Which Sponge Cells Most Closely Resemble The Animals' Closest Protist Relative?

Growth Factors in Development

Thomas Westward. Holstein , ... Suat Özbek , in Current Topics in Developmental Biology, 2011

2.1 Choanoflagellates

Choanoflagellates are small unicellular protists comprising both marine and freshwater species ( Fig. half dozen.1A). According to electric current molecular phylogenies, choanoflagellates are the closest unicellular relative of metazoans (King et al., 2008). The genome of the recently sequenced choanoflagellate Monosiga brevicollis contains approximately 9200 genes, including a number of genes that encode domains of metazoan-specific jail cell adhesion and signaling proteins (King et al., 2008). Choanoflagellates are morphologically like to the choanocytes of sponges and were therefore proposed to represent the closest living relatives of metazoans (King et al., 2008; von Salvini-Plawen, 1978).

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Metabolic and Genetic Features of Ancestral Eukaryotes versus Metabolism and "Master Pluripotency Genes" of Stalk Cells

Zoran Ivanovic , Marija Vlaski-Lafarge , in Anaerobiosis and Stemness, 2016

11.2.2 Choanoflagellate and Last Mutual Ancestor of Metazoan

Phylogenetic analysis indicates that choanoflagellates are the unicellular organisms phylogenetically closest to the Metazoa [71,72].

Comparison the choanoflagellate Monosiga brevicollis genome to early metazoan reveals the molecular and metabolic regulators presented in the concluding common antecedent of choanoflagellates and metazoans. Many pathways that are the hallmarks of the mammal stem cell maintenance are missing entirely; no receptors or ligands were identified from the Wnt, or Toll signaling pathway. Even so, homologs of the Notch, STAT, and Hedgehog signaling pathway components are present [72]. In addition, phosphotyrosine-based signaling is found in affluence in the M. brevicollis genome. TGF-β signaling is restricted to the Metazoa with neither ligand nor receptor molecules beingness constitute outside of the animal kingdom [73].

The core transcriptional apparatus of M. brevicollis is, in many ways, typical of about eukaryotes examined to appointment including, for example, all 12 RNA polymerase 2 subunits and most of the transcription elongation factors (TFIIS, NELF, PAF, DSIF, and P-TEFb) [72].

Most of TFs are C₂H₂-blazon zinc fingers, FOX TFs, otherwise known only from metazoans and fungi. One thousand. brevicollis contains a subset of the TF families previously thought to exist specific to metazoans. Members of the p53, MYC, and SOX/TCF families were identified as well every bit the homolog of MYC TF whose activeness is implicated in the regulation of the differentiation and mobilization of the hematopoietic stem cells (HSCs) [74]. Also, p53 regulates mammalian stalk cell self-renewal; the SOX family of TFs involve the members that are function of a core transcriptional regulatory network that maintains the pluripotent country of mammal stalk cells (run into Chapter 7). These information implicate that the abovementioned TFs took identify in the "stemness toolkit" at the choanoflagellate level.

Presence of all these factors in choanoflagellateas and the metazoan indicates that they evolved before the departure of the choanoflagellates and metazoan and further suggests that they were present in the final common ancestor of choanoflagellates and metazoans.

In contrast, many TF families associated with metazoan patterning and development (ETS, HOX, NHR, POU, and T-box) seem to be absent [72].

Comparison the genomes of basal metazoan anthozoan cnidarian Nematostella vectensis, the hydrozoan cnidarian Hydra magnipapillata, the placozoan Trichoplax adhaerens, the demosponge Amphimedon queenslandica, and the expanding list of bilaterian genomes revealed that genome organisation and content likewise as TFs and components of the signaling pathways are remarkably similar [75]. Also, comparison those genomes to the genome of the choanoflagellates helps to establish a gear up of TFs and signaling molecules that were present in the last common antecedent of metazoan before their departure [75]. This set is considered to accept been developed at the protometazoan phase. Also, it revealed the factors that were prerequisite for the evolution of metazoan multicellularity.

The ancestral metazoan genome included TFs that are members of the bHLH, MEF2, Fox, SOX, T-BOX, ETS, nuclear receptor, Rel/NF-kB, bZIP, Smad families, and homeobox-containing classes, including ANTP, PRD-like, PAX, POU, LIM-homeodomain, Half-dozen, and TALE. Some of these TFs have even more than ancient origins (eastward.k., FOX, bZIP, Rel/NK-kB, TALE, typical (non-TALE) homeoboxes, and T-box) [72]. These factors could be divided into iii categories. Creature TF genes that have no clear relatives outside the Metazoa are considered a type I novelty and currently include nuclear receptor families, ANTP homeobox. The POU, PAX, and Half dozen homeobox classes all can exist classified every bit type II novelties, where the beast restricted POU, SIX, and PAX domains are combined with the more ancient homeodomains to produce the metazoan novelty. In contrast, type III novelties are those where ancient premetazoan domains combine in novel ways to generate metazoan specific domain compages; an example is the beast-specific way in which the ancient LIM and homeodomain combine in the LIM-homeodomain form [76].

Also, functional Wnt, Notch, and TGF-β signaling pathways were developed on the protometazoan stage [73].

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Unicellular Eukaryotes as Models in Cell and Molecular Biological science

Martin Simon , Helmut Plattner , in International Review of Cell and Molecular Biology, 2014

2.2 Unicellular models: Examples, pitfals, and perspectives

Surprisingly lower eukaryotes, such as choanoflagellates and slime molds, possess precursors of cell adhesion molecules ( King et al., 2003, 2008; Shalchian-Tabrizi et al., 2008). With choanoflagellates this likewise includes signal transmission past changing the phosphorylation country of tyrosine residues—a crucial aspect of jail cell integration into tissues (Mayer, 2008; Miller, 2012; Shalchian-Tabrizi et al., 2008) upwardly to humans, with disregulation phenomena in cancer. In a recent congress report, Carpenter (2012) summarized work by Male monarch showing that some bacteria drive the formation of multicellular aggregates of choanoflagellates—a new epitome for an of import evolutionary stride. Already several generations ago some biologists propagated choanoflagellates equally precursors of metazoans. This is ane of these fascinating one-time hypotheses which now go increasingly supported by molecular biology (other examples are the symbiotic bacterial origin of chloroplasts and mitochondria). In Dictyostelium, a small molecular chemical compound (cyclic di-[3′:5′]-guanosine monophosphate) was identified equally the amanuensis that causes amoebae to aggregate to a multicellular "stem" (Chen and Schaap, 2012). All this shows old models in new calorie-free and their ongoing validity as model systems.

There are many other examples of the elucidation of signal transduction in unicellular models. As generally known, Ca^{2   +} is a universal second messenger and mediator of membrane–membrane interactions. Ca^{ii   +} may come from the outside medium or from internal stores (Berridge et al., 2003; Laude and Simpson, 2009; Petersen et al., 2005). In the field of Ca^{2   +} signaling, there be many examples supporting the value of such models. This is true specifically for ciliates, specially considering of the piece of cake applicability of conventional electrophysiology. Originally the low capacity/high affinity type of cytoplasmic Ca^{2   +}-binding protein, calmodulin, was plant in mammalian encephalon and testes (Cheung, 1980), only when it was detected in protozoa some crucial observations were made: Calmodulin in a complex with Ca^{2   +} activates a variety of plasmalemmal cation channels (Ehrlich et al., 1988; Saimi and Kung, 2002), just it shuts downwards conductivity of voltage-dependent Ca^{two   +}-influx channels in the ciliary membrane (Brehm and Eckert, 1978; Saimi and Kung, 2002). Such negative feedback of subplasmalemmal Ca^{2   +} had been searched for in brain cells. Only later the discovery of this phenomenon in Paramecium, the same inhibitory mechanism, based on a Ca^{2   +}/calmodulin complex, was found also in neuronal cells (Levitan, 1999; Xia et al., 1998). Recently an additional regulator for the inactivation of such channels has been plant in brain neurons, that is, Ca^{ii   +}-binding protein 1, which acts in competition with calmodulin (Oz et al., 2013).

Other molecules crucial for the brain office and immune defense, such as calcineurin (poly peptide phosphatase 2B, PP2B; Klee et al., 1998), surprisingly occur in protozoa. Interestingly plants (Angiosperms) only limited the B subunit as part of the stress-tolerance machinery (Gu et al., 2008). This exemplifies why institute cells are less suitable every bit full general models in cell biological science. The occurrence of both, the catalytic and regulatory subunits was well established for protozoa, including parasitic Apicomplexa and gratuitous-living ciliates (Fraga et al., 2010). Experiments with Paramecium taught united states its involvement in exocytosis (Momayezi et al., 1987) and this attribute has been followed up subsequently up to human. Nevertheless, this may encompass widely different aspects up to fusion pore expansion (Samasilp et al., 2012) and ensuing exocytosis-coupled endocytosis of empty "ghosts" (Lai et al., 1999). Manifestly such basic mechanisms are conserved from protozoa to man where the activity of calcineurin culminates in long-term potentiation, that is, learning (Mulkey et al., 1994), and immune defence via activation of transcription factor NFAT in T-cells (Bueno et al., 2002). However, as the function of calcineurin is not all the same fully understood in any system, farther experiments with protists may yield important clues.

Because the big number of genes, a prison cell is a complicated puzzle indeed. With ongoing development, the increase in the number of poly peptide-encoding genes is surprisingly moderate. Which advantages can protozoa offering along these lines? In college eukaryotes, culling splicing tin generate many more protein forms which tin besides help to fit building blocks more precisely and flexibly into the 4D puzzle. Moreover, different posttranslational modifications can increase the complexity of a cell. Estimates of the average number of splice variants in mammalian cells range from three to seven per transcript. In contrast, alternative splicing is about absent in ciliates, for example, Paramecium (Jaillon et al., 2008). Rare examples are certain types of intracellular Ca^{2   +}-release channels (CRCs) such equally PtCRC-Half-dozen-3, but here this may bespeak a pathway to pseudogenization (Ladenburger and Plattner, 2011). In principle, the general absenteeism of alternative splicing in ciliates can facilitate analyses of role and intracellular localization. This reward may be canceled whenever there occurred whole genome duplications; these can result in a number of like paralogs (also chosen ohnologs) as described below. In this regard, Tetrahymena is more than favorable than Paramecium. Posttranslational modifications tin can be manifold as well in protozoan cells. An example is glycination and acetylation of tubulin. This results in topologic diversification, that is, specific microtubule subpopulations achieve specific subcellular localization and stability in Paramecium every bit well equally in Tetrahymena (Adoutte et al., 1991; Libusová and Dráber, 2006; Wloga and Gaertig, 2010).

In Paramecium, the plethora of extensive gene families encoding many protein isoforms complicates the situation. These cells, in contrast to Tetrahymena, have numerous paralogs because of several rounds of whole genome duplications (Aury et al., 2006). Such paralogs/ohnologs are known from many Paramecium gene families. Examples are not just tubulin (Dutcher, 2001) merely also actin, with differential positioning of isoforms in the cell (Sehring et al., 2007a,b). This special situation makes the analysis of some aspects with this model rather intriguing. Nonetheless, in these cells, closely related paralogs from whole genome duplications can likewise provide admission to the analysis of further specification of poly peptide molecules. This can so get an culling to culling splicing. An example is the neofunctionalization of η-tubulin which may contribute to epigenetically controlled surface structuring (Ruiz et al., 2000). Thus, some models may entail specific problems but also open up new perspectives.

While this situation shows specific aspects of, and bug with some models, this can exist a chance for evolutionary studies. Moreover, it is possible to switch from Paramecium to a related model with less paralogs, such as Tetrahymena although this cell is less easily amenable to microinjection and electrophysiology. The same is true of yeast, with the advantage of still fewer genes. Thus, one has to find the proper rest between advantages and disadvantages to select the proper model for a specific problem. A model should have a special characteristic that makes it suitable for the assay of a specific aspect, thus following the postulate of the founding father of genetics, William Bateson, about 100 years agone: "Foster your exceptions." Toward the finish of Bateson'south life, Morgan established Drosophila every bit the most successful model in classical genetics. And so, there is no ane model for everything and different models should be available for different problems.

Allow u.s. at present illustrate the potential of unicellular organisms as models past some of the well-nigh contempo and dramatic discoveries in cell biology. Due to the special trait of chromosome fragmentation, Tetrahymena has allowed for the discovery of ribozymes, that is, catalytic self-splicing RNA (Kruger et al., 1982). This resulted in a Nobel Prize in 1989. Similarly it stimulated the recognition of telomeres and telomerase (Blackburn, 2010; Blackburn and Gall, 1978; Greider and Blackburn, 1985) honored with the 2009 Nobel Prize. All this was based on the knowledge of all-encompassing chromosome fragmentation which profoundly increases the number of telomeres in ciliates such as Tetrahymena (Katzen et al., 1981). Recently this advantage has facilitated the elucidation of the molecular structure of the telomerase holoenzyme (Jiang et al., 2013). Fragmentation of chromosomes is at present known to exist fifty-fifty much more abundant in the ciliate Oxytricha (Swart et al., 2013) which, thus, would exist an even ameliorate model. Currently regulation of epigenetic inheritance in ciliates is an example of rising interest (Section 4).

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Pre-ediacaran development

Nelson R. Cabej , in Epigenetic Mechanisms of the Cambrian Explosion, 2020

The choanocyte genome

Recently, biologists have sequenced the genome of a free-living choanoflagellate, Thousand. brevicollis (King et al., 2008), which consists of 9196 genes, which is unusually big for a unicellular organism. Most of these genes certainly derive from the common ancestor of choanoflagellates, placozoans, and eumetazoans in the late Precambrian menstruum, more than than 600   million years ago (King et al., 2008). Choanoflagellates possess a genetic toolkit that comprises genes for many families of metazoan transcription factors, prison cell signaling, cell adhesion, signaling pathways, Hox genes, etc., and a surprising number of tyrosine kinases and their downstream elements. No other known unicellular eukaryote has whatsoever of the metazoan developmental signaling pathways (Male monarch et al., 2008). It contains ii homeodomain proteins, previously thought to be specific to metazoans. M. brevicollis genome as well contains protein domains associated with jail cell adhesion in metazoans. All these genes seem to take been inherited from the unicellular common ancestor of choanoflagellates and metazoans. "The common ancestor of metazoans and choanoflagellates possessed several of the critical structural components used for multicellularity in modernistic metazoans" (King et al., 2008).

The fact that many metazoan genomic features (e.g., genes for metazoan-specific signaling pathways) are absent-minded in the choanoflagellate is believed to be result of the gene loss in Chiliad. brevicollis in the form of its evolution from the common ancestor of choanoflagellates and metazoans, just information technology seems more likely that these features were subsequently added to the genetic toolkit of metazoans.

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Gene Regulatory Networks

Douglas H. Erwin , in Current Topics in Developmental Biology, 2020

2 The origin of metazoan regulatory novelties

The closes living relatives of Metazoa are, successively, choanoflagellates, filastreans and ichthysporeans (collectively these 4 groups comprise the Holozoa). Comparative studies of representatives of these groups have provided insight into the early development of the metazoan regulatory genome ( Brunet & King, 2017; Richter, Fozouni, Eisen, & King, 2018; Richter & Male monarch, 2013; Sebe-Pedros et al., 2016; Sebe-Pedros, Degnan, & Ruiz-Trillo, 2017; Simakov & Kawashima, 2017), every bit studies of sponges, cnidarians and other taxa have illuminated the expansion of the metazoan regulatory genome. Cistron loss has been common, nonetheless, particularly in clades that have experienced morphologic simplification. Thus using the genome of a single species as an exemplar for a big clade can exist misleading.

Fig. 1 details the progressive expansion of the metazoan regulatory genome in a phylogenetic context (run into Erwin, 2020, for more detail). Three noteworthy points emerge from such a compilation. Showtime, many putatively "bilaterian" or "metazoan" elements have now been identified among the cousins of Metazoa and a plausible case has been avant-garde that they originally evolved to control temporal patterning in groups with complex life cycles (Arenas-Mena, 2017; Sebe-Pedros et al., 2017; Sogabe et al., 2019). Both complex life cycles and multicellular species are found inside each of the major clades of holozoans, although the class of multicellularity differs. With the advent of animals, regulators of temporal differentiation were repurposed for spatial control. 2d, during the early on diversification of animals the evolution of the metazoan genome included both the introduction of novel regulatory elements, including distal enhancers, new developmental transcription cistron families, a new type of promoter, and other elements, as well as continuing expansion of the developmental toolkit. Distal enhancers do not appear to have become widely deployed components of the regulatory genome until the diversification of Bilateria (Sebe-Pedros et al., 2018). (Distal enhancers accept recently been reported from plants also (Lu et al., 2019)). As genome size expanded, the complexities of regulatory control evidently increased as chromatin compages played an increasingly important part in regulating transcription in bilaterians, particularly through the appearance of CTCF sequences equally boundary elements for transcriptionally active domains (TADs) (Gaiti, Calcino, Tanurdzic, & Degnan, 2017). Finally, the phylogenetically early appearance of many deeply conserved regulatory elements reveals that the function of these elements changed over evolutionary fourth dimension. In contrast to the early days of "evo-devo" it is now articulate that fifty-fifty if genes are securely homologous and serve like functions in diverse living clades, this is not unambiguous show for the functions of these genes half a billion years ago.

Fig. 1. The history of acquisition of important parts of the holozoan and metazoan regulatory genomes, plotted on a phylogenetic tree. Red bars show the contained co-option of patterning elements in deuterostomes, ecdysozoans and lophotrochozoans associated with the generation of more hierarchically structured GRNs and other regulatory novelties as brute trunk sizes increased during the Ediacaran-Cambrian metazoan radiation (~   550–520 million years ago).

Based on references cited in text and in Erwin, D. H. (2020). Origin of animate being bodyplans: A view from the regulatory genome. Development 147, dev182899.

This final bespeak bears elaboration for it provides critical insight into the nature of regulatory development in animals. All-encompassing co-option of GRN subcircuits initially involved in cell type specification and simple embryonic patterning placed these subcircuits at the meridian of all-encompassing regulatory hierarchies responsible for regional patterning (Erwin, 2020; Erwin & Davidson, 2009). These considerations inform the following model (Erwin, 2020 ): temporal and spatial regulation is a mutual feature of Holozoa, and was co-opted for increased spatial differentiation within animals. The dominant course of regulatory control in basal metazoans (sponges, cnidarians and placozoa) is proximal via combinations of transcription factors at enhancers proximal to the coding sequence (Sebe-Pedros et al., 2018) (Fig. 1). Nearly GRNs in the earliest animals were likely relatively flat, at least in comparison to those developing amid bilaterians. The initial diversification of bilaterians involved the generation of more hierarchically structured GRNs through intercalation of new spatial and temporal regulators into the initial, flatter GRNs (Davidson & Erwin, 2006; Erwin & Davidson, 2009; Peter & Davidson, 2015). Extensive co-selection of subcircuits occurred, distal enhancers became more widespread, and new forms of control over chromatin were introduced. Finally, co-option leads to independent construction of many developmental processes, including partition, a tri-partite brain, appendages, regionalized gut and sensory systems. These processes generated new prison cell types, new developmental patterns and thus new phenotypes. Although the development of new cell types has been described equally bifurcating divergence (Arendt, 2008) other processes may be involved as well (Arendt et al., 2016).

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Metazoans, Origins of

M. Klautau , C.A.Chiliad. Russo , in Encyclopedia of Evolutionary Biology, 2016

Origin of the Sponges

Sponges were most possibly originated from a benthic colony of choanoflagellate-like organisms that might have had cell types with singled-out phases to feed (protochoanocytes) and to reproduce (protoarchaeocytes) ( Valentine, 2004). Notwithstanding, to go a metazoan, sponges had to develop multicellular bodies with differentiated cell types and an extracellular matrix.

Cleavage was probably a vital pace to a rapid development and to organize the differentiation and arrangement of cell lines (Valentine, 2004). Adding support to this early origin of eggs and cleaving embryos, fossils about 600 Mya have been establish (Xiao and Knoll, 2000). Indeed, fossils that were originally identified equally the protist group acanthomorphic (i.e., spinose) acritarchs were in fact hulls of diapause animal eggs (Yin et al., 2004, 2007). This finding might suggest that sponges, and perhaps other animals, could have appeared before 580 Mya (before the Ediacaran fauna!) (Telford and Littlewood, 2009). Recently, a fossil sponge in a geological stratum of 600 Mya was reported (Yin et al., 2015).

Although sponges are supposed to be morphologically uncomplicated, they have many features that are shared with metazoans. Apart from multicellularity, homebox genes, and task division, they exhibit collagen, septate junctions, integrins, fibronectins, and all appliance to link the extracellular matrix to the cytoskeleton (Valentine, 2004). Sponges besides adult cell specialization, although not rarely their cell differentiation is non last. The continuous totipotency/pluripotency of sponge cells provides these animals with a high level of plasticity that probably enabled them to survive along in the past 600 million years.

Sponges do not show nervous system or fifty-fifty organs. Basal lamina, a distinctive metazoan characteristic, is reported in a single out of the four extant Porifera classes. Currently, in that location are five recognized classes of sponges: Archaeocyatha, Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. Archaeocyatha is a class of marine sponges with calcium carbonate skeleton and that were reef builders in the Cambrian, when they became extinct. Calcarea is the only extant class that produce calcium carbonate spicules whereas the other three have silicium spicules. Demospongiae is the near speciose class, representing nearly 85% of the phylum. These are the only sponges that have invaded freshwater and that developed carnivory. Hexactinellida are mainly deep-bounding main sponges and are the only ones that present syncytial tissues. Until recently, Homoscleromorpha was considered a bracket of Demospongiae, but information technology was recently elevated to a course status.

In social club to abound in size, sponges adult a organization of inhalant and exhalant canals and chambers with flagellate cells (choanocytes) (Figure 10) that continuously pump h2o through the body of the sponge. Every bit the water goes through, the sponge is able to larn oxygen and food (bacteria and macromolecules) and too to eliminate residues. These continued channels and chambers consist of the aquiferous arrangement, the main synapomorphy of Porifera, simply some carnivorous sponges lack this characteristic (Vacelet and Boury, 1995).

Effigy 10. Schematic section of an asconoid sponge. Photo by E. Hajdu.

Sponges practice not possess truthful symmetry, merely they come up in different forms such as spherical, cylindrical, radial, and even some present bilateral symmetry (i.e., hexactinellids) and they are morphologically very unproblematic, as they practise not have organs, or sensorial cells not even nervous organization. Every bit sponges lack nervous cells, all communication between cells is mediated peradventure past chemical messengers. Additionally, sponge tissues are not considered to be true tissues, as nigh of the species exercise non have a basal lamina. The basal lamina is a membrane of proteins (laminins and collagens) working as a bulwark to avert that cells from one tissue infiltrates other tissues and, among sponges, it is encountered in Homoscleromorpha. Equally most sponges do not have this bulwark, neither strong cell junctions to isolate their epithelia, their cells can move freely throughout the sponge body. Associated with prison cell totipotency, this feature makes sponges very plastic animals a fundamental footstep that surely contributed for their adaptation and survival.

Although most sponges lack basal lamina, they share with other metazoans several characteristics, including multicellularity, type Four collagen, septate junctions, and work sectionalisation among cells. If in 1 paw the monophyletic origin of Porifera has been already questioned (eastward.grand., Lafay et al., 1992; Borchiellini et al., 2001; Sperling et al., 2009), on the other hand studies with very large matrices of Dna sequences are showing that sponges are monophyletic (Philippe et al., 2009, 2011; Choice et al., 2010), merely phylogenetic relationships among the classes are less articulate. Many recent molecular studies take shown that Calcarea is more related to Homoscleromorpha and Demospongiae is closely related to Hexactinellida (Philippe et al., 2009; Pick et al., 2010), merely more comprehensive studies are yet required to settle this affair.

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Phanerozoic evolution—Ediacaran biota

Nelson R. Cabej , in Epigenetic Mechanisms of the Cambrian Explosion, 2020

Synaptic proteins in unicellulars

Of all unicellulars, the most closely related to metazoans seem to be choanoflagellates ( Choanoflagellatea) (King et al., 2008) and protists living as unicellular and colonial forms in marine and freshwater. Cells resembling choanoflagellates are found in sponges (choanocytes) and other multicellular animals.

Among the synaptic proteins identified in the unicellular choanoflagellate, Monosiga brevicollis, which are absent-minded in metazoans, are a number of tyrosine kinases, which in this protist may play a role in detecting and adaptively responding to changes in environs, too as adhesion poly peptide homologs, such every bit cadherins, which may be used for prey capture (Male monarch et al., 2003). In choanoflagellates are identified neurally important synaptic proteins that are absent in nonmetazoans. However, the nearly ancient synaptic poly peptide families are conserved in unicellular eukaryotes, such as the yeast Saccharomyces cerevisiae and the ameba Dictyostelium discoideum. Over 21% of the mammalian MASC (MAGUK-associated signaling complexes) genes and 25% of the postsynaptic density genes have orthologs in these protosynaptic nonmetazoan organisms (Ryan and Grant, 2009). A general view of the evolution of postsynaptic components in the living globe is presented in Fig. 2.12.

Figure 2.12. Evolution of postsynaptic components. (A) Various poly peptide types (left column) that institute the postsynaptic density (PSD) and membrane-associated guanylate kinase (MAGUK) associated signaling complexes (MASCs) in unicellular eukaryotes (fungi), protostomes, and deuterostomes are ordered based on whether they take "upstream" or "downstream" signaling roles. Noncolored fields represent the absence of a given poly peptide. Dark grey rectangles stand for presence of protein. Grey rectangles stand for enrichment of a protein type in protostomes or protostomes and deuterostomes when compared with unicellular eukaryotes. Light gray rectangles represent enrichment of a protein type in deuterostomes when compared with protostomes. The diagrams to a higher place each column represent the molecular associates of MASC, in which upstream proteins (blue [gray in print version] circles) are continued to downstream proteins (yellow [white in impress version] triangles) through intermediate signaling proteins (carmine [dark grey in print version] rectangle). The relative proportions of these proteins in eukaryotes, protostomes, and deuterostomes are therefore illustrated. (B) The emergence of titular MASC components beyond clades is illustrated. Proteins are ordered based on whether they are located "upstream" or "downstream" in synaptic signal transduction pathways (Emes et al., 2008). Noncolored fields represent the absence of a given protein, whereas dark gray rectangles announce its presence. Diagrams of MASC structure are placed above each clade, along with an illustration of a representative model organism. AMPA, α-amino-three-hydroxy-5-methyl-4-isoxazolepropionic acid; CaMKII, calcium/calmodulin-dependent protein kinase II; Dlg, discs-big homolog; mGluRs, metabotropic glutamate receptors; NMDA, N-methyl-d-aspartate; NR2, NMDA receptor ii; SynGAP, synaptic Ras GTPase activating protein.

Diagrams in part (A) are modified, with permission, from REF. 16© (2008) Macmillan Publishers Ltd. All rights reserved. From Ryan, T.J., Grant, S.Yard.N., 2009. The origin and development of synapses. Nat. Rev. Neurosci. 10, 701–712.

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Microbial Globins - Status and Opportunities

Serge North. Vinogradov , ... David Hoogewijs , in Advances in Microbial Physiology, 2013

iv.half dozen Opisthokonta

The Opisthokonta contain metazoans (animals), fungi and several boosted microbial eukaryote lineages, including the Choanoflagellida, Ichthyosporea, Nucleariidae and Capsaspora. It is probable that the closest extant relative of both fungi and metazoans is a fellow member of the Opisthokonta. Furthermore, it appears that the closest relatives of metazoans are the choanoflagellates, followed past the Capsaspora and Ichthyosporea lineages ( Ruiz-Trillo, Roger, Burger, Grey, & Lang, 2008). Recently, the genomes of 5, close relatives of fungi and metazoa have been sequenced, as part of the Origin of Multicellularity Projection at the Wide Institute (Ruiz-Trillo et al., 2007). These include Capsaspora owczarzaki, an amoeboid parasite of the pulmonate snail Biomphalaria glabrata, which has a relatively small genome about 22–25   Mbp (Ruiz-Trillo, Lane, Archibald, & Roger, 2006), the apusozoan T. trahens (formerly Amastigomonas sp. ATCC 50062), two choanoflagellates, Salpingoeca rosetta (formerly Proterospongia sp. ATCC 50818) and Monosiga brevicollis, and two basal fungi, Allomyces macrogynus and Spizellomyces punctatus. Although these genomes contain globins, T. trahens has only one FHb, and A. macrogynus has two SSDgbs and a chimeric globin with a C-final TrHb1 domain. The ichthyosporean Sphaeroforma arctica has five SDgbs and an FHb, while C. owczarzaki has two chimeric two-SDgb domain proteins (Table 9.3). No nucleariid genomes are bachelor. The two available choanoflagellate genomes, each encode three SDgbs. It is worth pointing out that a hypothetical poly peptide of 1653 amino acids PTSG_01043 (EGD76343.one) can be identified in S. rosetta (Salpingoeca sp. ATCC 50818) via BLASTP search using Strongylocentrotus purpuratus androglobin (isoform i; XP_001186225.2). Whereas the N-terminal cysteine protease domain is present, the key globin domain is not identified by either BLASTP or FUGUE searches. Although this result is hardly surprising, given the fact that in multicellular metazoans, including all deuterostomes, androglobin appears to be predominantly expressed only in testis tissue (Hoogewijs, Ebner, et al., 2012), it suggests that androglobin was nowadays in the ancestor shared past metazoans and choanoflagellates.

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Phylogeny of Tec Family unit Kinases: Identification of a Premetazoan Origin of Btk, Bmx, Itk, Tec, Txk, and the Btk Regulator SH3BP5

Csaba Ortutay , ... C.I. Edvard Smith , in Advances in Genetics, 2008

VI THE ORIGIN OF PHOSPHOTYROSINE SIGNALING AND THE Office OF CYTOPLASMIC TYROSINE KINASES

The evolution of phosphotyrosine signaling suggests that more than 600 meg years agone there was a common ancestor for the unicellular choanoflagellates and for multicellular metazoans, which had already developed this power ( King and Carroll, 2001; King et al., 2008; Peterson and Butterfield, 2005; Pincus et al., 2008). In some species, such as in yeast, tyrosine phosphorylation appears at a very depression level, most likely due to promiscuity of serine/threonine kinases (Schieven et al., 1986). In a recent report, Pincus et al. (2008) suggest that phosphatases and SH2 domains appeared kickoff, whereas the enzymatic action of tyrosine kinases developed subsequently. The emergence of specific proteins resulted in the expansion of proteins and domains in cellular signaling. I 3rd of all domains found in combination with SH2 domains in choanoflagellates are unique while 38% are shared with metazoans.

Proteins agile in tyrosine kinase-related signaling are quite abundant in choanoflagellates (King et al., 2008). While most proteins in serine–threonine signaling pathways are common between metazoans and choanoflagellates, the opposite is true for tyrosine phosphorylation and several other intracellular signaling pathways, including many transcription factors.

Among choanoflagellates, SFKs and C-terminal Src kinase (Csk) were first reported in M. ovata (Segawa et al., 2006). The M. ovata Src has transforming capability only is not negatively regulated by Csk. Biochemical characterization of the M. brevicollis Src and Csk indicated that the putative, regulatory C-final tyrosine is not phosphorylated (Li et al., 2008).

Our report is the first to demonstrate the existence of a TFK in Yard. brevicollis, which has estimated to have 128 tyrosine kinase genes (Male monarch et al., 2008). Although the role of the 1000. brevicollis TFK is unknown, its mere beingness clearly suggests that it is functionally active in a unicellular organism. Since all domains of TFKs are conserved in this protein, it is possible that this kinase is already regulated by SFKs, PI3K, and PKC, like the metazoan counterparts. Yet, given the lack of Csk-induced control of SFKs in M. brevicollis every bit well as in K. ovata, it is equally possible that the regulation differs. Choanoflagellates besides encode an SH3BP5-related molecule, which has non been functionally characterized. Thus, it is also early say whether this molecule suppresses the corresponding TFK. Functional studies will be needed to resolve this consequence besides as the possibility that the choanoflagellate TFK can substitute for the loss of TFKs in metazoan cells.

Our written report of TFKs reveals that these enzymes are aboriginal and their ancestor appeared already in choanoflagellates. TFK members are regulated by several proteins and they control numerous signaling pathways. More studies will be needed to investigate how the pathways in which TFKs currently participate originally obtained this property.

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Cranial and Spinal Nerves of Fishes: Evolution of the Craniate Pattern☆

B. Fritzsch , D.C. de Caprona , in Reference Module in Life Sciences, 2017

Introduction

Understanding how the vertebrate's body and head, with its nervous organization and craniate senses, evolved requires a broad perspective taking into account the development of multicellular animals. A brief overview of animal evolution is therefore needed here to set the stage for our discussion of how the original, lengthened nervous systems, organized every bit nerve nets with no centralized nervous or sensory component, differentiated over fourth dimension into complex brains interconnected to the peripheral sensory organs and effector systems with peripheral sensory and motor nerves.

A new theory has recently emerged based on the insight that all multicellular organisms were derived from single-celled organisms called choanoflagellates, then named for their single flagellum surrounded by a collar of microvilli. The flagellum propels food caught by and ingested at the base of the microvilli. Aside from sponges (Porifera) and an odd species of an baggy colony-like aggregation of single-celled organisms (Placozoa), multicellular animals can be divided into 2 peachy clades ( Fig. 1), which are based on the type of body symmetry and the number of principal tissue layers:

Fig. 1. The simplified cladistic relationships of animals and some critical events apropos the evolution of neurons and sensory systems: (i) diffuse epithelial nerve net; (2) miR-124 specific for neurons appear; (three) miR-183 specific for sensory cells announced; (4) germination of a pentameric nerve cyberspace with rudimentary sensory organs; (5) partial dorsal invagination of the epithelial plexus and appearance of localized sensory cells; (half dozen) neurons are concentrated in a dorsal neural tube and blended sensory organs appear. Sensory neurons are inside the neural tube and motor neuron(s) are ventral in the neural tube. Possible precursors of neural crest, branchial motor neuron(south), and somatic motor neuron(s) appear; (7) branchial motor neuron(south) are present, get-go appearance of neural crest and perchance placodes; and (8) placode-derived sensory systems, cranial branchial motor neuron(due south), and spinal somatic motor neuron(south) announced. Eyes, ears, olfactory organ, gustatory modality buds, lateral line, electroreceptive ampullary organs, and ocular muscles appear.

ane.

Radially symmetrical animals are diploblasts. They take two primary tissue layers, the outer ectoderm and the inner endoderm. They include jellyfish and their relatives (Cnidaria) and the comb jellies (Ctenophora).

2.

Bilaterally symmetrical animals are triploblasts. They have three main tissue layers (intermediate layer of mesoderm in addition to the ectoderm and endoderm), and include all bilaterally symmetrical invertebrates and vertebrates. Amidst triploblasts, a group of flatworms lacking a gut are referred to as acoelomate flatworms (Acoela, Fig. i). These animals are no longer considered to exist true flatworms (the phylum Platyhelminthes) but rather are viewed as basal triploblasts. The rest of the bilaterally symmetrical animals, the Nephrozoa (animals with kidneys), comprise the Protostomes and Deuterostomes. The sometime include most species of invertebrates (including the "true" flatworms, Platyhelminthes), while the latter include the vertebrates and their deuterostome invertebrate relatives. The most basal extant deuterostomes (Fig. 1) are of the genus Xenoturbella, which comprises two species of marine worms, and the newly recognized clade Ambulacraria, which contains the echinoderms (starfish, sea urchins, etc.) and the hemichordates (acorn worms, with cylindrical bodies, and pterobranchs, with vase-shaped bodies). Chordates incorporate those animals which have a notochord at some point in their lifecycle and are divided into 3 major extant clades: the cephalochordates (amphioxus), the urochordates (bounding main squirts or tunicates), and the craniates (vertebrates) (Fig. 1).

Of the numerous theories most the origin of the craniate head, brain, and peripheral nerves which have been proposed over the past 150 years, most assumed a transformation of either an amphioxus-like (cephalochordate) organism into a craniate with a clearly delineated head (the new head hypothesis), or that tunicates (urochordates) class the outgroup of craniates and consequently, many head organs, jaws, and the craniate spinal cord had to evolve afresh. Ideas that the last common bilaterian ancestor of nephrozoans had a highly developed primal nervous system (CNS) and was segmentally organized require the supposition that many basal protostomes and deuterostomes have secondarily lost all of these features and devolved into a more ancestral-like organization.

As all multicellular organisms derive from unmarried-celled choanoflagellates, they share molecular synapomorphies at the level of diploblasts and triploblasts such as dorso-ventral patterning of epithelial cells, indicating that the ectodermal nervus internet constitute in diploblasts may likewise accept been the primitive land in basal triploblasts. In fact, inside basal deuterostomes, the marine worm in the genus Xenoturbella has an epithelial nervus plexus with no recognizable sensory system. Among Ambulacralia, acorn worms (hemichordates, Fig. 1) have some centralization of nervous elements without germination of a true CNS and single sensory cells instead of sensory organs. Likewise, what appears to be basal protostomes, the arrow worms (Chaetognathans), mostly accept an epidermal nerve plexus with little concentration into a ganglion, like to basal triploblasts, the acoel worms. The wide phylogenetic distribution of these shared features indicates that the common triploblast antecedent, and therefore likely also the common deuterostome ancestor, may have had an epithelial nervus plexus with little, if whatsoever, metameric (segmental) organization. If the morphology of the nervous system in Xenoturbella is indeed close to the basal deuterostome organization and not secondarily derived through regressive development, the antecedent of craniates could accept had a nerve net, avoiding the need to transform an already existing CNS, as in amphioxus or tunicate larvae, into a craniate-like CNS (Fig. i).

For this article, nosotros assume that the deuterostome and craniate ancestor had a nerve net very much like the Xenoturbella's unorganized nerve internet with little to no metameric arrangement. It seems unlikely that a metameric organization of a well-adult CNS, one time formed, devolved to form the epithelial nerve nets at present establish in diploblasts and basal triploblasts.

Following already well-entrenched arguments for the evolution of optics and ears, the unlike patterns of nervous system establish in deuterostomes are considered here to be independently derived forms reflecting either dissimilar and independent steps in forming a CNS (acorn worms, cephalochordates, urochordates, and craniates) or a transformation into a pentameric (five-portioned) nerve cyberspace (echinoderms). A consequence of this hypothesis is that many problems in establishing homology of peripheral nerves and sensory organs across deuterostomes become obsolete, leading to the more than parsimonious idea that the germination of a CNS comes about by aggregating a baso-epidermal nerve net and condensation of distributed sensory cells into sensory organs. Such centralization may take happened at to the lowest degree twice in deuterostomes (acorn worms, chordates) and at to the lowest degree twice in protostomes (insects and related species; mollusks, and related species).

A consequence of this condensation is the formation of connecting strands of the emerging CNS with effector and sensory organs, referred to here every bit "nerves." Apparently, in club to communicate with the remaining parts of the torso, such nerves have to carry both afferents (for sensory input from the periphery) and efferents (for motor output to the periphery). A by-product of this condensation of the CNS out of a lengthened nerve cyberspace is the condensation of various sensory cells with more or less discrete segregation of sensory input into organs dedicated to a specific fix of stimuli such as photic, mechanical, or chemical stimulation besides equally the evolution of the neural crest (embryonic tissue giving ascension to most of the peripheral nervous system (PNS)).

For this idea to be truthful, each of the deuterostomes having a CNS will have to show sure similarities in its overall development and its molecular basis which in office volition reverberate the patterning already plant in the more or less centralized nerve plexus of acorn worms. In detail, genes related to neurulation should exist mutual among all deuterostomes with a CNS, but may reflect common co-option or a prototypical partial aggregation into a CNS that does not reach the level of obvious homology due to lack of topological landmarks. Patently, only craniates accomplished the aggregation of all neurons into a CNS, aggregated all peripheral sensory systems into discrete sensory organs, and developed a unique set of innervation via neural-crest-derived sensory neurons and neurogenic-placode-derived sensory organs. Neurogenic placodes, like neural crest, are ectodermally derived structures just are nowadays just in the caput; along with neural crest, they produce the sensory neurons of the PNS in the head region. The formation of these special embryonic tissues – neurogenic placodes and neural crest – is due to the newly evolved developmental mechanism which achieves this aggregation of sensory cells in craniates' embryos.

Whereas much piece of work has concentrated on the specific gene networks promoting embryonic formation of placodes or neural crest, few studies take been focusing on the molecular mechanism of suppression of neuronal fate determination in the remaining ectoderm. In other words, the ectoderm in a developing embryo will become nervous-arrangement tissue unless this fate is repressed; in the parts of the ectoderm where this repression occurs, the ectoderm forms epidermis, developing into the outer layer of the skin. It is at present articulate that fibroblast growth cistron (Fgf) upregulation, combined with express to no expression of bone morphogenetic proteins (BMPs) induce a neuronal fate in the ectoderm of chordates (ie, it becomes neuroectoderm) but patently not in Xenoturbella. That the default state of ectoderm for chordates is indeed neurogenic has been demonstrated by overexpressing proneuronal basic helix-loop-helix (bHLH) genes in the ectoderm, revealing a transformation into neurons.

Another emerging concept is the importance of micro-RNA (miR) for the evolution of neurons and sensory cells. Recent data have shown that specific miRs (miR-124) are evolutionary conserved and evolved commencement with triploblasts. MiR-124 has been shown to exist essential for neuronal evolution through the regulation of chromatin remodeling and absence of miR-124 (and other miR species) causes rapid degeneration of developing neurons. Another set of miR, miR-183,182, and 96, is evolutionary conserved amongst nephrozoa (ie, bilaterians excluding acorn worms). It has been shown to be associated with mature sensory cells and is crucial for their development. In fact, fifty-fifty single-base mutations in miR-96 pb to deafness. Obviously, the ability of miR to regulate large sets of transcription factors could be perceived as a prerequisite for the evolution of a more than complex neuronal and sensory arrangement.

This commodity provides an overview of the evolutionary steps and their underlying developmental transformations toward the formation of a PNS such equally the spinal and cranial sensory nerves, molecular mechanisms of suppressing neurogenesis in the ectoderm, invagination of the neuroectoderm, and induction and initial differentiation of placodes and of neural crest. It is organized to reverberate the currently accepted phylogenetic relationships among deuterostomes, with Xenoturbella (Fig. one) as a likely antecedent type for all deuterostomes as far as its nerve organization is concerned – information technology has a uncomplicated epithelial nerve net with no evidence of concentration of neurons or sensory cells. Unfortunately, beyond the knowledge of an unusual Hox lawmaking, no molecular data exist on these animals to illustrate what might be the original molecular lawmaking for the evolution of a nerve net.

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