DiscoveryEditPlacozoans were first discovered on the walls of an aquarium in 1883. The were found by German scientist F. E. Schulze. Schulze's observations were skewed in textbooks and journals after his initial discovery. This was due to no other Trichoplax being found until 70 years later. "Then, in 1969, the noted German protozoologist Karl Grell redis-covered Trichoplax in Red Sea algal samples." Since Grell's find, many scientists have gathered more Trichoplax, but no solid effort has been made to make additional clones in the lab. (1)
Trichoplax adhaerens is the only species of the Placozoan phylum. However, due to difference in the placozoan lines, more species are estimated to exist. Trichoplax has no organs or tissues and it lacks symmetry in its body. (2) In appearence it seems to be flat discus composed of two skin layers with a layer of multinucleate fibre cells inbetween. The organism moves in no regular manner via cilia. Trichoplax is composed of only 4 somatic cells and lacks any nerve, sensory, or muscle cells. (3) Placozoan are also able to regenerate themselves from a very small amount of cells when seperated. Trichoplax is asexual. To reproduce, it either ultilizes budding (rare) or more commonly implements fission. (4)
The genome of Trichoplax was sequenced using whole genome shotgun sequencing methods. These methods found that the genome has about 98 million base pairs and 11514 protein coding genes. (3) 80% of the protein coding genes are shared with cnidarians and bilaterians. Of the introns in the genome, ~80% are shared with humans. It is interesting that Trichoplax has retained introns since other organisms with small genomes have evolved to minimize non-coding regions and to a state that does not retain ancestral genome organization. (5) Nature has asserted that the introns shared with humans "have orthologous counterparts with the same position and phase in Trichoplax."
(The following subheadings have come from Nature. The body of each is a summary of the content in the article.)
Putative Developmental Transcription FactorsEdit
Trichoplax has a significant amount of developmental transcription factors included in its genome that are "commonly associated with ... eumetazoan development." Transcription factor families include: Homeobox, Helix-loop-helix, Zinc Finger, Sox, Fox, T-box, bZip, ETS, along with corresponding subfamiles. (3)
- Homeobox factors: involved in regulating morphogenesis
- Sox factors: "involved in the regulation of embryonic development" (3)
- T-box factors: involved in limb and heart development.
- Helix-loop-helix factors: involved in neural and muscle cell regulation
- Zinc Finger factors: "participate in the specification of endodermal, cardiac and blood cell fates" (3)
- Fox factors: regulates expression of cell growth, proliferation, differentiation, longevity, and sometimes embryonic development
- ETS factors: involved in tissue development
Putative Developmental Signaling PathwaysEdit
In Trichoplax, multiple pathways are present despite its small genomic size. The Wnt /b-catenin signaling pathway is fully present within Trichoplax adhaerens. This pathway is “used for axial patterning in bilaterians and cnidarians.” It is interesting that this pathway is complete in the organism since it does not possess symmetry in its body. The transforming growth factor beta (TGF-b ) signaling pathway is also included in the Trichoplax genome. It is not complete, however, all the essential components are present. This pathway is involved in many cellular processes including (but not limited to) apoptosis, cell division/growth, and homeostasis. Notch and JAK/STAT pathways are also present but are not complete in the genome. Both of these pathways “lack molecular components critical to signal transduction.” It is noted in Nature that the hedgehog pathway may be included in the Trichoplax genome but it is not functional. The assumption that the pathway is non-functioning lies in the observations that no “hedgehog ligand, patched or smoothened receptors, or Gli-like transcription factor” are apparent.
It is important to note the differences in pathways present in Trichoplax relative other organisms. The lack of certain animal signaling pathways in comparison to the presence of pathways that are/were contained in cnidarians/bilaterians suggests, according to Nature scientists, that Trichoplax “branched off from an ancestor that either did not possess all animal [signaling] pathways or that these genes were lost in the placozoan lineage.”
Elements Associated with Neuroendocrine FunctionEdit
Trichoplax’s neuroendocrine system is interesting since Trichoplax has no nervous system. All environmental reactions are in response to signal responses outside of the nervous system. Trichoplax contains ion channels. These channels are “implicated in the neural signaling in animals.” The channels included are: Members of the Kv family of voltage dependent potassium channel a-subunits and b-subunits, inward rectifier potassium channels, homologues of voltage-gated sodium channels, and voltage-gated L-type calcium channels.
Other elements associated with neuroendocrine function are neurotransmitter biosynthesis, vesicle transport systems, and putative neuroendocrine-like secretory apparatuses. Only certain components of these systems are present. The present components include: DOPA decarboxylase, DBH-like monooxygenase, and vesicular amine transporters.
The final elements are the neurotransmitter and neuropeptide receptors. Those present include: seven transmembrane G-protein- coupled receptors (GPCRs), four opsin genes, eighty-five members of the class 3 GPCR family, transmembrane proteins, and synapse formation proteins.
Extracellular Matrix and Cell AdhesionEdit
The Tricoplax genome encodes extracellular matrix proteins. The proteins encoded include: collagen IV, laminin-a, -b and -c, and nido- gen. Typical ECM proteins fibronectin, fibrin, elastin and vitronectin are, however, not encoded for.
It is significant that Trichoplax encode for ECM proteins because it does not have an extracellular matrix. It is however possible, according to Nature scientists, that the ECM may dodge traditional histological stains and still be present in Trichoplax.
- Briggs, D. (2204). The Peabody Meseum home to the Trichoplax genome project. Yale Environmental News, 9(2), 8.