere, the attachment point for the spindle fibers in cell division, is located at the center, or acrocentric, with the centromere at an extreme end of each chromosome. The cells of the fiber syncytium can be tetraploid, i.e. contain a quadruple complement of chromosomes.

A single complement of chromosomes in Trichoplax adhaerens contains a total of fewer than fifty million base pairs and thus forms the smallest animal genome; the number of base pairs in the intestinal bacterium Escherichia coli is smaller by a factor of only ten.

The genetic complement of Trichoplax adhaerens has not yet been very well researched; it has, however, already been possible to identify several genes, such as Brachyury and TBX2/TBX3, which are homologous to corresponding base- pair sequences in eumetazoans. Of particular significance is Trox-2, a placozoan gene known under the name Cnox-2 in cnidarians and as Gsx in the bilaterally symmetrical Bilateria. As a homeobox or Hox gene it plays a role in organization and differentiation along the axis of symmetry in the embryonic development of eumetazoans; in cnidarians, it appears to determine the position of mouth-facing (oral) and opposite-facing (aboral) sides of the organism. Since placozoans possess no axes of symmetry, exactly where the gene is transcribed in the body of Trichoplax is of special interest. Antibody studies have been able to show that the gene’s product occurs only in the transition zones of the dorsal and ventral sides, perhaps in a fifth cell type that has not yet been characterized. It is not yet clear whether these cells, contrary to traditional views, are stem cells, which play a role in cell differentiation. In any case, Trox-2 can be considered a possible candidate for a proto-Hox gene, from which the other genes in this important family could have arisen through gene duplication and variation.

Initially, molecular-biology methods were applied unsuccessfully to test the various theories regarding Placozoa’s position in the Metazoa system. No clarification was achieved with standard markers such as 18S rDNA/RNA: the marker sequence was apparently “garbled”, i.e. rendered uninformative as the result of many mutations. Nevertheless, this negative result supported the suspicion that Trichoplax might represent an extremely primitive lineage of metazoans, since a very long period of time had to be assumed for the accumulation of so many mutations.

Of the 11,514 genes identified in the six chromosomes of Trichoplax, 87% are identifiably similar to genes in cnidarians and bilaterians. In those Trichoplax genes for which equivalent genes can be identified in the human genome, over 80% of the introns (the regions within genes that are removed from RNA molecules before their sequences are translated in protein synthesis) are found in the same location as in the corresponding human genes. The arrangement of genes in groups on chromosomes is also conserved between the Trichoplax and human genomes. This contrasts to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non- coding regions and a loss of the ancestral genome organizations.[6]

Written by M. //