Lamellar

Franz Schulze

Trichoplax was first discovered by the Austrian zoologist Franz Schulze in 1883 on the walls of a marine aquarium at the University of Graz Zoological Institute, containing plant and animal samples found in the Gulf of Trieste of the Adriatic Sea . Schulze called this strange organism in the form of whitish plates of an irregular shape Trichoplax adhaerens . The name of the genus is derived from other Greek. θρίξ (genus τριχός ) “hair” and πλάξ “plate”, and lat. adhaerens "sticking" ^{[4] [5]} . At the same time, Schulze found that the side of the animal’s body facing the substrate ( ventral ) is covered with a high ciliary epithelium, and the opposite ( dorsal ) side is covered with a flat ciliary epithelium (the use of the term “ epithelium ” in the case of trichoplax is not entirely correct; see below ) ^{[ 6]} .

Shortly after the publication of Schulze's works, the Italian scientist F. Monticelli discovered a form close to trichoplax in the marine aquarium of the Neapolitan biological station and called it Treptoplax reptans . The main difference between Treptoplax reptans and Trichoplax adhaerens was the absence of cilia on the dorsal side. However, nobody found Treptoplax after Monticelli, and the very existence of this organism is doubtful: Monticelli simply could not notice the sparse cilia located on the dorsal side of trichoplax on histological sections ^[6] .

The discovery of Trichoplax adhaerens aroused great interest, and several new works on its morphology and biology were soon published. Many biologists considered trichoplax as an intermediate form between unicellular and multicellular organisms. Some considered it close to the ancestors of the intestinal turbellaria ( Acoela ) ^{[7] [8]} .

In 1907, the authoritative German researcher T. Krumbach discovered trichoplax in his aquarium just when the hydrated jellyfish Eleutheria krohni contained in it reached puberty . This coincidence allowed Krumbach to conclude that Trichoplax adhaerens is not an independent biological species, but an aberrant planus E. krohni . By that time, sexual reproduction in Trichoplax was not described, and, in view of the authority of Krumbach, his findings were taken without due criticism and began to be published as a fact in many zoological publications (although already in 1912 G. Schubbotz conducted a histological analysis of the structure and cellular composition of T adhaerens and E. krohni planula and showed that there is no similarity between these organisms ^[9] ). Because of this, over the next 60 years, Trichoplax was practically not studied; after an article by F. Schulze in 1914, almost a half century has not appeared in zoological journals an article devoted to the study of trichoplax ^{[7] [10]} .

Research on trichoplax in the 1960s resumed. In 1963 and 1966, articles were published by the spouses V. and G. Kul from Frankfurt University on the movement and regeneration of Trichoplax. A little later, the cultivation of Trichoplax adhaerens was taken up by A. V. Ivanov and other Soviet biologists who studied its structure and biology ^{[11] [12]} .

In 1971, C. Grell described the eggs of Trichoplax adhaerens and studied its structure at the electron microscope level. He also developed methods for cultivating Trichoplax in Petri dishes . The existence of eggs in Trichoplax adhaerens served as evidence of its independence as a biological species. Grell proposed isolating Trichoplax adhaerens into a separate type of Placozoa. After that, cytological , molecular biological , behavioral and other features of Trichoplax began to be actively studied ^[13] . In 2006 and 2008, respectively, its mitochondrial and nuclear genomes were sequenced ^{[14] [15]} .

A flattened, lacking symmetry ^[16] Trichoplax body reaches 2-3 mm in diameter and only 25 microns in thickness. Only 4 types of cells are organized in the Trichoplax body, organized in three layers: the upper and lower layers of cells, as well as the connective tissue that separates them. The outer layer of cells is very similar to the epithelium due to the presence of typical cellular contacts between neighboring cells (tape desmosomes in the apical region of cells associated with bundles of actin filaments, as well as contacts similar to septated desmosomes), therefore it resembles Metazoa epithelium more than sponges . However, he does not underlie the , like all real epithelia. The cells of the upper (dorsal) surface are flat, bear one cilium, their nucleated parts extend deep into the body. In the upper layer of cells are the so-called shiny balls ( German Glanzkugeln - a term coined by F. Schulze in 1891 ^[7] ), which are strongly lipid bodies refracting light and are the remains of degenerated cells (they probably come from cells of the median fibrous syncytium migrated to the upper layer). Cilia are located at the bottom of the structurally formed recess. Microtubules and bundles of transversely striated fibrils that support the cilia ^{[17] [18]} extend from the basal body of each cilium.

The lower (ventral) surface faces the substrate, the cells covering it are represented by flagellate-free glandular cells and cells bearing one cilium. Strong attachment to the substrate (hence the species epithet adhaerens ) is provided by outgrowths of ciliary cells resembling microvilli . These cells are tall and narrow, so the cilia are located close to each other, due to which a dense ciliary cover is formed on the lower surface of the animal’s body, providing locomotion ^{[17] [19]} .

The space between the upper and lower layers of cells is filled with connective tissue. It is represented by fibrous syncytium, forming a complex three-dimensional network; it lies in the liquid component, which is close in composition to seawater due to weaker intercellular contacts than other lower multicellular ones. It cannot be considered a real extracellular matrix , characteristic of all other multicellular animals, because it lacks collagen - proteoglycan - glycoprotein complex. Numerous mesenchymal fibrous syncytium nuclei are not separated by membranes , but by intracellular septa (septa). Similar septa are found in the syncytia of glass sponges and mushrooms. However, according to the latest data, the syncytial structure of the network of fibrous cells is not clearly confirmed ^[20] . The nuclei of these cells are tetraploid , in contrast to the diploid nuclei of cells of the upper and lower epithelium. Their mitochondria are connected into a single complex located near the nucleus; this complex also includes bubbles of unknown nature. In addition, fibrous cells contain endosymbiotic bacteria and, unlike cells in the upper and lower layers, are capable of phagocytosis . It is believed that fibrous syncytium is reducible; it contains actin, microtubules, and probably myosin . Probably, fibrous cells functionally correspond to both muscle and nerve cells. In addition to fibrous syncytium, germ cells lie in the intermediate layer ^{[17] [21]} .

With the growth of the Trichoplax body, mitoses occur in all three layers of the cell layers, and cells of each type, except glandular, are formed of their own kind (glandular cells are formed from ciliary). in trichoplax are absent. Trichoplax regeneration experiments have shown some degree of cell differentiation . So, from the isolated central region of the plate and the edge bead with a width of about 20 μm, formed by somewhat smaller cells, the restoration of the whole plate is impossible. When artificially connecting the edge and central fragments, excess material is rejected, which indicates the existence of a certain balance between the cells of the edge and central parts of the plates. Perhaps the process of dividing Trichoplax is triggered by a violation of this balance. With an unbalanced growth of both layers of cells, certain structural anomalies appear. So, with a lack or complete absence of the dorsal layer of cells, large hollow balls form from the ventral layer of cells, which exhibits the ability to pinocytosis and is lined with fibrous cells from the inside. If there is no ventral epithelium, then continuous balls of dorsal epithelium are formed, filled with fibrous syncytium ^[22] .

In 2014, a review of the cellular composition of Trichoplax was carried out. According to the latest ideas, 6 types of somatic cells are distinguished in the body of trichoplax: ciliary epithelial cells, newly identified lipophilic cells filled with lipid granules, and glandular cells in the ventral layer of cells; in the intermediate layer, fibrous cells; in the dorsal layer of cells - dorsal epithelial cells, newly identified crystalline cells. Functional and other features of Trichoplax cell types are listed in the table below ^[20] .

Sometimes in old cultures nonviable spherical forms of Trichoplax appear. When these spherical forms form, the dorsal layer of cells loses contact with fibrous syncytium, and the layer of connective tissue is filled with fluid. If at the same time the ventral layer of cells is separated from the substrate, then the spherical shape of Trichoplax can float in culture as a small ball for some time. Two types of spherical shapes are distinguished : hollow , the outer wall of which is represented by ciliary ventral epithelium, and fibrous cells protrude into the cavity, and ciliary dorsal cells may also be contained in the cavity; dense , the outer wall of which is formed by the dorsal epithelium, and the inner part is densely packed with fibrous cells (in addition, it may contain a compartment of ventral cells) ^[23] .

By the method of movement, Trichoplax resembles an amoeba : it slowly glides along the substrate, constantly changing shape due to the presence of bonds between cells of the upper and lower layers. The constant front and rear ends of the body are not expressed, so Trichoplax can change the direction of movement without turning. When you try to start crawling in two opposite directions at the same time, the body of Trichoplax may break into two parts. Narrowing and breaking of the constriction during division also occurs due to contacts between cells in the upper and lower layers ^{[21] [24]} .

Trichoplax adhaerens feeds on algae and other food particles on the substrate. Extracellular digestion occurs outside the body of Trichoplax, between its ventral surface and substrate. During feeding, Trichoplax bends, raising the central part above the substrate, and food is digested in the closed pocket that has formed; this process is sometimes called “temporary gastrulation ”, and the ventral epithelium of Trichoplax functionally corresponds to the cervix . Digestive enzymes are probably secreted by glandular cells. Digestion products are absorbed by ciliary cells, which can form endocytotic carinated vesicles. Apparently, a small flat body makes transport possible by ordinary diffusion ; therefore, Trichoplax does not need a circulatory system ^{[25] [19]} .

Fibrous cells contain calculus (digestive) vacuoles , in which food particles can be detected at different stages of digestion. Apparently, the processes of fibrous cells penetrate through the upper layer of cells and food particles are phagocytosed there ^[26] .

In 2015, an article was published that reported on an interesting detail of the eating behavior of Trichoplax. It turned out that the more algae appeared in the food pocket of Trichoplax, the more lipophilic cells secrete their secretions outside, causing the algae cells to lyse . In addition, only those lipophilic cells that are located at a distance of 10−20 μm from the food particle are discharged. The mechanisms for ensuring such precise regulation are unknown ^[27] .

There is evidence of the existence of group behavior in Trichoplax. According to some reports, when living in large aquariums that simulate natural conditions, Trichoplax exists in the form of clusters, the development of which passes through certain stages ^[28] .

Trichoplax adhaerens reproduces predominantly asexually (by fragmentation or budding ). With fragmentation, the body of trichoplax with the help of the resulting constriction is divided into two approximately equal halves. The fragmentation process can take several hours, and a thin multicellular bridge remains between diverging individuals for a long time ^[29] .

When budding, rounded kidneys (tramps) with a diameter of 40-60 microns are formed on the dorsal surface, they contain all types of cells, including ventral ciliary cells and connective tissue. The formation of the kidneys is as follows. A bud of the kidney is formed in the inner layer of the body from cells that are expelled inward from the upper and lower layers, while losing cilia. Subsequently, these cells form the ventral epithelium of the daughter individual. Before the kidneys are separated, these cells form cilia directed into the cavity and, thus, reverse the polarity. As the kidney grows, it forms a protrusion on the dorsal side of the body and, in the end, is disengaged from it along with part of the dorsal layer of cells. The budding process lasts about 24 hours. Separated, ciliated kidneys float away, swim for about a week and settle to the bottom. A hole is formed on the side facing the substrate, which gradually expands, so that the tramp becomes cup-shaped, and then it spreads over the substrate and is attached to it by the ventral epithelium. Smaller kidneys formed by cells of the dorsal and ventral layers and filled with fibrous cells can be separated from the marginal roller. Such smaller kidneys are not capable of swimming in the water column and performing the resettlement function; they gradually flatten and turn into young individuals ^{[25] [30]} .

Trichoplax does not yet have a reliable description of the sexual process , however, under laboratory conditions, eggs were described and, at the ultrastructural level, sperm . The existence of the sexual process is also confirmed by the distribution of single nucleotide polymorphisms ^[16] . The eggs are formed from cells of the lower layer, which are dedifferentiated and immersed in a layer of connective tissue. Cells of fibrous syncytium play the role of lactating, part of their processes, along with endosymbiotic bacteria, phagocytize the eggs. Trichoplax germ cells are formed irregularly; Factors that trigger sexual reproduction include an increase in population density, lack of food, and an increase in water temperature to 23 ° C or more. Both ova and sperm can form on one individual. The nature of meiosis is unknown. The egg is fertilized upon reaching a size of 70-120 microns. After that, the egg is covered with the so-called protective “fertilization membrane” and undergoes complete uniform crushing . The embryo develops inside the mother's body until the latter is completely destroyed and the new organism comes out. Under laboratory conditions, the embryos are destroyed at the stage of 64 blastomeres for unknown reasons (the maternal individual also dies), so the embryonic development of Trichoplax is not studied. A possible cause of death of the embryo may be uncontrolled DNA replication , in which there is no transition of cells from the S phase to the G ₂ phase ^{[30] [31]} .

Chromosome set Trichoplax adhaerens 2n = 12 ^[21] . Three pairs are represented by two-arm meta- or submetacentric chromosomes , while the other three pairs of chromosomes are smaller and possibly acrocentric. The absolute size of chromosomes does not exceed 2-3 microns ^[32] . Diploid epithelial cells contain only 0.08 pg of DNA , which is less than the same value for any Metazoa representative ^[30] .

Sequencing of the Trichoplax adhaerens nuclear genome was preceded by sequencing of its mitochondrial genome , carried out in 2006. The results were unexpected: it turned out that the mitochondrial genome of T. adhaerens has the largest size among all Metazoa and contains more than 43 thousand pairs of nucleotides (while the sizes of mitogenomes of other Metazoa usually lie in the range from 15 to 24 thousand pairs of nucleotides). Its distinctive features include the presence of numerous spacers between sections of transcribed DNA, several introns , and also ORFs with functions as yet unclear (all this is not typical for other Metazoa), and DNA sections encoding proteins are on average 10% larger than others animals. The listed features of the mitochondrial genome of trichoplax should be considered as traits inherited from the common ancestor of Metazoa, since among the opistocontes not related to Metazoa, these features are not uncommon. So, in the choanoflagellate Monosiga brevicollis, the mitochondrial genome contains approximately 76.5 thousand pairs of nucleotides, having large areas occupied by spacers and several introns (and the mitochondrial genome of the ascomycete consists of 94-100 thousand pairs of nucleotides ^[33] ) ^{[34] [35]} .

Taking into account the fact that among the lamellar ones, the presence of significant genetic diversity was revealed, which was reflected in the isolation of several fairly isolated clades , the researchers in 2007 sequenced mitochondrial genomes from three other laboratory samples obtained in Belize . Among them, the sample under the symbol BZ2423 represented Clade II, BZ10101 - Clade III, and BZ49 - Clade V ( T. adhaerens himself belongs to Clade I). An analysis of these genomes showed that they share the features previously identified in the mitochondrial genome of T. adhaerens : in all four genomes there are numerous spacers, several ORFs with unknown functions, introns, and a common set of genes: 12 genes of the respiratory chain , 24 tRNA genes, 2 rRNA genes ( rnS and rnL ), but the at8 and atp9 genes were not detected. At the same time, a significant difference in the genome size was observed: in the BZ49 sample it contained 37.2 thousand pairs of nucleotides, in BZ2423 - 36.7 thousand pairs, and in BZ10101 - 32.7 thousand pairs. The proportion of coding and structural nucleotide sequences ranged from 55% in T. adhaerens to 67% in BZ49. The average spacer length varied from 101 pairs for BZ10101 and 105 pairs for BZ49 to 154 pairs for BZ2423 and 209 pairs for T. adhaerens ^{[36] [34]} .

The nuclear genome of Trichoplax was sequenced in 2008. It contains about 98 million nucleotide pairs and 11,514 protein coding genes, of which about 87% show significant similarity with the corresponding genes of other animals. Most (about 83%) of the approximately 7,800 conserved in sea ​​anemones and bilaterally symmetrical animals have homologs in trichoplax. The density of introns in Trichoplax is comparable to that in vertebrates and sea anemones. The Trichoplax nuclear genome retains many ancient introns lost in other animal lines, and for 82% of human introns there are corresponding introns with the same arrangement in the trichoplax genes ^[15] . Interestingly, Trichoplax acquired some genes from its bacterial gram-negative endosymbiont ( rickettsia ) ^[37] . The nuclear genome of trichoplax is rather small compared to the Metazoa genomes, but this can hardly be explained by secondary simplification: the absence of secondary simplification is indicated by the preservation of ancient introns and gene order ( synteny ) ^[38] .

Despite the relative simplicity of the structure, the Trichoplax adhaerens genetic material contains many genes inherent in animals with a higher level of organization ^[38] . For example, the secondary structure of the 16S rRNA molecule in trichoplax is much more complicated than in any representatives of the cnidaria type ^[39] . Trichoplax nuclear DNA contains the genes of proteins (especially transcription factors ) necessary for differentiation of various types of cells, development and functioning of the nervous system, isolation of germ line cells and other aspects of the development of a bilaterally symmetrical organism. It contains the Hox and genes and other homeobox genes: , Brachyury and . Trichoplax also has genes encoding the protein signaling pathways - and Wnt , which are extremely important for determining body axes during the development of a bilaterally symmetrical organism ^[38] .

In addition, genes encoding proteins of the basal plate, molecules necessary for intercellular adhesion and attachment of cells to the extracellular matrix were found in Trichoplax. Trichoplax also has genes critical for the functioning of the nervous system - such as genes encoding voltage-dependent , potassium and calcium channels , an almost complete set of synaptic , proteins of the synthesis and transport systems of monoamines , and many and G-linked receptors ( GPCR ), axon maintenance factors and neuron migration. It has been established that trichoplax has visual pigments - opsins ^[40] . There is evidence of the presence of genes of insulin and insulin receptors in Trichoplax, as well as genes of various regulatory peptides and their receptors, which indicates the presence of rudiments of hormonal regulation in Trichoplax ^[16] . Trichoplax adhaerens has a gene for the precursor of the major histocompatibility complex (proto-MHC), which is necessary for the functioning of the vertebrate immune system ; this is considered as an argument in favor of the assumption of the early occurrence of proto-MHF in animal evolution. In Trichoplax, proto-MHC is involved in defense processes (antiviral immunity, stress response, ubiquitin – proteasome protein cleavage pathway) ^[41] .

Trichoplax revealed a special membrane-active antimicrobial peptide, trichoplaxin, which can be used to develop a new series of antimicrobial agents with a wide spectrum of action ^[42] .

For a long time, the species Trichoplax adhaerens was considered the only representative of the Placozoa type, since under a light microscope all lamellar from different parts of the world look the same. A number of recent studies have disproved this view. It turned out that among the lamellar species there is significant genetic diversity, and as of 2015, the Trichoplax adhaerens taxon actually includes 19 species (so far called haplotypes ), which comprise at least 7 well-isolated clades . The description of individual species in the case of Trichoplax seems extremely complicated due to the uniform morphology of these animals ^[43] .

In 2018, a new lamellar species was identified - Hoilungia hongkongensis . Sequencing of the genome of this organism was carried out, and its genome was compared with the genome of T. adhaerens . It turned out that H. hongkongensis has many inversions and translocations that are absent in the T. adhaerens genome, which indicates a deep separation between the two species. Analysis of sequence divergence also indicated an unexpectedly large evolutionary distance between the two lamellar species. It was also shown that the key mechanism of speciation in lamellar (that is, the separation of the species into two new ones) is gene duplication ^[2] .

Very little is known about the ecology and biogeography of trichoplaxes. Lamellar are found only in calm coastal waters and avoid deep waters, as well as waters with strong currents. They were also found in stagnant waters to a depth of 20 m. It is believed that the main factors limiting the distribution of trichoplaxes are the temperature of the water and its salinity. Lamellar were found in tropical and sub-tropical waters of the Indian , Pacific and Atlantic oceans, namely, near Bermuda , in the Caribbean , off the coast of East Australia , in the Great Barrier Reef , near the island of Guam , Hawaii , Japan , the Mediterranean Sea , and islands Palau , Papua New Guinea , in the Red Sea , near Vietnam and East Samoa ^[44] . Members of some treasures are very widespread, while the range of others is more limited ^[43] .

Lamellar prefer warm water with temperatures up to 32 ° C, and sometimes they were found in water with a temperature slightly above 10 ° C. Most often lamellar occur in places with calm waters on solid substrates (stones, coral reefs , mangrove roots ). The diversity of lamellar habitats may underlie their great species diversity. Lamellar can adapt to muddy waters, as well as low salinity of water, which, for example, is observed in mangroves ^[4] .

The number of trichoplaxes depends on the time of year. As a rule, lamellar are most numerous in the warm season (from June to October in the subtropical and temperate zones of the Northern Hemisphere ) ^[4] .

Observations of the connections of trichoplaxes with other living organisms show that they are often associated with sedentary filtrators, for example, sedentary ciliates and polychaete worms . Due to their small size and benthic lifestyle, lamellae should often become victims of predators, but so far only one case of eating trichoplax is known. Cases have been described when potential predators (for example, gastropod mollusks or codling) quickly bounced after contact with Trichoplax, which indicates the possible existence of chemical defense mechanisms in these animals. Maybe they are mediated by shiny balls located in the upper layer of cells ^[4] .

There are two main methods for collecting plate from natural habitats. The first of them involves the collection of stones, fragments of coral reefs and other solid objects from a depth of 5 m. In the second method, ordinary object glasses are used, which are placed in a plastic bag open on both sides, so that sea water can flow through it. Bags with glass are immersed to a depth of 2 to 20 m. Soon the glass is covered with a coating of algae, which serve as food for lamellar ^[4] . Under laboratory conditions, trichoplaxes are fed with unicellular algae and Chlorella ^[44] .

There is no consensus on the simplicity of the Trichoplax structure: it is not known whether it is primary, or whether it is the result of secondary simplification. There are even suggestions that Trichoplax is a progenetic larva of some completely extinct taxon ^[30] . According to mitochondrial DNA analysis, Placozoa were the first group to separate near the base of the evolutionary tree of animals. However, analysis of several nuclear genes has yielded conflicting results. According to some data, the lamellar should be considered as the Metazoa basal group, according to others - as the second branch, separated from the tree of animals after separation of the sponges, according to the third - as the sister group Bilateria . Analysis of 18S rRNA and Hox / ParaHox genes is in favor of monophilia of the group including taxa Placozoa, Cnidaria and Bilateria and named . Recent paleontological findings that shed light on the nutritional model of large soft-bodied Ediac animals (such as Dickinsonia ) allow us to conclude that these organisms are close to modern lamellar, although no reliable fossil remains of lamellar ones were found ^{[45] [16]} .

Many scientists regarded Trichoplax as an intermediate between unicellular and multicellular animals. Under the influence of the first information about Trichoplax, O. Buchley put forward the hypothesis of the origin of multicellular from placula - a hypothetical bilayer colony. Its lower layer was facing the substrate and performed a trophic function (future endoderm ), and the upper one performed a protective function and gave rise to ectoderm . The digestive role of Trichoplax ventral epithelium is in good agreement with this hypothesis. However, the ability of fibrous cells to transepithelial phagocytosis is more consistent with the phagocytella model proposed by I. I. Mechnikov ^[30] .

The name of the type Placozoa was proposed by C. Grell in 1971. In contrast, A. V. Ivanov , who preferred not the placula model, but the phagocytella model, suggested isolating the Phagocytellozoa type with the only genus Trichoplax ^{[7] [30]} . Somewhat later, Ivanov (although with remarks about the “failure” of the name) recognized the Placozoa type, but singled out the Phagocytellozoa subdivision in the Metazoa system, including the only Placozoa type (the Russian equivalent of the name is Lamellar ) ^[46] . However, later studies revealed that Trichoplax and the phagocytella model have nothing in common ^[47] . Sometimes the genus Trichoplax is isolated into a separate family Trichoplacidae as a part of the Placozoa type ^[48] .

The following is a brief overview of currently existing hypotheses regarding the evolutionary position and origin of lamellar ones.

Functional morphological hypothesis

Because of its simple structure, Placozoa was often considered as a model organism, illustrating the transition from unicellular organisms to multicellular animals (Metazoa), and therefore Placozoa can be considered a sister group in relation to all other metazoans ^[15] :

This idea, which remains purely speculative at present, is called the functional-morphological hypothesis. She suggests that all or most of the animals came from a gallertoid (from German Gallert 'jelly, jelly') - a free-living marine pelagic spherical organism. It consisted of a single layer of ciliary cells supported by a thin non-cellular separation layer — the basal lamina. The inner part of the sphere was filled with capable of contracting fibrous cells and a jelly-like extracellular matrix ^{[15] [49]} .

Modern Placozoa and other animals allegedly descended from this ancestral form, passing one of two paths. The first way - the invasion of the epithelium - led to the development of an internal system of canals, and such a modified gallertoid with an internal system of canals gave rise to sponges, creeping and combing . The second path included the transition from a pelagic lifestyle to a benthic one . Unlike seawater, where food, potential predators and sexual partners can meet on all sides, when living near a substrate, a distinction arises between the side facing the substrate and the side facing the open water. Under these conditions, flattening of the body has a selective advantage, and, in fact, a flattened body is inherent in many benthic animals. According to the functional morphological model, lamellar and, possibly, some other groups, known only from fossil remains, evolved from such a benthic life form called a placuloid . According to this model, the dorsal and ventral layers of Trichoplax cells are analogues , but not homologs of the ectoderm and endoderm of the eumetazoids ^{[15] [49]} .

In the case of the truth of the above model, Trichoplax adhaerens would turn out to be the oldest branch of multicellular organisms and a relic of the Ediakar (or even pre-Ediakar) fauna. Due to the lack of an extracellular matrix and basal lamina, the development of these animals, which were very successful in their ecological niche, was, of course, limited, which explains the low rate of evolution of laminae and their almost complete invariance during evolution ^[15] .

The functional-morphological hypothesis is confirmed by the analysis of mitochondrial DNA ^{[35] [36]} , however, it does not find proper confirmation in the statistical analysis of the trichoplax genome compared with the genomes of other animals ^[15] .

Hypothesis Epitheliozoa

According to the Epitheliozoa hypothesis, based on purely morphological features, Placozoa are the closest relatives of animals with true tissues (Eumetazoa). The taxon (Epitheliozoa) formed by them turns out to be a sister group with respect to the sponges ^[15] :

The main confirmation of this hypothesis is the presence of special cellular contacts - tape desmosomes, which are found in Placozoa and all other animals, with the exception of sponges. In addition, Trichoplax adhaerens combines the presence of glandular cells on the ventral side with eumethasoids. In the framework of this hypothesis, a possible scenario for the origin of monociliary cells similar to the epithelium (epithelioid) of Trichoplax adhaerens tissues is a reduction in the collar of choanocytes in the sponge group, which has lost the ability to filter filtration and has become an ancestral plate-like group. The epithelioid tissue of trichoplax subsequently gave rise to the epithelium of eumetazoids ^[15] .

According to the Epitheliozoa model, the dorsal and ventral layers of Trichoplax cells are homologues of the ectoderm and endoderm, and the layer of fibrous cells gave rise to the connective tissues of other animals. Statistical analysis of the trichoplax genome compared with the genomes of other animals speaks in favor of this model ^[15] .

Eumetazoa hypothesis

The Eumetazoa hypothesis, which is mainly based on molecular genetics, considers lamellar as extremely simplified eumetazoids . According to this hypothesis, Trichoplax evolved from much more complex animals that had muscle and nerve tissue . These two types of tissues, as well as the basal lamina and epithelium, were lost during subsequent radical simplification. Many attempts have been made to determine the sister group of Placozoa: according to one data, they are the closest relatives of the bowing, according to others, they are the sister group of ctenophores, and sometimes they are placed in the group of bilaterally symmetrical ( Bilateria ) ^[15] :

The main drawback of this hypothesis is that it practically does not take into account the morphological features of lamellar ones. The extreme degree of simplification that, according to this model, occurs in Placozoa, is known only in animals leading a parasitic lifestyle and, apparently, is unlikely in free-living species, such as Trichoplax adhaerens . Statistical analysis of the Trichoplax genome compared with the genomes of other animals rejects the hypothesis ^[15] .

Diploblasta Hypothesis

A different picture of the phylogenetic relationships between the main Metazoa units is provided by the results of a combined analysis based on the joint use of morphological data, the secondary structure of mitochondrial rRNA genes and nucleotide sequences from the mitochondrial and nuclear genomes. On the basis of these results, lamellar groups are grouped together with sponges, ctenophores, and those clinging to the treasure Diploblasta, sister to Bilateria ^{[50] [51]} :

The combination of the four types mentioned (Placozoa, Porifera, Ctenophora, Cnidaria) into a single taxon of the highest rank (“Radiata kingdom ”) was proposed by T. Cavallier-Smith in his six-kingdom system of the organic world of 1998 (however, he did not insist on the monophilia of this taxon) ^{[ 52]} . In the system of the kingdom of Metazoa, proposed in 2015 by Michael Ruggiero et al, and included as an integral part of the macro-system of living organisms that they represent, this kingdom includes two sub-kingdoms: not received a name (corresponds to the Diploblasta group in the given cladogram) and Bilateria. This classification option is also suitable for the adoption of any of the three previous phylogenetic hypotheses, but then the indicated nameless kingdom will no longer be a treasure, but a paraphyletic group ^{[53] [54]} .

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