Choanoflagellate

These colonial choanoflagellates illustrate the plausibility of the theory that a choanoflagellate-similar organism might have made the very first steps to colonial and multicellular animal life.

From: Current Topics in Developmental Biology , 2016

Growth Factors in Development

Thomas Due west. 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 current molecular phylogenies, choanoflagellates are the closest unicellular relative of metazoans (Rex 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 cell adhesion and signaling proteins (King et al., 2008). Choanoflagellates are morphologically similar 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 "Main Pluripotency Genes" of Stem Cells

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

eleven.ii.2 Choanoflagellate and Last Common Antecedent of Metazoan

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

Comparing the choanoflagellate Monosiga brevicollis genome to early metazoan reveals the molecular and metabolic regulators presented in the last common ancestor 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 Cost signaling pathway. However, homologs of the Notch, STAT, and Hedgehog signaling pathway components are present [72]. In addition, phosphotyrosine-based signaling is found in abundance in the M. brevicollis genome. TGF-β signaling is restricted to the Metazoa with neither ligand nor receptor molecules being institute outside of the brute kingdom [73].

The cadre transcriptional apparatus of G. brevicollis is, in many ways, typical of most eukaryotes examined to engagement including, for instance, all 12 RNA polymerase Two subunits and most of the transcription elongation factors (TFIIS, NELF, PAF, DSIF, and P-TEFb) [72].

About of TFs are C2H2-blazon zinc fingers, FOX TFs, otherwise known only from metazoans and fungi. M. brevicollis contains a subset of the TF families previously idea to exist specific to metazoans. Members of the p53, MYC, and SOX/TCF families were identified besides as the homolog of MYC TF whose action is implicated in the regulation of the differentiation and mobilization of the hematopoietic stalk cells (HSCs) [74]. Too, p53 regulates mammalian stem cell cocky-renewal; the SOX family unit of TFs involve the members that are office of a core transcriptional regulatory network that maintains the pluripotent state of mammal stem cells (meet Affiliate 7). These data implicate that the abovementioned TFs took place in the "stemness toolkit" at the choanoflagellate level.

Presence of all these factors in choanoflagellateas and the metazoan indicates that they evolved before the divergence of the choanoflagellates and metazoan and further suggests that they were present in the last mutual antecedent of choanoflagellates and metazoans.

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

Comparing 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 organization and content too as TFs and components of the signaling pathways are remarkably similar [75]. Too, comparing those genomes to the genome of the choanoflagellates helps to establish a set of TFs and signaling molecules that were nowadays in the last common ancestor of metazoan before their deviation [75]. This set is considered to have been developed at the protometazoan stage. Also, information technology 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, SIX, and TALE. Some of these TFs have even more than aboriginal origins (e.g., Play a trick on, bZIP, Rel/NK-kB, TALE, typical (non-TALE) homeoboxes, and T-box) [72]. These factors could be divided into three categories. Animal TF genes that take no articulate relatives outside the Metazoa are considered a type I novelty and currently include nuclear receptor families, ANTP homeobox. The POU, PAX, and SIX homeobox classes all can be classified equally type II novelties, where the animal restricted POU, 6, 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 architecture; an instance is the fauna-specific way in which the aboriginal LIM and homeodomain combine in the LIM-homeodomain class [76].

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

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Unicellular Eukaryotes every bit Models in Cell and Molecular Biology

Martin Simon , Helmut Plattner , in International Review of Prison 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 ( Male monarch et al., 2003, 2008; Shalchian-Tabrizi et al., 2008). With choanoflagellates this also includes indicate transmission by irresolute the phosphorylation state of tyrosine residues—a crucial aspect of jail cell integration into tissues (Mayer, 2008; Miller, 2012; Shalchian-Tabrizi et al., 2008) upwards to humans, with disregulation phenomena in cancer. In a recent congress report, Carpenter (2012) summarized piece of work by King showing that some leaner bulldoze the germination of multicellular aggregates of choanoflagellates—a new paradigm for an important evolutionary stride. Already several generations ago some biologists propagated choanoflagellates as precursors of metazoans. This is 1 of these fascinating former hypotheses which now become increasingly supported by molecular biology (other examples are the symbiotic bacterial origin of chloroplasts and mitochondria). In Dictyostelium, a small molecular compound (circadian di-[3′:5′]-guanosine monophosphate) was identified as the amanuensis that causes amoebae to aggregate to a multicellular "stem" (Chen and Schaap, 2012). All this shows old models in new light and their ongoing validity equally model systems.

There are many other examples of the elucidation of signal transduction in unicellular models. As generally known, Catwo   + is a universal 2d messenger and mediator of membrane–membrane interactions. Ca2   + may come up from the exterior medium or from internal stores (Berridge et al., 2003; Laude and Simpson, 2009; Petersen et al., 2005). In the field of Ca2   + signaling, at that place exist many examples supporting the value of such models. This is truthful specifically for ciliates, particularly because of the easy applicability of conventional electrophysiology. Originally the depression capacity/high analogousness type of cytoplasmic Ca2   +-binding protein, calmodulin, was constitute in mammalian brain and testes (Cheung, 1980), but when information technology was detected in protozoa some crucial observations were made: Calmodulin in a circuitous with Ca2   + activates a diversity of plasmalemmal cation channels (Ehrlich et al., 1988; Saimi and Kung, 2002), but information technology shuts downwards electrical conductivity of voltage-dependent Ca2   +-influx channels in the ciliary membrane (Brehm and Eckert, 1978; Saimi and Kung, 2002). Such negative feedback of subplasmalemmal Ca2   + had been searched for in encephalon cells. Just after the discovery of this phenomenon in Paramecium, the aforementioned inhibitory mechanism, based on a Ca2   +/calmodulin circuitous, was found as well in neuronal cells (Levitan, 1999; Xia et al., 1998). Recently an additional regulator for the inactivation of such channels has been found in brain neurons, that is, Ca2   +-binding protein 1, which acts in contest with calmodulin (Oz et al., 2013).

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

Because the large number of genes, a cell is a complicated puzzle indeed. With ongoing evolution, the increase in the number of protein-encoding genes is surprisingly moderate. Which advantages can protozoa offer along these lines? In higher eukaryotes, culling splicing can generate many more protein forms which tin can also help to fit building blocks more precisely and flexibly into the 4D puzzle. Moreover, different posttranslational modifications tin increment the complexity of a prison cell. Estimates of the average number of splice variants in mammalian cells range from three to seven per transcript. In contrast, alternative splicing is nigh absent in ciliates, for case, Paramecium (Jaillon et al., 2008). Rare examples are certain types of intracellular Ca2   +-release channels (CRCs) such as PtCRC-VI-three, but here this may indicate a pathway to pseudogenization (Ladenburger and Plattner, 2011). In principle, the general absenteeism of alternative splicing in ciliates can facilitate analyses of function and intracellular localization. This reward may be canceled whenever there occurred whole genome duplications; these can result in a number of similar paralogs (also called ohnologs) as described below. In this regard, Tetrahymena is more favorable than Paramecium. Posttranslational modifications can exist manifold also in protozoan cells. An example is glycination and acetylation of tubulin. This results in topologic diversification, that is, specific microtubule subpopulations accomplish specific subcellular localization and stability in Paramecium as well every bit in Tetrahymena (Adoutte et al., 1991; Libusová and Dráber, 2006; Wloga and Gaertig, 2010).

In Paramecium, the plethora of extensive factor families encoding many poly peptide isoforms complicates the state of affairs. 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 only tubulin (Dutcher, 2001) but 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. However, in these cells, closely related paralogs from whole genome duplications can also provide access to the analysis of further specification of protein molecules. This tin can then become an culling to alternative splicing. An instance 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 state of affairs shows specific aspects of, and issues with some models, this can be a run a risk for evolutionary studies. Moreover, it is possible to switch from Paramecium to a related model with less paralogs, such equally Tetrahymena although this jail cell is less easily amenable to microinjection and electrophysiology. The aforementioned is truthful of yeast, with the advantage of still fewer genes. Thus, one has to find the proper balance between advantages and disadvantages to select the proper model for a specific trouble. A model should have a special feature that makes it suitable for the analysis of a specific aspect, thus following the postulate of the founding father of genetics, William Bateson, about 100 years ago: "Foster your exceptions." Toward the end of Bateson's life, Morgan established Drosophila every bit the most successful model in classical genetics. So, there is no one model for everything and different models should be available for different bug.

Let u.s. now illustrate the potential of unicellular organisms as models by some of the most recent 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 extensive chromosome fragmentation which greatly increases the number of telomeres in ciliates such every bit 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 now known to be even much more than abundant in the ciliate Oxytricha (Swart et al., 2013) which, thus, would be an even meliorate model. Currently regulation of epigenetic inheritance in ciliates is an case of rising interest (Department iv).

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Factor Regulatory Networks

Douglas H. Erwin , in Current Topics in Developmental Biological science, 2020

2 The origin of metazoan regulatory novelties

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

Fig. 1 details the progressive expansion of the metazoan regulatory genome in a phylogenetic context (meet Erwin, 2020, for more detail). Iii noteworthy points emerge from such a compilation. Get-go, many putatively "bilaterian" or "metazoan" elements have at present been identified among the cousins of Metazoa and a plausible case has been advanced that they originally evolved to control temporal patterning in groups with circuitous life cycles (Arenas-Mena, 2017; Sebe-Pedros et al., 2017; Sogabe et al., 2019). Both complex life cycles and multicellular species are constitute within each of the major clades of holozoans, although the form of multicellularity differs. With the advent of animals, regulators of temporal differentiation were repurposed for spatial control. Second, during the early 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 standing 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 have recently been reported from plants too (Lu et al., 2019)). As genome size expanded, the complexities of regulatory control evidently increased as chromatin architecture played an increasingly of import role in regulating transcription in bilaterians, particularly through the appearance of CTCF sequences as 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 time. In contrast to the early on days of "evo-devo" information technology is now clear that even if genes are deeply homologous and serve like functions in diverse living clades, this is non unambiguous evidence for the functions of these genes half a billion years agone.

Fig. 1

Fig. 1. The history of conquering of important parts of the holozoan and metazoan regulatory genomes, plotted on a phylogenetic tree. Reddish bars show the independent co-choice of patterning elements in deuterostomes, ecdysozoans and lophotrochozoans associated with the generation of more hierarchically structured GRNs and other regulatory novelties as animal body sizes increased during the Ediacaran-Cambrian metazoan radiation (~   550–520 million years agone).

 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 signal bears elaboration for information technology provides disquisitional insight into the nature of regulatory development in animals. All-encompassing co-choice of GRN subcircuits initially involved in cell type specification and elementary embryonic patterning placed these subcircuits at the acme 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 common characteristic of Holozoa, and was co-opted for increased spatial differentiation within animals. The ascendant class 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). Virtually GRNs in the earliest animals were likely relatively apartment, at least in comparison to those developing among 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-option of subcircuits occurred, distal enhancers became more widespread, and new forms of control over chromatin were introduced. Finally, co-option leads to contained construction of many developmental processes, including division, a tri-partite brain, appendages, regionalized gut and sensory systems. These processes generated new cell types, new developmental patterns and thus new phenotypes. Although the evolution of new cell types has been described as bifurcating difference (Arendt, 2008) other processes may be involved too (Arendt et al., 2016).

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Metazoans, Origins of

Chiliad. Klautau , C.A.M. 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 take had cell types with distinct phases to feed (protochoanocytes) and to reproduce (protoarchaeocytes) ( Valentine, 2004). However, to get 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 organization of cell lines (Valentine, 2004). Adding back up to this early on origin of eggs and cleaving embryos, fossils about 600 Mya take been found (Xiao and Knoll, 2000). Indeed, fossils that were originally identified every bit the protist group acanthomorphic (i.eastward., spinose) acritarchs were in fact hulls of diapause animal eggs (Yin et al., 2004, 2007). This finding might suggest that sponges, and perchance other animals, could have appeared before 580 Mya (earlier 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 simple, they take many features that are shared with metazoans. Apart from multicellularity, homebox genes, and task division, they exhibit collagen, septate junctions, integrins, fibronectins, and all apparatus to link the extracellular matrix to the cytoskeleton (Valentine, 2004). Sponges also developed prison cell specialization, although non rarely their prison cell differentiation is not final. 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 evidence nervous organisation or even organs. Basal lamina, a distinctive metazoan characteristic, is reported in a single out of the four extant Porifera classes. Currently, there are v 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 just extant grade that produce calcium carbonate spicules whereas the other 3 have silicium spicules. Demospongiae is the most speciose class, representing almost 85% of the phylum. These are the just sponges that have invaded freshwater and that developed carnivory. Hexactinellida are mainly deep-ocean sponges and are the merely ones that present syncytial tissues. Until recently, Homoscleromorpha was considered a subclass of Demospongiae, but it was recently elevated to a class status.

In order to grow in size, sponges developed a system of inhalant and exhalant canals and chambers with flagellate cells (choanocytes) (Figure 10) that continuously pump h2o through the trunk of the sponge. As the water goes through, the sponge is able to acquire oxygen and food (leaner and macromolecules) and likewise to eliminate residues. These connected channels and chambers consist of the aquiferous organization, the main synapomorphy of Porifera, only some carnivorous sponges lack this feature (Vacelet and Boury, 1995).

Figure 10. Schematic section of an asconoid sponge. Photograph past E. Hajdu.

Sponges do non possess true symmetry, but they come in different forms such as spherical, cylindrical, radial, and even some present bilateral symmetry (i.due east., hexactinellids) and they are morphologically very simple, as they do non have organs, or sensorial cells not even nervous system. As sponges lack nervous cells, all communication between cells is mediated perchance by chemical messengers. Additionally, sponge tissues are non considered to be true tissues, as near of the species do non have a basal lamina. The basal lamina is a membrane of proteins (laminins and collagens) working equally a bulwark to avert that cells from 1 tissue infiltrates other tissues and, among sponges, it is encountered in Homoscleromorpha. As about sponges do not have this barrier, neither strong cell junctions to isolate their epithelia, their cells tin movement freely throughout the sponge body. Associated with cell totipotency, this feature makes sponges very plastic animals a central step that surely contributed for their adaptation and survival.

Although most sponges lack basal lamina, they share with other metazoans several characteristics, including multicellularity, blazon Iv collagen, septate junctions, and work sectionalisation among cells. If in one paw the monophyletic origin of Porifera has been already questioned (e.yard., 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; Option et al., 2010), simply phylogenetic relationships amidst the classes are less clear. Many recent molecular studies have shown that Calcarea is more than related to Homoscleromorpha and Demospongiae is closely related to Hexactinellida (Philippe et al., 2009; Pick et al., 2010), simply more comprehensive studies are yet required to settle this matter.

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Microbial Globins - Condition and Opportunities

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

4.6 Opisthokonta

The Opisthokonta comprise metazoans (animals), fungi and several additional microbial eukaryote lineages, including the Choanoflagellida, Ichthyosporea, Nucleariidae and Capsaspora. It is likely 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, Greyness, & Lang, 2008). Recently, the genomes of 5, close relatives of fungi and metazoa have been sequenced, equally part of the Origin of Multicellularity Projection at the Broad Institute (Ruiz-Trillo et al., 2007). These include Capsaspora owczarzaki, an amoeboid parasite of the pulmonate snail Biomphalaria glabrata, which has a relatively minor genome most 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 incorporate globins, T. trahens has but one FHb, and A. macrogynus has two SSDgbs and a chimeric globin with a C-last TrHb1 domain. The ichthyosporean Sphaeroforma arctica has 5 SDgbs and an FHb, while C. owczarzaki has two chimeric two-SDgb domain proteins (Table 9.3). No nucleariid genomes are bachelor. The ii available choanoflagellate genomes, each encode iii SDgbs. It is worth pointing out that a hypothetical protein of 1653 amino acids PTSG_01043 (EGD76343.one) can be identified in Southward. rosetta (Salpingoeca sp. ATCC 50818) via BLASTP search using Strongylocentrotus purpuratus androglobin (isoform 1; XP_001186225.2). Whereas the North-terminal cysteine protease domain is present, the central globin domain is not identified by either BLASTP or FUGUE searches. Although this effect 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 present 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 ROLE OF CYTOPLASMIC TYROSINE KINASES

The evolution of phosphotyrosine signaling suggests that more than 600 million years ago there was a common ancestor for the unicellular choanoflagellates and for multicellular metazoans, which had already adult this power ( King and Carroll, 2001; Rex et al., 2008; Peterson and Butterfield, 2005; Pincus et al., 2008). In some species, such as in yeast, tyrosine phosphorylation appears at a very low 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 first, whereas the enzymatic activity of tyrosine kinases adult after. The emergence of specific proteins resulted in the expansion of proteins and domains in cellular signaling. One third of all domains institute 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 well-nigh proteins in serine–threonine signaling pathways are common between metazoans and choanoflagellates, the reverse 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 adequacy simply is not negatively regulated past Csk. Biochemical characterization of the M. brevicollis Src and Csk indicated that the putative, regulatory C-terminal tyrosine is not phosphorylated (Li et al., 2008).

Our report is the first to demonstrate the being of a TFK in Thousand. brevicollis, which has estimated to have 128 tyrosine kinase genes (King et al., 2008). Although the office of the Grand. brevicollis TFK is unknown, its mere beingness clearly suggests that information technology 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 past SFKs, PI3K, and PKC, like the metazoan counterparts. However, given the lack of Csk-induced control of SFKs in M. brevicollis besides as in Grand. ovata, it is equally possible that the regulation differs. Choanoflagellates also encode an SH3BP5-related molecule, which has not been functionally characterized. Thus, it is also early say whether this molecule suppresses the corresponding TFK. Functional studies will be needed to resolve this issue equally well every bit the possibility that the choanoflagellate TFK tin can substitute for the loss of TFKs in metazoan cells.

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

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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'south body and head, with its nervous organisation and craniate senses, evolved requires a broad perspective taking into business relationship the evolution 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 circuitous 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 chosen choanoflagellates, so named for their single flagellum surrounded by a neckband of microvilli. The flagellum propels food caught by and ingested at the base of operations of the microvilli. Aside from sponges (Porifera) and an odd species of an amorphous colony-like aggregation of single-celled organisms (Placozoa), multicellular animals can be divided into two great clades ( Fig. 1), which are based on the type of trunk symmetry and the number of main tissue layers:

Figure 1

Fig. 1. The simplified cladistic relationships of animals and some disquisitional events concerning the evolution of neurons and sensory systems: (i) diffuse epithelial nerve net; (2) miR-124 specific for neurons appear; (3) miR-183 specific for sensory cells appear; (4) formation of a pentameric nerve net with rudimentary sensory organs; (5) partial dorsal invagination of the epithelial plexus and appearance of localized sensory cells; (vi) neurons are full-bodied in a dorsal neural tube and composite sensory organs appear. Sensory neurons are within the neural tube and motor neuron(s) are ventral in the neural tube. Possible precursors of neural crest, branchial motor neuron(s), and somatic motor neuron(s) appear; (7) branchial motor neuron(s) are nowadays, first appearance of neural crest and mayhap placodes; and (8) placode-derived sensory systems, cranial branchial motor neuron(s), and spinal somatic motor neuron(s) appear. Eyes, ears, nose, taste buds, lateral line, electroreceptive ampullary organs, and ocular muscles appear.
1.

Radially symmetrical animals are diploblasts. They have 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 iii primary tissue layers (intermediate layer of mesoderm in addition to the ectoderm and endoderm), and include all bilaterally symmetrical invertebrates and vertebrates. Amid triploblasts, a group of flatworms defective a gut are referred to as acoelomate flatworms (Acoela, Fig. ane). These animals are no longer considered to be true flatworms (the phylum Platyhelminthes) only rather are viewed every bit basal triploblasts. The residuum of the bilaterally symmetrical animals, the Nephrozoa (animals with kidneys), comprise the Protostomes and Deuterostomes. The former include most species of invertebrates (including the "truthful" flatworms, Platyhelminthes), while the latter include the vertebrates and their deuterostome invertebrate relatives. The well-nigh 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 comprise those animals which take a notochord at some point in their lifecycle and are divided into three major extant clades: the cephalochordates (amphioxus), the urochordates (sea squirts or tunicates), and the craniates (vertebrates) (Fig. 1).

Of the numerous theories most the origin of the craniate caput, brain, and peripheral nerves which have been proposed over the by 150 years, near 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) grade the outgroup of craniates and consequently, many head organs, jaws, and the craniate spinal string had to evolve anew. Ideas that the last common bilaterian ancestor of nephrozoans had a highly developed key nervous organisation (CNS) and was segmentally organized require the assumption that many basal protostomes and deuterostomes have secondarily lost all of these features and devolved into a more bequeathed-like arrangement.

Equally all multicellular organisms derive from single-celled choanoflagellates, they share molecular synapomorphies at the level of diploblasts and triploblasts such every bit dorso-ventral patterning of epithelial cells, indicating that the ectodermal nerve cyberspace found in diploblasts may also accept been the primitive state in basal triploblasts. In fact, within basal deuterostomes, the marine worm in the genus Xenoturbella has an epithelial nerve plexus with no recognizable sensory system. Among Ambulacralia, acorn worms (hemichordates, Fig. 1) accept some centralization of nervous elements without formation of a true CNS and unmarried sensory cells instead of sensory organs. Likewise, what appears to be basal protostomes, the arrow worms (Chaetognathans), generally have an epidermal nerve plexus with little concentration into a ganglion, like to basal triploblasts, the acoel worms. The broad phylogenetic distribution of these shared features indicates that the common triploblast antecedent, and therefore probable besides the common deuterostome ancestor, may have had an epithelial nerve plexus with little, if whatsoever, metameric (segmental) organization. If the morphology of the nervous system in Xenoturbella is indeed shut to the basal deuterostome organisation and not secondarily derived through regressive evolution, the ancestor of craniates could have 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. 1).

For this article, we assume that the deuterostome and craniate antecedent had a nervus cyberspace very much similar the Xenoturbella's unorganized nerve net with little to no metameric arrangement. It seems unlikely that a metameric organization of a well-developed CNS, once formed, devolved to form the epithelial nerve nets now found in diploblasts and basal triploblasts.

Following already well-entrenched arguments for the development of eyes and ears, the different patterns of nervous system institute in deuterostomes are considered here to be independently derived forms reflecting either different and independent steps in forming a CNS (acorn worms, cephalochordates, urochordates, and craniates) or a transformation into a pentameric (five-portioned) nerve net (echinoderms). A consequence of this hypothesis is that many bug in establishing homology of peripheral fretfulness and sensory organs across deuterostomes become obsolete, leading to the more parsimonious idea that the formation of a CNS comes about by aggregating a baso-epidermal nerve net and condensation of distributed sensory cells into sensory organs. Such centralization may have happened at to the lowest degree twice in deuterostomes (acorn worms, chordates) and at least 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 as "nerves." Manifestly, in order to communicate with the remaining parts of the torso, such nerves take 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 diffuse nerve internet is the condensation of various sensory cells with more or less discrete segregation of sensory input into organs dedicated to a specific set of stimuli such as photic, mechanical, or chemical stimulation equally well every bit the development of the neural crest (embryonic tissue giving rise to about of the peripheral nervous system (PNS)).

For this thought to be true, each of the deuterostomes having a CNS will have to prove certain similarities in its overall evolution and its molecular basis which in part will reverberate the patterning already found in the more or less centralized nerve plexus of acorn worms. In item, genes related to neurulation should be common amidst all deuterostomes with a CNS, simply may reflect mutual co-option or a prototypical fractional aggregation into a CNS that does not achieve the level of obvious homology due to lack of topological landmarks. Plainly, but craniates accomplished the aggregation of all neurons into a CNS, aggregated all peripheral sensory systems into discrete sensory organs, and adult a unique ready of innervation via neural-crest-derived sensory neurons and neurogenic-placode-derived sensory organs. Neurogenic placodes, like neural crest, are ectodermally derived structures but are present only in the caput; along with neural crest, they produce the sensory neurons of the PNS in the caput region. The germination 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 work has full-bodied on the specific factor networks promoting embryonic formation of placodes or neural crest, few studies have been focusing on the molecular mechanism of suppression of neuronal fate decision in the remaining ectoderm. In other words, the ectoderm in a developing embryo will become nervous-organisation 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 clear that fibroblast growth gene (Fgf) upregulation, combined with limited to no expression of bone morphogenetic proteins (BMPs) induce a neuronal fate in the ectoderm of chordates (ie, it becomes neuroectoderm) but apparently 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.

Some other emerging concept is the importance of micro-RNA (miR) for the evolution of neurons and sensory cells. Recent data take shown that specific miRs (miR-124) are evolutionary conserved and evolved first with triploblasts. MiR-124 has been shown to be essential for neuronal development through the regulation of chromatin remodeling and absenteeism of miR-124 (and other miR species) causes rapid degeneration of developing neurons. Some other set up of miR, miR-183,182, and 96, is evolutionary conserved amidst 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. Evidently, the ability of miR to regulate large sets of transcription factors could be perceived every bit a prerequisite for the evolution of a more complex neuronal and sensory system.

This commodity provides an overview of the evolutionary steps and their underlying developmental transformations toward the germination of a PNS such as 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 reflect the currently accepted phylogenetic relationships among deuterostomes, with Xenoturbella (Fig. 1) as a probable ancestor type for all deuterostomes equally far as its nerve organization is concerned – information technology has a unproblematic epithelial nerve net with no evidence of concentration of neurons or sensory cells. Unfortunately, beyond the knowledge of an unusual Hox code, no molecular information exist on these animals to illustrate what might exist the original molecular code for the evolution of a nervus net.

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WHAT IS EVOLVABILITY?

Kim Sterelny , in Philosophy of Biological science, 2007

Publisher Summary

This affiliate discusses evolvability. Somewhere between 700 and 800 one thousand thousand years ago, in a protist lineage closely related to the living choanoflagellates, cell division resulted in cell aggregation. That lineage prospered and became a source for Metazoa: the lineage of multicelled animals. About 75 million years agone some other experiment in collective life began: that of the volvocaceans. The founder of this lineage lived in shallow imperceptible ponds (if the environmental of its descendants is whatever guide). The volvocaceans are a good probe for investigating this problem, because at that place is a concrete hypothesis which explains the limits of volvox disparity. Recent literature on bacterial evolvability the focus has been on the development of mutation rates; in particular, whether elevated mutation rates show that there has been selection for increased mutation itself, or merely declining investment in error correction in impoverished environments. Hence it can exist concluded that Evolvability is a holding of a lineage.

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Evolution of Sox2 and Functional Redundancy in Relation to Other SoxB1 Genes

Yusuke Kamachi , in Sox2, 2016

Evolution of the Sox Family unit Genes

When did the ancestral Sox genes arise during development? Two Sox/Tcf-like genes were identified in the genome of the unicellular choanoflagellate Monosiga brevicollis (Male monarch et al., 2008). Nevertheless, the HMG domain sequences of these Sox/Tcf-like proteins showroom low levels of identity (<forty%) with metazoan Sox proteins (Zhong et al., 2011), which is significantly lower than the levels of identity observed among the metazoan Sox proteins (≥46%) (Lefebvre et al., 2007). The Sox/Tcf-like proteins likewise practise not include the signature Sox family sequence (RPMNFMVW) in the N-terminal part of the Sox HMG domain (Bowles et al., 2000) and more strongly resemble Tcf and Capicua families (Figure one). These data advise that the choanoflagellate Sox/Tcf-similar genes may share ancestry with the Sox/Tcf family but cannot exist categorized within the current Sox family unit. Interestingly, Sox/Tcf-similar genes are not plant in the genome of the unicellular holozoan Capsaspora owczarzaki, which has been putatively assigned to the sister group of metazoans and choanoflagellates (Sebe-Pedros et al., 2011), which suggests that a proto-Sox/Tcf gene may have appeared in the common antecedent of metazoans and choanoflagellates (Figure three).

Effigy 3. Evolution of HMG domain and Sox family unit. Hypothetical origins of major Sox groups (red dots), minor Sox groups (pinkish dots), Sox/Tcf-like (imperial dot), MATA_HMG domain (gray dot) and HMG domain (black dot) are plotted on a simplified cladogram showing the phylogenetic relationship of representative lineages of Opisthokonta. Duplication events (blue dots) are also plotted. 2R-WGD, two rounds of whole-genome duplication; TSGD, teleost-specific genome duplication.

In the past decade, the genomes of species in early-branching metazoan phyla, including poriferans (sponges), placozoans, ctenophores (comb jellies), and cnidarians (anemones and jellyfish), have been sequenced. Analyses of these genomes take revealed that the genome system and contents are similar among metazoans including more complex bilaterians (Degnan et al., 2009; Larroux et al., 2008). Remarkably, a vast majority of transcription factor families, including Sox, T-box, Ets and nuclear receptors, and signaling pathway components, including Wnt, transforming growth factor-β and Hedgehog, be in poriferans, placozoans, ctenophores, and cnidarians, although these factor families were considered in association with complex bilaterian development. Analogous to the example of Sox, these regulatory proteins lack directly orthologs in choanoflagellates (Male monarch et al., 2008). This finding suggests that a set of these regulatory proteins was established after the difference of the choanoflagellate and metazoan lineages but before the deviation among metazoan lineages. However, further analysis will be required to confirm these evolutionary scenarios because a more than contempo assay of the C. owczarzaki genome revealed that some transcription factors formerly thought to be metazoan specific, such every bit T-box and Runx, accept a more ancient origin and were specifically lost in the choanoflagellate lineage (Sebe-Pedros et al., 2011).

The phylogenetic placement of poriferans, placozoans, ctenophores, and cnidarians has not been fully resolved (Marlow and Arendt, 2014). Ctenophore genome analyses have suggested the placement of ctenophores as the about basal brute lineage (Moroz et al., 2014; Ryan et al., 2013), whereas other studies have suggested the placement of either poriferans or placozoans at the primeval-branching animal lineage (Nosenko et al., 2013; Schierwater et al., 2009). In any case, genome analyses of these potentially basal animal species from the poriferan, placozoan, and ctenophore lineages suggest that a basic fix of Sox family genes was already established in Urmetazoa, the hypothetical beginning multicellular animals or the final common antecedent of Metazoa (Jager et al., 2006). Among poriferan species, four and vii Sox genes were identified in the demosponge Amphimedon queenslandica (Larroux et al., 2008) and the calcareous sponge Sycon ciliatum (Fortunato et al., 2012), respectively, whereas v Sox genes were identified in the placozoan Trichoplax adhaerens (Srivastava et al., 2008), and half dozen Sox genes in the ctenophore Mnemiopsis leidyi (Schnitzler et al., 2014). These Sox genes have been classified into 3 major groups (B, C, and F) in the demosponge A. queenslandica (Larroux et al., 2008), four groups (B, C, E, and F) in the calcareous sponge Due south. ciliatum (Fortunato et al., 2012), three groups (B, C, and East) in Trichoplax (Schnitzler et al., 2014; Srivastava et al., 2008), and four groups (B, C, E, and F) in the ctenophore Grand. leidyi (Schnitzler et al., 2014). Some Sox genes from these species have not been classified into the major Sox groups and are likely lineage specific. The variable numbers of Sox genes among these species likewise suggest the occurrence of lineage-specific factor duplication and/or loss during evolution.

From these analyses, it appears that urmetazoans harbored at least a prototype set of genes composed of SoxB, SoxC, SoxE, and SoxF (Figure 3). Interestingly, considering the genomes of Amphimedon and Trichoplax include both SoxB1-like and SoxB2-like genes, information technology appears that the duplication of a hypothetical epitome SoxB gene and subsequent divergence into prototype SoxB1 and SoxB2 genes occurred before the establishment of Urmetazoa (meet too Section 6.5). Genes that can be conspicuously classified into the SoxD group accept not been found in poriferans, placozoans, ctenophores, or cnidarians but take been found in nigh bilaterian metazoans (Larroux et al., 2008). Taken together, this suggests that the total complement of the major six Sox groups (B1, B2, C, D, Eastward, and F) was established in Urbilateria, the hypothetical terminal mutual ancestor of bilaterians (Figure 3).

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