Are protists part of the monophyletic group

Origin of animals and fungi - PROTISTEN.DE

Published in MIKROKOSMOS 102, 321 - 330 (2013) The evolutionary origin of animals and fungi For a long time, evolutionary research focused mainly on the analysis of structural body features and their changes in fossil finds that can build up millions of years. With the possibility of analyzing gene sequences and using their similarity or difference in interaction with so-called molecular clocks as developmental features, evolution research has become a powerful additional instrument since the 1980s, which this branch of research has developed Life sciences and has helped to change the face of biology from a mainly descriptive to a measuring science (Jahn 2000). At the beginning of the 1990s there was an increasing number of studies based on molecular biology, which came closer to the exciting question of which protist group could mark the origin of the animals, the Metazoa. Or should there have been several points of origin? Christen et al. Presented in 1991. The well-known zoologist and ultrastructural researcher Karl Gottlieb Grell (T & uuml; bingen), who has published the standard work "Protozoology" since the 1950s, and who has long and intensively dealt with the small, very simply structured marine organism called Trichoplax adhaerens, can also be found there as a co-author. the last representative of the Placozoa. First of all, some basic remarks on the groups of organisms discussed. From a morphological point of view, the Metazoa are divided into three main groups: - Species with poorly developed tissue differentiation: Porifera (sponges) and Placozoa, - Diploblasts, species with two embryonic tissue layers (cotyledons): Cnidaria (cnidarians) and ctenophores , - Triploblasts, species with three cotyledons: all other animal groups. In order to introduce the other zoological terms that appear in the course of the article and to cover them with example organisms, the modern subdivision of triploblasts is briefly referred to here. Also known as bilateria, they are bilaterally symmetrical and have real muscle tissue. The simplest, the acoelomats, do not have a shell with the exception of the digestive tract. They are the flatworms (platelet mints), to which for example the leeches and tapeworms belong. The structure of the pseudocoelomatics is somewhat more complicated, with the two groups of caterpillars (Rotifera) and roundworms (Nematoda). The Coelomats are divided into the Protostomier with the groups Molluska (including snails), Annelida (calendula worms) and Arthropoda (including insects). Finally, the deuterostomies are divided into the echinodermata (e.g. starfish) and the chordata, to which the vertebrates also belong. In the world of mushrooms, a broad distinction was made around the turn of the millennium - Chytridiomycota (droplet fungi, mostly microscopic and unicellular) - Zygomycota (yoke fungi, including mycorrhizal species, the root symbionts of many vascular fungi, molds, plants) - Ascomycota Morels) - Basidiomycota (cap mushrooms, rust mushrooms). For the nomenclature of the protist groups mentioned and the terms relating to the structure of the phylogenetic trees, see Bettighofer (2013) and the description of the overview graphic at the end of this report. First a few words about the terms heterokonta, Chromista and Chromalveolata, which are often used in protistology: In 1981 Thomas Cavalier-Smith described the eukaryotic realm Chromista with the sub-realms Cryptophyta and Chromophyta. The heteroconta and the haptophyta were located in the chromophyta. For some time the Chromista and Alveolata (ciliates, dinoflagellates, etc.) were combined to form the Chromalveolata supergroup (Cavalier-Smith 1999). Since 2009, however, there has been an increasing number of studies showing that haptophytes and cryptophytes are not closely related to hetero accounts and alveolates. Their placement in the phylogenetic tree is now considered uncertain. The group of straight accounts had to be divided as well. Of these, only those are closely related to the alveolates, whose flagella or gamete flagellum are constructed in three parts (tubular shaft as the main axis, lateral appendages, which in turn have appendages Hausmann et al. 2003). 2 These are gold algae, diatoms, brown algae, Oomycetes, Opalinaden and other, small flagellate groups, which together form a monopyhlylic group called stramenopilates (Latin stramen = straw, pilus = hair). On the other hand, it has been found that the rhizaria, consisting of Cercozoa (to which, for example, most filose ambes and many parasites belong), of foraminifera and of the radiolarian groups Acatharia and Polycystinea, are closely related to and with stramenopilates and alveolates form them the monophyletic group SAR (Stramenopila, Alveolata, Rhizaria). Are the Metazoa monophyletic? Back to the question posed by Christen et al. Did the three basic blueprints of the Metazoa evolve independently, or did they have a common origin? This was discussed controversially at the time. The analysis of the extensive molecular biological data set that the authors had compiled and that included all strains of diploblasts at the time (placozoas, sponges, cnidaries and ctenophores), combined with an even more extensive database with sequences of fungi, protists, triploblasts and vessels Planting brought interesting results (see Fig. 1): - Early branches in the tree: flagellates and slime molds (Amoebozoa I) - Next branches: ciliates, dinoflagellates, stramenopilates and ascomycetes. - A little later, diploblasts, triploblasts and viridiplantae (green algae and vascular plants) branched off as monophyletic substructures. An exciting sub-question initially remained unanswered: Which protist group formed the origin of the Metazoa? Classically, there were two main theories on this question, Haeckel's Gastraea theory, formulated in 1866, and Hadzi's ciliate theory from 1963. Haeckel took the view that the ontogenesis of animals, i.e. their embryonic development, their phylogeny (their phylogenetic development) reflects. He interpreted the early embryonic cup germ stage (gastraea stage) as an indication that the multicellular cells had developed from colony-forming single cells with a division of labor. Haeckel concluded from this that the coelenterates (Cnidaria) must have had similar beings at the origin of the multicellular cells. Fig. 1: The diploblasts and triploblasts appear monophyletic, as do Viridiplantae (green algae and taller plants). The investigated fungi also form a clade, but the protist groups have inconsistent origins. Trypaosomes (Discoba) served as the outer group. According to Christen et al. (1991), simplified. Hadzi, on the other hand, took the view that in multinucleated ciliates, cell walls would have formed around the nuclei. From this point of view he deduced that it was not representatives of the diploblasts (cnidaria), but the intestinal flatworms (platelmintes, the most original triploblasts) that should have stood at the beginning of the history of the Metazoa. The fact that the cnidariums would have developed from the flatworms, on the other hand, should be shown by their so-called planula larva, which is similar to flatworms. Thus Hadzi again resorted to Haeckel's basic theory “ontogenesis reflects phylogenesis”. Still no trace of the origin of the Metazoa back to the 1990s. Christen and co-workers made it clear that the first available molecular biological phylogenesis studies by Field et al. (1988) as well as their own results could not shed light on the question of which protist group was the original for the Metazoa. However, it was interesting to observe that although it was never possible to connect a protist with the clades of diploblasts or triploblasts, the unicellular green algae Chlorogonium and Pyramimonas always formed a monophyletic unit with the vascular plants. This was undoubtedly an interesting discovery that still proves to be correct today (Review in Bettighofer 2013). With the many, very different groups of protists who settled in these early molecular biologically sound phylogenetic analyzes in many different places in the tree and refused to form clean groups, the researchers had to leave it open whether there might not be more than just one The origin of the multicellular organization in animals could have been given. Such concepts had already been discussed by B & uuml; tschli (1910) and were based on observations that a number of different protists form cell networks based on division of labor. Animals and mushrooms are closest relatives! For a long time there was no doubt that fungi were part of the family tree of plants. It was assumed that they would have lost the ability to assimilate secondarily when they became native to the ecological niche of saprophytism, the recovery of dead organic material. For example, the 30th edition of the botanical standard work Strasburger (1971) classified the mycophyta (fungi) as the third division after the schizophyta (bacteria) and the phycophyta (algae in the broader, morphological sense) in the plant kingdom. In the introduction to the Mycophyta, however, it was already clearly worked out how they differ from the other plants: - They live heterotrophically, they have no plastids. - Your cell walls are mostly made of chitin, not cellulose. - Your reserves are glycogen and fat, never starch. One could have added: What they have in common with plants is that they do not run away. So it was quite a bang when Sandra Baldauf and Jeffrey Palmer headlined in the Proceedings of the National Academy of Sciences USA in 1993: Animals 5 and fungi are each other’s closest relatives: Congruent evidence from multiple proteins. This bang was felt to be so loud mainly in conservative botanical circles, because the botanist Robert H. Whittaker had already in 1959 instead of Haeckel's three realms of living nature (Haeckel 1894: Protista, Plantae, Animalia) based on fundamentally different building plans ; Five kingdoms described: He divided the Protista into living beings with and without a nucleus (Monera, Protista), the Plantae into Fungi and Plantae. But just as Haeckel's separation of unicellular organisms from animals and plants was not accepted in the bioscientific world, Whittaker's theses were also only noted with interest. Now, with the development of molecular diagnostic tools, the scientific world has had a harder time ignoring the identified groupings and the genetic divisions between them. A few years earlier, Thomas Cavalier-Smith (1987) had worked out that animals and fungi may have a closer relationship than previously assumed on the basis of ultrastructural and biochemical similarities. According to his view at that time, the last common ancestors of both large groups should have been similar to the Choanoflagellates living today. Fig. 2: In the tree by Baldauf and Palmer (1993), all the animals and fungi examined are grouped in a sub-tree, both groups form sister trees. The supergroup SAR with representatives of apicomplexa (parasites), ciliates, brown algae and other stramenopilates also form a coherent group. Here, too, trypaosomes (Discoba) served as an outside group. 6 Results The study covered a variety of species: 7 archaea, 27 protists, 12 plants including green algae, 21 fungi and 26 animals. Sequences from 25 proteins were used with the aim of testing four different groupings of species. All the resulting phylogenetic trees had one thing in common: - Animals and fungi were closely connected to one another at their base, with one tree alternative the fungi even grouped themselves as a sub-tree of the animals! - Plants including green algae formed a coherent sub-tree in all cases, which arose clearly at a distance from those of the fungi and animals. - The selected protists, with the exception of the algae, were again grouped inconsistently and also not in stable subsurface. Fig. 2 shows an example of one of the tree alternatives in a simplified form. The closest unicellular relatives of animals and fungi In the following years there have been many studies with conflicting results on the question of which protists are to be found at the roots of the phylogenetic trees of animals and fungi. However, the view was established that all living beings, which, like animals, developed unicellular stages with a single pusher at the rear end of the cell, belonged to the monophyletic group of the opisthokonta. In 2006 Steenkamp, ​​Wright and Baldauf presented a broad study with the aim of shedding light on the branches of the Metazoa and the Fungi from the group of opistho accounts. The focus was on genes encoding four to five core proteins with slow rates of change. The following protist groups from the opistho accounts were included in the investigation, each with several species: - Choanoflagellates: heterotrophic flagellates living in water, - Nucleariids: nude females with filopodia, - Ministeriids: small, marine, heterotrophic protists with radial, immobile pseudopodia. One species, Ministeria vibrans, shows a flagellum in the vegetative stage (Tong, 1997), 7 - Corallochytria: small, amol; biode, flagelless, marine, saprophytic protists, - Ichthyosporea: animal parasites with amoid and flagellate stages. With the exception of the Choanoflagellates and the Ichthyosporea, all the other groups mentioned above are very poor in species. These newly obtained values ​​were compared with genetic data from a large number of animals, fungi, amoebozoa (lobose amberis), alveolata (including ciliates and dinoflagellates), stramenopilata (including diatoms), euglenozoa (including euglenes and trypanosomes) and viridiplantae (greens and dinoflagellates) , taller plants). The result showed a strongly supported grouping of animals, fungi and opisthokonten protists. All of the other large groups mentioned above were also clearly visible, and the phylogenetic closeness of the Amoebozoa to the Opisthokonta on the one hand and the Alveolata to the Stramenopila on the other hand (now united in the SAR supergroup) became apparent. In detail, the Choanoflagellaten, Ministeria, Corallochytrium and the Ichthyosporea together with the animals formed a common sub-tree, which today represents the taxonomic group of the Holozoa, while Nuclearia is grouped with the fungi (Fig. 3). Fig. 3: The phylogenetic overview tree by Steenkamp et al. (2006) spans almost all large eukaryotic groups and shows the opistho accounts monophyletically. The family tree of the Holozoa The protist sister groups of the animals and the mushrooms that were worked out in this way were now used alternately as so-called outer groups, which each determined the tree roots. This is methodologically beneficial. The closer an outside group is to the tree to be examined, the more precisely the relationships at its roots can be worked out. As expected, the monophyly of the animals was confirmed (Fig. 4). As in Fig. 3, choanoflagellates, ministeriids and ichthyosporea were found to be sister groups of the animals, but it was also evident that these unicellular, close relatives did not form a separate, monophyletic sister group to the clade of the animals. The closeness of Ministeria to the animals was very noticeable, but this position turned out to be unstable with the different evaluation methods used, so that the authors warned against overestimating their results. In the animals, the higher bilaterally symmetrical, triplobastic protostomia and deuterostomia formed a robust phylogenetic unit, the lower groups (Porifera, Ctenophora and Cnidaria) branched off constantly in front of the bilateria. The branching off of the protostomia in the middle of the deuterostomia was remarkable. But this branch formation also varied with the different evaluation methods, the diagnosis had to be put into perspective and showed that more data from more related bilaterals would be needed here in order to be able to generate clearer statements. Fig. 4: The family tree of the Holozoa shows the group of choanoflagellates as sister groups of the animals. According to Steenkamp et al. (2006). - Fig. 5: The family tree of the mushrooms with the filose nude lady Nucelaria simplex as its closest relative. According to Steenkamp et al. (2006). The Fungi Family Tree and the Relationship to Nuclearia To determine the roots of the fungi family tree, Monosiga brevicollis and Corallochytrium limacisporum, two protists in the immediate vicinity of the roots of the higher animals, were used.All calculations of the resulting trees 9 placed Nuclearia stably at the root of a monophyletic fungal sub-tree (Fig. 5). The higher fungi (Ascomycetes, Basidiomycetes) formed stable, monophyletic sub-spaces, the Zygomycetes and Chytridiomycetes grouped in nested sub-spaces, so they were paraphyletic. As an overall result, the authors pointed out that the supergroup Opisthokonta is an important taxon, as it includes unicellular and multicellular organisms. They suggested that in further phylogenetic studies on the roots of the animals or fungi, opisthocontal protists should always be included in the study. The results of earlier work, according to which plants and fungi (Philip et al. 2005) or animals and plants (L & ouml; ytynoja and Milinkovitch 2001) should form common groups, were impressively refuted by the available data. It was noteworthy that these earlier works had excluded both the opisthocontal protists and representatives of lower fungi or animals from consideration. The investigation also showed plausibly that the old idea that the choanoflagellates could have been the common origin of animals and fungi (Cavalier-Smith 1987) was no longer tenable. More precise details on the roots of the Metazoa and Fungi In the meantime, the genus Ministeria has become part of the small amoflagellate group Filasterea. The Choanoflagellaten now appear in many analyzes as a real sister group of the animals, Filasterea and Ichthyosporea settle basal in the Holozoa (Fig. 6). Fig. 6: The current phylogenetic tree of the Holozoa. Mushrooms make up the outer group. According to Torruella et al. (2012). Are the Choanoflagellates our ancestors now? Sponges form chamber systems through which they pump water. In the so-called collar vulture chambers there are specially constructed cells, the choanocytes, which have a collar made of microvilli in which a vulture sits centrally. The coordinated beats of these whips cause a directed flow of the water, suspended particles such as bacteria and other small eukaryotic plankters as well as organic, colloidal macromolecules are filtered out in the sponge body (Westheide / Rieger 1996). Choanoflagellates are very similar to the choanocytes of the sponges, both under the light microscope and ultrastructurally. Did the common ancestors of animals and choanoflagellates look like this? Carr et al. (2008) examined four genes from 16 very differently structured choanoflagellates and examined the molecular biological characteristics obtained together with ultrastructural characteristics such as the materials and construction plans of the differently designed sheaths or housings, the cell division characteristics and ecological data processed and assessed. They came to the conclusion that the common ancestors should have differed significantly from today's choanocytes and choanoflagellates. Animals and choanoflagellates showed each other as clearly delimited, monophyletic sister groups. There were no signs anywhere that the trees were interwoven, i.e. paraphyletic, which could have indicated that choanoflagellates could have originated from Metazoa or vice versa. It let & szl; it can be deduced that the common ancestor was a sessile, marine unicellular organism. The animals developed from this as multicellular, tissue-forming organisms, while the choanoflagellates basically remained in the unicellular organization, although they often developed colony-forming forms of life. Even without real multicellularity, the choanoflagellates were and are very successful.They inhabit a whole range of different habitats in the sea and in fresh water as plankton as well as in growth, including in bacterial biofilms. The situation with the fungi With the fungi it was confirmed that the Zygomycota and Chytridiomycota of old strain were paraphyletic. The Blastocladiales were separated from the Chytridiomycota, the Zygomycota fell into a number of groups, whose positions in the taxonomic tree are currently being discussed controversially. Previously, under the umbrella of the Zygomycota, the Glomeromycota, the mycorrhiza-causing group of the root symbionts of the higher plants, were united with the Mucoromycotina, which contains saprophytic molds. More details can be found in Kwon-Chung (2012) and Adl et al. (2012). The latter publication shows a clear graphic (Fig. 7) on the phylogenetic relationships between the large groups of living things with cell nuclei that are valid today. 12 The current classification of all eukaryotic groups Fig. 7: Overview of the phylogenetic relationships between the large groups of living things with cell nuclei that are valid today. According to Adl et al. (2012). A global working group led by Sina M. Adl (Saskatoon, Canada) had already published a revision of the eukaryote classification for the International Protistological Society in 2005. As in 2005, it was the declared goal of the scientists in 2012 to find, as they put it, a “conservative balance” between the inclusion of new, mainly molecular biological data and extensively tested and recognized groups . The classification should only be updated where well-founded, multiple verified data was available. 13 Overall, it can be stated that the majority of the supergroups, which had already been included in the system in 2005, were retained. The chromium alveolata were an important exception. As already mentioned at the beginning, it is very likely that they are an evolutionarily incoherent group, that is, polyphyletic. In their place is now SAR, which connects a large part of the earlier Chromalveolata with the Rhizaria. In the graphic, the coloring of the branches indicates that they belong to a large group. Many branches are gray in the lower part of the circle. This should reflect that their origin in the phylogenetic tree is roughly known, but the data are currently not sufficient for a reliable assignment. The two black lines between the three large group clusters are intended to symbolize that the tree is rootless, i. H. There is still too little reliable data on the evolutionary sequence of the emergence of the first representatives of the large groups. Outlook Life on earth has independently invented multicellularity at different points in the family tree of the eukaryotes. The species-rich groups of animals and fungi arose from the opisthoconts, the slime molds from the ambozos, the taller plants, the red algae and individual green algae lines with work-sharing multicellularity from the archaeplastida. The brown algae from the stramenopilates also developed complex, sometimes meter-long thalli ("giant sea kelps", e.g. Laminaria). In the shallow marine water they form dense forests with species-rich communities. The yellow-green algae (Xanthophyceen) and the cellulose fungi (Oomycetes), also groups from the stramenopilates, should not be forgotten. Today's understanding of the biology of living things with cell nuclei arises mainly from studies on animals, fungi and vascular plants. However, it is the single-celled organisms that show the most diversity! Many of these lineages have so far been little explored, but precisely they could illustrate a lot about the developmental forces and currents that have produced the diversity of eukaryotes. This diversity means, among other things, that for every rule of the life sciences there are some species in the realm of the protists 14 that break or at least strongly bend this rule. Knowing about these aspects could teach us how nature works (Baldauf 2008). It should therefore take some time before the life sciences can draw stable phylogenetic trees of all groups of organisms with cell nuclei. Much is not yet understood about the unicellular organisms, and one is only just beginning to discover the species-rich and individual-rich groups of nano- and pico-eucaryotes in the oceans. 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