| Trichoplax | |
|---|---|
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| Light microscope image ofTrichoplax (specimen ca. 0.5 mm across) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Placozoa |
| Class: | Uniplacotomia |
| Order: | Trichoplacea Tessleret al., 2022 |
| Family: | Trichoplacidae Bütschli & Hatschek, 1905 |
| Genus: | Trichoplax Schulze, 1883 [1] |
| Species: | T. adhaerens |
| Binomial name | |
| Trichoplax adhaerens Schulze, 1883 | |
| Synonyms | |
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Trichoplax adhaerens is one of the four named species in thephylumPlacozoa. The others areHoilungia hongkongensis,Polyplacotoma mediterranea andCladtertia collaboinventa. Placozoa is a basal group of multicellularanimals, possible relatives ofCnidaria.[2]Trichoplax are very flat organisms commonly less than 4 mm in diameter,[3] lacking anyorgans 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 possesscilia used in locomotion, and gland cells that lack cilia.[4] Between these layers is the fibresyncytium, a liquid-filled cavity strutted open by star-like fibres.
Trichoplax feed by absorbing food particles—mainlymicrobes—with their underside. They generallyreproduce asexually, bydividing orbudding, but can alsoreproduce sexually. ThoughTrichoplax has a small genome in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.
Trichoplax was discovered in 1883 by the German zoologistFranz Eilhard Schulze, in a seawater aquarium at the Zoological Institute inGraz, Austria. The generic name is derived from the classical Greekθρίξ (thrix), "hair", andπλάξ (plax), "plate". The specific epithetadhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.[5]
Although from the very beginning most researchers who studiedTrichoplax in any detail realized that it had no close relationship to other animal phyla, the zoologist Thilo Krumbach published a hypothesis thatTrichoplax is a form of the planula larva of theanemone-likehydrozoanEleutheria krohni in 1907. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals aboutTrichoplax until the 1960s. In the 1960s and 1970s a new interest among researchers led to acceptance ofPlacozoa as a new animal phylum. Among the new discoveries was study of the early phases of the animals' embryonic development and evidence that the animals that people had been studying are adults, not larvae. This newfound interest also included study of the organism in nature (as opposed to aquariums).[6]
Trichoplax generally has a thinly flattened, plate-like body in cross-section around half a millimetre, occasionally up to two or three millimetres. The body is usually only about 25 μm thick. Because they are so thin and fragile, and because the cilia which they use for locomotion are only loosely coordinated, they are constantly being split into two or three separate clones when their cilia moves in opposite directions, causing microfractures in the animal’s epithelium. One hypothesis is that the larger a motile animal lacking a nervous system is, the less coordinated its locomotion becomes, placing an upper limit on their possible size.[7][8] These colorlessly gray organisms are so thin they are transparent when illuminated from behind, and in most cases are barely visible to the naked eye. Like the single-celledamoebae, which they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form. These may facilitate movement to new habitats.
Trichoplax lacks tissues and organs; there is also no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers: dorsal epithelia cells and ventral epithelia cells, each with a single cilium ("monociliate"), ventral gland cells, syncytial fiber cells, lipophils, and crystal cells (each containing a birefringent crystal, arrayed around the rim). Lacking sensory and muscle cells, it moves usingcilia on its external surface.[9] The collective movements of the cilia are completely coordinated by mechanical interactions.[10]
There are no neurons present, but in the absence of a nervous system the animal uses short chains of amino acids known aspeptides for cell communication, in a manner resembling the way animals with neurons useneuropeptides for the same purpose. These specialized cells are called peptidergic cells, but unlike neurons they don't use electrical impulses and their messaging is restricted to sending signals to other nearby cells only, as they're unable to both send and receive signals.[11] Individual cells contain and secrete a variety of small peptides, made up of between four and 20 amino acids, which are detected by neighbouring cells. Each peptide can be used individually to send a signal to other cells, but also sequentially or together in different combinations, creating a huge number a different types of signals. This allows for a relatively complex behavioural repertoire, including behaviours such as "crinkling", turning, flattening, and internal "churning".[12] The genome of Trichoplax codes for eighty-five neurotransmitter receptors, more than in any other sequenced animal.[13]
Both structurally and functionally, it is possible to distinguish a back or dorsal side from a belly or ventral side inTrichoplax adhaerens. Both consist of a single layer of cells coated on the outside with slime and are reminiscent ofepithelial tissue, primarily due to the junctions—beltdesmosomes—between the cells. In contrast to trueepithelium, however, the cell layers of the Placozoa possess nobasal lamina, which refers to a thin layer of extracellular material underlying epithelium that stiffens it and separates it from the body's interior. The absence of this structure, which is otherwise to be found in all animals except the sponges, can be explained in terms of function: a rigid separating layer would make the amoeboid changes in the shape ofTrichoplax adhaerens impossible. Instead of an epithelium, therefore, we speak of anepitheloid in the Placozoa.
A mature individual consists of up to a thousand[clarification needed] cells that can be divided into four different cell types. The monociliated cells of the dorsal epitheloid are flattened and containlipid bodies. The cells on the ventral side likewise possess a single cilium, while their elongated columnar shape, with a small cross section at the surface, packs them very closely together, causing the cilia to be very closely spaced on the ventral side and to form a ciliated "crawling sole". Interspersed among these ventral epithlioid cells are unciliated gland cells thought to be capable of synthesizingdigestive enzymes.
Between the two layers of cells is a liquid-filled interior space, which, except for the immediate zones of contact with the ventral and dorsal sides, is pervaded by a star-shaped fibresyncytium: a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls (septa), do not have truecell membranes between them. Similar structures are also found in the sponges (Porifera) and manyfungi.
On both sides of the septa are liquid-filled capsules that cause the septa to resemblesynapses, i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues (Eumetazoa). Striking accumulations of calcium ions, which may have a function related to the propagation of stimuli, likewise suggest a possible role asprotosynapses. This view is supported by the fact that fluorescent antibodies againstcnidarian neurotransmitters, i.e. precisely those signal carriers that are transferred in synapses, bind in high concentrations in certain cells ofTrichoplax adhaerens, and thus indicate the existence of comparable substances in the Placozoa. The fibre syncytium also contains molecules ofactin and probably also ofmyosin, which occur in the muscle cells of eumetazoans[citation needed]. In the placozoans, they ensure that the individual fibres can relax or contract and thus help determine the animals' shape.
In this way, the fibre syncytium assumes the functions of nerve and muscle tissues. Moreover, at least a portion of digestion occurs here. On the other hand, no gelatinous extracellular matrix exists of the kind observed, inmesoglea, incnidarians andctenophores.
Pluripotent cells, which can differentiate into other cell types, have not yet been demonstrated unambiguously inT. adhaerens, in contrast to the case of the Eumetazoa. The conventional view is that dorsal and ventral epithelioid cells arise only from other cells of the same type.
TheTrichoplax genome contains about 98 millionbase pairs and 11,514 predicted protein-coding genes.[14]
All nuclei of placozoan cells contain six pairs[verification needed] ofchromosomes that are only about two to three micrometres in size. Three pairs aremetacentric, meaning that thecentromere, the attachment point for the spindle fibers in cell division, is located at the center, oracrocentric, with the centromere at an extreme end of each chromosome. The cells of the fiber syncytium can betetraploid, i.e. contain a quadruple complement of chromosomes.
A single complement of chromosomes inTrichoplax 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 bacteriumEscherichia coli is smaller by a factor of only ten.
The genetic complement ofTrichoplax adhaerens has not yet been very well researched; it has, however, already been possible to identify several genes, such asBrachyury andTBX2/TBX3, which are homologous to corresponding base-pair sequences in eumetazoans. Of particular significance isTrox-2, a placozoan gene known under the nameCnox-2 in cnidarians and asGsx in the bilaterally symmetricalBilateria. As a homeobox orHox 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 ofTrichoplax 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, arestem 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 as18S 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 thatTrichoplax 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 ofTrichoplax, 87% are identifiably similar to genes in cnidarians and bilaterians. In thoseTrichoplax genes for which equivalent genes can be identified in thehuman genome, over 80% of theintrons (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 theTrichoplax 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.[15]
The phylogenetic relationship betweenTrichoplax and other animals has been debated for some time. A variety of hypotheses have been advanced based on the few morphological characteristics of this simple organism that could be identified. More recently, a comparison of theTrichoplaxmitochondrial genome suggested thatTrichoplax is a basalmetazoan—less closely related to all other animals including sponges than they are to each other.[16] This implies that the Placozoa would have arisen relatively soon after the evolutionary transition from unicellular tomulticellular forms. But an even more recent analysis of the much largerTrichoplax nuclear genome instead supports the hypothesis thatTrichoplax is a basaleumetazoan, that is, more closely related toCnidaria and other animals than any of those animals are to sponges.[14] This is consistent with the presence inTrichoplax of cell layers reminiscent of epithelial tissue (see above).
Trichoplax was first discovered on the walls of a marine aquarium, and is rarely observed in its natural habitat.[17]Trichoplax has been collected, among other places, in the Red Sea, the Mediterranean, and the Caribbean, off Hawaii, Guam, Samoa, Japan, Vietnam, Brazil, and Papua New Guinea, and on the Great Barrier Reef off the east coast of Australia.[18]
Field specimens tend to be found in the coastaltidal zones of tropical and subtropical seas, on such substrates as the trunks and roots of mangroves, shells of molluscs, fragments of stony corals or simply on pieces of rock. One study was able to detect seasonal population fluctuations, the causes of which have not yet been deduced.
Trichoplax adhaerens feeds on small algae, particularly on green algae (Chlorophyta) of the genusChlorella, cryptomonads (Cryptophyta) of the generaCryptomonas andRhodomonas, and blue-green bacteria (Cyanobacteria) such asPhormidium inundatum, but also on detritus from other organisms. In feeding, one or several small pockets form around particles of nutrients on the ventral side, into which digestive enzymes are released by the gland cells; the organisms thus develop a temporary "external stomach", so to speak. The enclosed nutrients are then taken up bypinocytosis ("cell-drinking") by the ciliated cells located on the ventral surface.
Entire single-celled organisms can also be ingested through the upper epitheloid (that is, the "dorsal surface" of the animal). This mode of feeding could be unique in the animal kingdom: the particles, collected in a slime layer, are drawn through the intercellular gaps (cellular interstices) of the epitheloid by the fibre cells and then digested byphagocytosis ("cell-eating"). Such "collecting" of nutrient particles through an intact tegument is only possible because some "insulating" elements (specifically, a basal lamina under the epitheloid and certain types of cell-cell junctions) are not present in the Placozoa.
When the concentrations of algae are high the animals are more likely to engage in social feeding behavior.[19]
Not all bacteria in the interior of Placozoa are digested as food: in the endoplasmic reticulum, anorganelle of the fibre syncytium, bacteria are frequently found that appear to live insymbiosis withTrichoplax adhaerens.[20] Theseendosymbionts, which are no longer able to survive outside its host, are transferred from one generation to the next through both vegetative and sexual reproduction.[21]
Placozoa can move in two different ways on solid surfaces: first, their ciliated crawling sole lets them glide slowly across the substrate; second, they can change location by modifying their body shape, as an amoeba does. These movements are not centrally coordinated, since no muscle or nerve tissues exist. It can happen that an individual moves simultaneously in two different directions and consequently divides into two parts.[22]
It has been possible to demonstrate a close connection between body shape and the speed of locomotion, which is also a function of available food:
Since the transition is not smooth but happens abruptly, the two modes of extension can be very clearly separated from one another. The following is a qualitative explanation of the animal's behavior:
The actualdirection in whichTrichoplax moves each time is random: if we measure how fast an individual animal moves away from an arbitrary starting point, we find a linear relationship between elapsed time and mean square distance between starting point and present location. Such a relationship is also characteristic of randomBrownian motion of molecules, which thus can serve as a model for locomotion in the Placozoa.
Small animals are also capable of swimming actively with the aid of their cilia. As soon as they come into contact with a possible substrate, adorsoventral response occurs: the dorsal cilia continue to beat, whereas the cilia of ventral cells stop their rhythmic beating. At the same time, the ventral surface tries to make contact with the substrate; small protrusions and invaginations, themicrovilli found on the surface of the columnar cells, help in attaching to the substrate via their adhesive action.
UsingT. adhaerens as a model, 0.02–0.002 Hz oscillations in locomotory and feeding patterns were observed, and taken as evidence of complex multicellular integration, dependent on endogenous secretion of signal molecules. Evolutionarily conserved low-molecular-weight transmitters (glutamate, aspartate, glycine, GABA, and ATP) acted as coordinators of distinct locomotory and feeding patterns. Specifically, L-glutamate induced and partially mimicked endogenous feeding cycles, whereas glycine and GABA suppressed feeding. ATP-modified feeding is complex, first causing feeding-like cycles and then suppressing feeding. Trichoplax locomotion was modulated by glycine, GABA, and, surprisingly, by animals’ own mucus trails. Mucus triples locomotory speed compared to clean substrates. Glycine and GABA increased the frequency of turns.[13]
A notable characteristic of the Placozoa is that they can regenerate themselves from extremely small groups of cells. Even when large portions of the organism are removed in the laboratory, a complete animal develops again from the remainder. It is also possible to rubTrichoplax adhaerens through a strainer in such a manner that individual cells are not destroyed but are separated from one another to a large extent. In the test tube they then find their way back together again to form complete organisms. If this procedure is performed on several previously strained individuals simultaneously, the same thing occurs. In this case, however, cells that previously belonged to a particular individual can suddenly show up as part of another.
The Placozoa normally propagate asexually, dividing down the middle to produce two (or sometimes, three) roughly equal-sized daughters. These remain loosely connected[clarification needed] for a while after fission. More rarely,budding processes are observed: spherules of cells separate from the dorsal surface; each of these combines all known cell types and subsequently grows into an individual on its own.[citation needed]
Sexual reproduction is thought to be triggered by excessive population density. As a result, the animals absorb liquid, begin to swell, and separate from the substrate so that they float freely in the water. In the protected interior space, the ventral cells form an ovum surrounded by a special envelope, the fertilisation membrane; the ovum is supplied with nutrients by the surrounding syncytium, allowing energy-rich yolk to accumulate in its interior. Once maturation of the ovum is complete, the rest of the animal degenerates, liberating the ovum itself. Small, unciliated cells that form at the same time are interpreted to be spermatozoa. It has not yet been possible to observe fertilisation itself; the existence of the fertilisation membrane is currently taken to be evidence, however, that it has taken place.[citation needed]
Putative eggs have been observed, but they degrade, typically at the 32–64 cell stage. Neither embryonic development nor sperm have been observed. Despite lack of observation of sexual reproduction in the lab, the genetic structure of the populations in the wild is compatible with the sexual reproduction mode, at least for species of the analysed genotype H5.[23]
Usually even before its liberation, the ovum initiates cleavage processes in which it becomes completely pinched through at the middle. A ball of cells characteristic of animals, theblastula, is ultimately produced in this manner, with a maximum of 256 cells. Development beyond this 256-cell stage has not yet been observed.[14]
Trichoplax lack ahomologue of theBoule protein that appears to be ubiquitous and conserved in males of all species of other animals tested.[24] If its absence implies the species has no males, then perhaps its "sexual" reproduction may be a case of the above-described process of regeneration, combining cells from two separate organisms into one.[citation needed][original research?]
Due to the possibility of its cloning itself by asexual propagation without limit, the life span of Placozoa is infinite; in the laboratory, several lines descended from a single organism have been maintained in culture for an average of 20 years without the occurrence of sexual processes.[citation needed]
Long ignored as an exotic, marginal phenomenon,[tone]Trichoplax adhaerens is today[when?] viewed as a potential biological model organism.[citation needed] In particular, research is needed to determine how a group of cells that cannot be considered full-fledged epithelial tissue organizes itself, how locomotion and coordination occur in the absence of true muscle and nerve tissue, and how the absence of a concretebody axis affects the animal's biology. At the genetic level, the way in whichTrichoplax adhaerens protects against damage to its genome needs to be studied, particularly with regard to the existence of special DNA-repair processes.T. adhaerens can tolerate high levels of radiation damage that are lethal to other animals.[25] Tolerance to X-ray exposure was found to depend on expression of genes involved inDNA repair andapoptosis including the geneMdm2.[25] Complete decoding of the genome should also clarify the placozoans' place in evolution, which continues to be controversial.
Its ability to fight cancer through a combination of aggressive DNA repair and ejection of damaged cells makes it a promising organism for cancer research.[26]
In addition to basic research, this animal could also be suitable for studying wound-healing and regeneration processes; as yet unidentified metabolic products should be researched. Finally,Trichoplax adhaerens is also being considered as an animal model for testing compounds and antibacterial drugs.[27]
The related lineageTrichoplax sp. H2 has been suggested to be a more suitable model organism thanT. adhaerens, due to its abundance and ease of culture.[28]
Francesco Saverio Monticelli described another species in 1893, which he found in the waters around Naples, naming itTreptoplax reptans. However, it has not been observed since 1896, and most zoologists today doubt its existence.[citation needed]
Significant genetic differences have been observed between collected specimens matching the morphological description ofT. adhaerens, leading scientists to suggest in 2004 that it may be acryptic species complex.[29] At least twenty haplotypes have since been assigned based on the 16S mitochondrial DNA fragment, withT. adhaerens being equated to the lineage H1. While most haplotypes have not been formally described as species, they have been (with the exception of the morphologically distinct H0,Polyplacotoma mediterranea) provisionally placed into seven distinct clades. The genusTrichoplax was redefined as comprising clades I and II, including haplotypes H1, H2, H3 and H17.[30] A later study definedTrichoplax more restrictively as only clade I (haplotypes H1, H2 and H17), with H3 being suggested to belong to a separate undescribed genus in the family Trichoplacidae.[31]
Placozoan haplotypes are not necessarily equivalent to species, and several haplotypes of the related placozoan genusHoilungia have been found to belong to the same species.[31] Nonetheless, haplotype H2 is usually considered to be a separate undescribed species, referred to asTrichoplax sp. H2.[32] It has been reported to be more robust and abundant thanT. adhaerens, and easier to culture, making it a better fit for experimental research.[33]Trichoplax sp. H2 is also distinguished by the presence of an additional cell type, termed "epithelia upper-like", giving it a total of 29 cell types compared to the 28 inT. adhaerens.[34]
Comparative genetic studies ofTrichoplax adhaerens and the Panama strain ofTrichoplax sp. H2 have suggested that their genetic similarity might be due to an interbreeding event having happened in the wild at least several decades ago, with one of them being the result of hybridization between the other and a third unknown strain.[28] Analysis of bacterial endosymbionts supports this as a possible hypothesis, as the endosymbiont found in the Panama strain is closer to the one inT. adhaerens than to the one in the Hawaii strain ofTrichoplax sp. H2.[30]
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| Uniplacotomia |
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