|Bütschli , 1910|
Tissue animals (Eumetazoa) (old Gr. Εὖ eu good, genuine + μετά (da) after + ζῷον [zóon], living being, animal) are a hypothetical taxon in the systematics of multicellular animals . This includes all multicellular animals (Metazoa), with the exception of the sponges and the Placozoa (together formerly called tissue-less or Parazoa). The name refers to a feature (the possession of real cell tissue ) which, according to the theory, should not occur in sponges (for an alternative theory see).
The most important argument for the grouping, however, is the similarity of the collared flagellate cells or choanocytes of the sponges with the cells of the unicellular collared flagellates , which would provide a plausible explanation for the development of unicellular cells to multicellular cells . The Eumetazoa hypothesis thus states that the sponges are the sister group of all other multicellular animals, i.e. the group that first split off from the common parent group. For a long time it was considered the almost unchallenged standard hypothesis , but today it is doubted by numerous researchers on the basis of phylogenomic data, in which the relationship is estimated based on the comparison of homologous DNA sequences, but is still supported by others.
Some zoologists use the name Animalia ( Linnaeus , 1758) for this grouping instead . For the taxon in which the Placozoa and the Eumetazoa are combined, the name Epitheliozoa was coined by Peter Ax . Later authors have partially included the placozoa in this grouping, as they consider the placozoa tissue to be homologous to the epithelia of the other epitheliozoa. According to this view, Eumetazoa and Epitheliozoa would then be synonymous with one another.
Other researchers dispute the basal position of the sponges and see a different group as being basal: many the comb jellyfish, some the bilateria. For these, the Eumetazoa are no longer an independent taxon, Eumetazoa would then be a synonym for Metazoa.
The taxon of tissue animals is characterized by specialized cell types and real tissues such as sensory cells , nerve or muscle tissue . Epithelium and other cells are connected by special cell-cell connections known as gap junctions . In addition, during the development of the embryo , gastrulation causes the cells to split into at least two cell layers ( germ layers ), the endoderm and the ectoderm . Other common characteristics of the Eumetazoa to the exclusion of the Placozoa are a body with a mouth and a bowel, the possession of radial or bilateral symmetry of the body and a body axis with front and rear ends.
A possible phylogeny of multicellular animals, taking into account the Eumetazoa hypothesis, could look like this:
Compare below some currently discussed alternative hypotheses.
The relationship of the four basic, basal groups of multicellular animals to one another, the sponges (Porifera), Placozoa, rib jellyfish (Ctenophora), cnidarians (Cnidaria) to the bilateria , the "higher" animals with a bilateral symmetry of the body plan in the basic state, which alone make up more than 99 percent of the animal kingdom has not yet been conclusively clarified and is one of the fundamental problems of the phylogeny of organisms. There is now broad consensus that the sponges and the bilateria are monophyletic groups - this was controversial for some time, especially in the case of the sponges. The order of the evolutionary development of the other groups is extremely problematic, as there are a number of fundamental studies on this, which are extremely convincing in themselves, but have come to fundamentally different results.
The only scientific approach that is currently being pursued to solve the problem is the cladistics method , also called phylogenetic systematics in Germany. In contrast to earlier approaches, which were largely based on the authority and intuitive view of famous and recognized researchers, cladistics is based on strictly formalized decision algorithms . This broke a decade-long standstill, which was based on the one hand on the formation of national scientific schools and on the other hand on the fact that it was frowned upon for young researchers to dare to contradict established experts, and that corresponding views were mostly not even published. The morphological investigation methodology made great progress in the decades since around 1980, in which cell morphological investigations in particular on the formation of the cotyledons, the development of the nervous system and the structure of the sperm in the various animal phyla were refined. However, the investigation of the relationship based on the comparison of homologous DNA segments became of fundamental importance .
The results obtained with these methods are, however, contradicting to this day. The results naturally depend on the choice of the respective trait or gene. Often, different family trees result if certain key groups are included in the analysis or left out; It may be advisable to leave out groups with extremely divergent combinations of characteristics, i.e. high evolutionary speeds, as they can greatly distort the analysis. However, it is not easily possible to recognize such cases in advance. The sorting algorithm used also has a major influence. Determining the relationship of numerous groups to one another by simple trial and error ( permutation ) is impossible even with supercomputers, since the computational effort is too high. Optimization methods must therefore be chosen in order to simplify the problem. Various methods are used for this. Some procedures initially group similar cases into pairs and then gradually add the others. Others try to create family trees ( called cladograms here ) with a minimum length. Today optimization methods from Bayesian statistics are mostly used. Each method can provide a different result for the same data records. The optimization method finally delivers a tree as a result , but initially no indication of the order in which the branches occurred. For this purpose, the group examined in each case is to be compared with outer groups (called "rooting") so that the knot that is most similar to the outer group can be set at the base. The result now also depends on the choice of the outgroup (s). It is often appropriate to weight characteristics differently, as complex characteristics such as the possession of certain organs are more meaningful than simple ones, such as the presence of a certain base at a position in the DNA sequence (which, with four bases, already has a random probability of 25 Percent owns). Morphological features can also be compared with those on fossils , the age of which may provide further information. Each of the methodological decisions mentioned here and many others have advantages and disadvantages, so it is not easy to choose the best of them.
The results so far have shown that morphological features (even those that appear very basic) can be completely regressed or converged in different groups in a similar form. It is discussed, for example, that the comb jellyfish and the rest of the eumetazoa convergent and independently developed a nervous system. Gene sequences can be changed so much by mutations that their similarity no longer extends beyond a chance match. Certain gene families can contain orthologous genes and paralogue genes with different evolutionary speeds or, in extreme cases, go back to horizontal gene transfer .
The methodological problems mentioned lead to different hypotheses about the relationships. Some of these are shown below. In addition to this selection, there are numerous other hypotheses.
Many recent works that confirm the existence of Eumetazoa lead to a grouping of cnidarians with the comb jellies, a taxon that in classical systematics as Hohltiere (Coeneterata) was already the default hypothesis, but was barely represented for several decades. An example would be phylogeny in a review by Maximilian J. Telford and colleagues. This would result in the following phylogeny:
Rib jellyfish instead of sponges as the first branch
While for a long time the sponges (or more rarely the Placozoa) were the only candidates for the basal group of the Metazoa because of their simple and deviant organization, this position is assigned to the comb jellyfish according to a number of very influential recent genetic hypotheses. This result was surprising according to morphological standards, but according to a certain genetic method it is relatively robust and statistically well secured. The following phylogeny would result:
According to this hypothesis, the Eumetazoa would not be a valid taxon. A big problem of the hypothesis would be to explain the great similarity of the choanocytes of the sponges with the cells of the unicellular collared flagellates (Choanoflagellata). According to consistent results, the Choanoflagellata are considered to be the best secured sister group of the Metazoa, the existence of the choanocytes has been a strong indicator for over 100 years how a multicellular animal could possibly have evolved from a colony of protozoa. It is conceivable that both cell types are actually homologous, but have been lost in the comb jellyfish and all metazoa except the sponges (symplesiomorphism), but such a complicated hypothesis is always an indication against a certain theory. In fact, arguments have also been put forward that make a convergent formation of the cell types that initially look so similar seem at least conceivable.
An alternative hypothesis, represented, for example, in a review by Gonzalo Giribet, based on a particularly highly weighted trait, the possession of genes from the family of the Hox genes , which are fundamental to the body plan of most animals, the Placozoa, Cnidaria and Bilateria in a taxon called Parahoxozoa.
Even according to this hypothesis, a taxon Eumetazoa would not exist.
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