Note: This page is a draft and currently under development.
Attribution: User Jtwsaddress42 (discuss • contribs) created this resource and is actively using it. Please coordinate future development with this user if possible.
Phylogenetic classification of animals and their unicellular relatives.[c][2]Choanoflagellate and choanocyte
Animals are multicellular colonies and pattern formation is at the heart of embryology.
One of the great problems that evolutionary developmental biologists have been trying to solve since the time of Darwin is - how unicellular organisms living in a colonial environment made the transition from cells being selected as individuals within the population of cells - to, the entire multicellular colony being selected as an individual, an event that marks the transition to multicellularity. This threshold has been achieved at least three times in evolution of the eukaryotic domain,[d] yielding three of the seven major biological kingdoms[e] - plants, animals, and fungi, each with its own historically contingent outcome.
We can rephrase the problem for animals as - how unicellular organisms living in a clonal colony, along with other potential microfluoral and faunal symbionts, made the transition from cells being selected as intracellularly-digesting phagocytic individuals within the population - to, the entire multicellular colony being selected as a single extracellularly-digesting individual functioning as a unitary whole; an event that marks the transitional endpoint in the origin of metazoans as a firmly established biological form of organization.
For animals with their unicellular ancestral roots in the relatively sophisticated choanoflagellate protozoa, the problem becomes one of understanding how they made the transition to Porifera, Cnidaria, Bilateria, Deuterostome, and Chordate - and, onward to vertebrates and mammals where Edelman will pick up the story. Edelman investigates the problem of pattern formation from the perspective of evolutionary developmental morphology, but enters the game with the vertebrates.
In 1987, Yale professor Leo W. Buss published The Evolution of Individuality (1987).[3] Buss was thinking about the early conditions surrounding metazoan origins with regard to selection and the development of metazoans. The blurb on the back of his book summarizes Buss' take on the problem elegantly:
"Leo Buss expounds a general theory of development through a simple hierarchical extension of the synthetic theory of evolution. He perceives innovations in development to have evolved in ancestral organisms where the germ line was not closed to genetic variation arising during the course of ontogeny. Variants that favor both the proliferation of the cell lineage and the organism harboring them were sequentially incorporated in an increasingly sophisticated epigenetic program. In contrast, variants that favor the replication of cell lineage at the expense of the individual were eliminated and ultimately favored the fixation of variants that limited the production and/or expression of subsequent variation, creating a stable developmental system."[4]
This vision of the evolution of the developmental process, by Buss, is very compatible with the concepts of Neural Darwinism and Somatic Selection put forth by Gerald Edelman around the same time- each authors work informing the others in an indirect manner. Perhaps, even offering the opportunity to develop a clear understanding of how a bottom up process can lead to the organized diversity and complexity we see around us in the animal kingdom. As we will see, many unicellular organisms took the first path to multicellularity, but it is animals that took the later path by "creating a stable developmental system."[4]
Multicellular colonies following the first pathway where variants that favor both proliferation of the cell and the colony that harbors it - are capable of organizing themselves through genetically constrained epigenetic routines that organize it into a multicellular whole - but, they are reversibly committed to such endeavors, defaulting to unicellularity for each cell in the colony for sexual reproductive purposes. They are also intracellularly digesting via phagocytosis.
To protect the species integrity of colonies where variants that are parasitic to the colony as a whole arise, the second pathway eventually constrains the preexisting epigenetic routines of the cells via somatic selection and topobiology. These constraints become increasingly stringent and inhibitory on the ancestral pattern of eternal cell division as they are required to produce stable and reliable ontogeny. In these colonies the ontogenetic development of a sacrificial somatic population that supports the reproductive germ line arises and protects it from damage. Responsibility for carrying the genome into the gene pool is relegated to the germ line at this point.
Once a stable and adaptive ontogenetic program of epigenetic behaviors has been established, further variation that favors uncontrolled proliferation takes us away from the adaptive solution that successfully results in the genetic recombination and proliferation of the germ line. Further variation often results in positive selective pressure for the acquisition of masking agents that buffer the system by suppressing or masking the expression of non-lethal, but destabilizing variants. From the perspective of a somatic selective systems and population biology, this store of variation can be unmasked under stress conditions to instantaneously offer a much broader range of adaptive options in the moment of need than could be acquired by gradual mutational pressure over time.
Animal Cells - Divide Or Differentiate, But Not Both At The Same Time
Cytoskeleton of a transporting epithelial cell showing the cell-cell and cell-substrate junctions connecting the actin, tubulin, and intermediate filament networks.[5]
Eukaryotic mitotic spindle with microtubules, microtubule organizing complex, and kinetochores.
Multicellular lifecycle diagrams[g] Plants (A), Animals (B) and Fungi (C)
Animal cells only have one microtubule organizing center (MTOC) per cell, and a choice in how it can be used at any given time. The MTOC can either nucleate microtubules to be used to differentiate the cytoskeletal architecture; or, the MTOC can be used to form the meiotic and mitotic spindles that participate in the separation of the chromosomes during cell division. In order for an animal cell to divide, it must retract its cytoskeleton and become ameoboid, so that it can redirect its MTOC to the process of division instead. This restriction has interesting consequences for animal evolution as a multicellular organism.
The cytoskeleton is comprised of a variety of microfilaments, intermediate filaments and microtubules - all of which can undergo dynamic assembly and/or disassembly to control fiber length during morphological transformations of the cells overall shape. It is the mutual requirement of the microtubules in cytoskeletal differentiation and cellular replication that sets the constraints on animal body plan organization. It appears that this very constraint may have been the necessary requirement to direct the bifurcation of the early animal bodyplan into germ and soma.
Buss traces the ideas of somatic selection in animals to the great nineteenth century biologist August Weismann (January 17, 1834 – November 5, 1914) and this theory of germ plasm, where the cells of the body follow one of two distinct paths: Germ or Soma.[h]
In animals the meiotic pathway is the path of the germ line - and, leads to the production of genetically recombinant individuals in the next generation. Aside from amplifying the germ line population in anticipation of reproduction; the mitotic pathway is clonal and the destiny of the Soma. Within the developing animal these cells in the mitotic pathway will experience increasingly reproductively-inhibitory mechanistic interaction with their neighbors that are the result of somatic selection; essentially limiting their reproductivity by inducing them to differentiate into the morphologically specialized cytoskeletal architectures that define the cell types of the animal body plan.
The Soma will provided a protective cordon through the environment for the germ line to successfully reproduce without degradation from environmental damage, which has been borne by the Soma instead.
Individuals are selected, species evolve. That is the relationship between the individual phenotype exemplified by the Soma or somatic line; the species genome, or gene pool, which resides in the Germ or germ line; and, what we mean by the word "evolution" in a biological context.
Ultimately, Buss explains, in early animals, as the early animal body plan complexifies, the openness of the germ line to developmental variation in the course of embryogenesis becomes more refractory; and the only way that the body plan can continue to complexify is for the germ line to be sequestered so as to protect its own genomic integrity between recombination events. Clonal replication is directed into the soma such that the soma protects the germ line from destabilizing influences. At this point the soma, or body, has essentially become a protective transport mechanism for the germ line through the environment until such time as the germ line can consummate a successful sexual recombination and progeny.
Cells of the soma are under pressure to differentiate, specialize, and forgo reproduction. To achieve this, cellular populations exert mutual reproductivity-inhibiting mechanisms on the neighboring cells in their community - and instead, direct them along a specialization trajectory that is the result of somatic selection during the developmental sequence.
Essentially the body is a spaceship for the germline, where the cells of the body forgo their ancient impetus towards reproduction via binary fission, and instead differentiate and participate in the communal activity of protecting the species germ line from environmental damage and mutation - thereby, protecting the species genome from deleterious mutations to the course of developmental ontogeny.
For animals, the soma, or body is meant to be discarded - it is for the good of the species integrity. The accumulation of physical damage, and environmental mutation, is discarded with the soma; and, the genome of the species is protected from the potential destabilizing effects it would have had on the developmental program already established. Most unicellular organisms can be killed, but they don't die of old age - they divide. For metazoans, the soma is the origin of mortality - and, the inevitability of death in animals. This is the cost of being a true metazoan.[j]
The old age senescence of the soma is adaptive - and, meant to prevent the introduction of mutation into the species gene pool. These cell have sacrificed themselves for the good of the species, which is carried in the germ line. It is no surprise that pre-programmed cell death and apoptosis occur within the developmental ontogeny, selection is for the benefit of the germ line and any benefit to the soma is incidental.
Interestingly, cells that disengage from the inhibitory mechanisms on reproduction within the soma, can become cancerous as they resort to their ancestral pattern of reproductive fission with a complete disregard to the physiological and adaptive integrity of the body they are embedded within.
There are many forms of multicellularity that have arisen in all the major biological domains, but most of these have exhibited reversible developmental dynamics such that the multicellular colony, and its individual members or groups, could associate and disassociate in accordance with changing conditions; and thereby, oscillate between the collective and the individual, with selection still centered on the unicellular phenotype and the behavioral repertoire that facilitates multicellular association.
Even in this architecturally primitive form, we can see the potential dynamism of multicellularity for creating new ecological niches - and, see the sophistication of the cellular behavioral repertoire on the individual, as well as collective, level. The late[9] pioneering British eukaryotic evolutionary microbiolist and theorist Thomas Cavalier-Smith (October 21, 1942 - March 19, 2021), devoted a lifetime to organizing our understanding of the eukaryotic world and detailed countless examples of multicellularity in various unicellular species.
Evolving Animals From Multicellularity
"Evolving multicellularity is easy, especially in phototrophs and osmotrophs whose multicells feed like unicells. Evolving animals was much harder and unique;"[10] - Thomas Cavalier-Smith (2017)
Topobiology & Epigenesis - Mechanically Linking The Cell & Animal To The Historical Contingencies Of Life
Much of the molecular machinery, and the genes associated with it, that guided and patterned the first animal embryos emerged from the systems of epigenetic behavioral response that had developed in protozoan choanoflagellates. These genes that enable and constrain epigenetic behavioral routines are integrated into a far more ancient genome that organizes and constrains the eukaryotic metabolism and basic physiology. These are the systems linked to the cell surface and its interactions with the environment and other cells. They represent the primary cognitive and behavioral systems of the cell.
Key to the emergence of animals is a transformation of the genetic elements of the epigenetic behavior system into a stable, and relatively consistent, developmental ontology that guides the formation of the animal body plan. It is the acquisition of new repertoires of genes associated with the restructuring of the epigenetic system, along with their respective proteins, transcription factors, and regulatory regions, that eventually results in the emergence stable patterned tissue formation - an absolute prerequisite for animals to emerge.
"The establishment and maintenance of distinct cell identities are ultimately controlled by epigenetic mechanisms: the stable expression of defined transcriptional networks."[15] - M. Albert Basson (2012)
Gerald Edelman surmised that there are three major types of genetic regulation associated with establishing the morphology of an animal:[16]
Historegulatory genetic pathways - responsible for tissue-specific cell differentiation, specialization, and maintenance of basic cellular physiology and behavior.
Selector genes - responsible for determining tissue types via the activation or suppression of specific historegulatory gene pathways. (i.e. homeotic transcription factors, HOX genes, diffusable morphogens...)
Morphoregulatory genes - genes for the regulation of CAMs, SAMs, CJMs expression in a time- and place-dependent fashion.
The Role Of Apoptosis & Growth Factors In Stabilizing The Patterning Of The Body Plan
All cells in the animal organism contain the same genome, perhaps with the exception of the unfertilized sperm and eggs which contain a half-set of alleles, but not all populations of cells in the organism are identical. Each of the somatic cells in the developing organism has an initial competence to be induced to commit and differentiate into any number of specific cellular phenotypes. This process of commitment gradually constrains and reduces the totipotency of the cellular mitosis - and, ultimately leads to the final determination the differentiated cellular phenotype and the cessation of the mitotic sequence.
Totipotency is the ability of a cell to give rise to multiple cell types via mitosis, or even ultimately an entire multicellular organism - perhaps even one that metamorphosizes between two life stages. The genomic implications of this are that the genome of a cell possesses much more than it phenotypically expresses at any given point in space and time.
Not only does the genome possess the genes necessary for all the tissue types, but it can carry the genes for instructing the formation of more than just one body plan - a phenomena we clearly witness in the metamorphosis of many organisms from juvenile to adult body plans. Indeed, genome duplication events can facilitate the emergence of multiple body plan routines, if they don't disrupt the already established ontogeny.
The genome can also contain vestigial elements of past body plans that are no longer expressed because they have been developmentally bypassed under the pressures of somatic selection over evolutionary time scales as a result of adaptive changes to the body plan. Vestigial elements can lay dormant for long periods of time in the genome and will eventually accumulate a high mutation load eventually degrading the system, but there is the possibility of their reappearance in ontogeny in response to the right inductive signals.
There are a number of pre-requisites for multicellularity, not the least of which is the ability to stick together. But just because cells stick together, doesn't mean cells stay stuck together and protozoan cells possess mechanisms for selectively engaging and disengaging adhesion mechanisms via the mechano-chemistry of membrane-embedded proteins.
Multicellularity Evolves In Two Ways - Naked and Walled Cells
"Multicellularity evolves in two ways. Naked cells, as in animals and slime moulds, evolve glue to stick together. Walled cells modify wall biogenesis to inhibit the final split that normally makes separate unicells, so daughters remain joined. The ease of blocking that split allowed almost every group of bacteria, fungi and plants (and many chromists) to evolve multicellular walled filaments, more rarely two-dimensional sheets, most rarely three-dimensional tissues. Tissues require more geometric control of daughter wall orientation, as in embryophyte green plants and chromist brown algae; both can grow longer than blue whales. Evolving tissues is selectively harmful to many walled multicells whose filaments are best for reproductive success. Almost all multicells retain unicellular phases (eggs, sperm, zygotes), so adhesion is temporally controlled and developmentally reversible—except for purely clonal vegetatively propagating plants or ‘colonial’ invertebrates (evolutionarily transient) the only organisms that are never unicellular."[17] - Thomas Cavalier-Smith (2017)
Important adhesion molecules that needed to be acquired by our protozoan ancestors prior to the origins of animals include: catenins, integrins, and cadherins that facilitate cell-cell adhesion. Also important are substrate adhesion molecules used for selective-anchoring to surfaces in the environment, as well as extracellular matrix (ECM) proteins that provide additional scaffolding for the multicellular population organize itself on. Cavalier-Smith will trace these innovations to adaptations in locomotion, feeding, reproduction, social, and life-cycle organization.
Integrins are used by epithelial cells to attach themselves to the extracellular matrix and appear to have emerged in the line of Sulcozoa with psuedopodia that leads to Choanocytes and animals.[18]
Cavalier-Smith postulates that the ancestral choanoflagellate organized itself into a benethic and pelagic stage, a theme to be returned to in our evolutionary story.
Trophic Continuity
"Evolving animals was much harder and unique; probably only one pathway via benthic ‘zoophytes’ with pelagic ciliated larvae allowed trophic continuity from phagocytic protozoa to gut endowed animals."[10] - Thomas Cavalier-Smith (2017)
From Phagocytosis To Gastric Provisioning - The Evolution Of Cellular Individuality Within The Somatic Population
Although the popular Darwininistic recapitulist interpretations of embryology championed by Ernst Haeckel dominated the discussion of biological origins at the time, there were dissenters of all kinds. But within the realm of those committed to a scientific and biological explanatory theory of animal origins, Elie Metchinikoff was a prominent dissenter.[25]Mechanistic continuity and physiological viability are the necessary requirements Metchnikoff demanded of each stage of an embryological and evolutionary transition. Metchinikoff challenged the orthodoxy of Haeckels Gastraea theory of animal origins on the basis that it was not mechanistically continuous and physiologically viable in all its intermediate stages between unicellularity and multicellularity. He argued that there must have been a preceding historical period leading to the extracellularly-digesting "gastraea" - and, that was the colonial ancestors of the mesoderm-derived lmyphocytes which exhibit the ancestral mode of intracellular digestion through phagocytosis - Phagocytella.
In an attempt to explain what was required to make the transition from individual intracellularly-digesting phagocytic choanoflagellate cells to organized and differentially specialized tissues reliant on extracellular digestion and nutritional provisioning, Cavalier-smith proposed that "Epithelia and connective tissue could evolve only by compensating for dramatically lower feeding efficiency that differentiation into non-choanocytes entails. Consequentially, larger bodies enabled filtering more water for bacterial food and harbouring photosynthetic bacteria, together adding more food than cell differentiation sacrificed."[10]
This requirement for mutual nutrient provisioning is the necessary pre-condition for beginning the pathway to cellular specialization within the multicellular population. Some cells will forgo the essential task of harvesting food to specialize, in return their services will be rewarded by other cells becoming super harvesters which will provision the specialized non-digestive cells with their nutritive needs. Resource provisioning pathways are fundamental to multicellular organization, but the specialized cells within animals are critically dependent upon them.
Metchnikoff's accidental discovery of the white blood cell, or lymphocyte, in a star fish that he had skewered with a thorn inspired his research into phagocytosis; and, lead him to the concept of a system of innate host immunity in animals where the white blood cells, phagocytes, patrol the body looking for foreign invaders and damaged cells to consume. He reasoned that they were a cell lineage that had retained their ancestral role of distinguishing self from non-self in early animal colonies - and, therefore represented a primitive state for that cellular population in animals.
This population of cells was not only responsible for ejecting foreign invaders but was also responsible for dealing with sick and damaged cells of the body - as well as those emitting apoptotic signals. In addition to facilitating the process of defense, these cells participate in the processes of metamorphosis.
The Events Of Embryological Development: Historical Contingency & The Emergence Of The Body Plan
Differential Positional Provisioning Of mRNA, Protein, & Other Factors In The Egg - Maternal Factors, Continuity Of The Metabolism, & The Bootstrapping Of The Genome
Fertilization - Acrosomal Reaction, Penetration Of The Zona Pellicuda, Fusion Of Sperm & Egg
Oogenesis & The Polar Bodies - Sorting Chromosomes & Microtubule Organizing Centers During Sexual Recombination
Cleavage & The Modes Of Early Animal Development
Morula, Blastula, Gastrula, & Neurula - Population Growth & The Migration In The Developing Embryo
Ectoderm, Mesoderm, Endoderm - Adapting To The Cellular Ecology
Radial & Bilateral Symmetry - Developing An Anterior-Posterior & Dorsal-Ventral Axial Distribution Of Functional Differentiation
↑Subject to major change, revision ,and/or retraction at any moment.
↑Portions of this essay started as a contributions by myself, User:Jtwsaddress42, to the article Neural Darwinism on Wikipedia.
↑Original description: "Figure 1. Phylogenetic classification of animals and their unicellular relatives. (a) A timeline of different events during early animal evolution. The transition to animal multicellularity, and therefore the origin of the first animals, occurred sometime at the end of the Tonian period, according to molecular clock estimates. The oldest fossil or geological evidence of recognizable animals dates back to the Ediacaran period, with molecular clocks extending the emergence of different animal phyla back to the Cryogenian. Time units are million years ago (Ma). (b) Cladogram representing the major clades of the tree of animals and the major groups of unicellular relatives of animals: choanoflagellates, filastereans, ichthyosporeans and corallochytreans-pluriformeans. Coloured nodes indicate different ancestors that we can reconstruct and that are important to understand the transition to animal multicellularity; the highlighted internal branch (from the Urchoanozoan to the animal LCA) indicates the animal stem. Uncertain positions within the animal tree and within Holozoa are represented with polytomies."[1]
↑The cytoskeleton is a dynamic structure and microtubules play an important role in determining the architectural state of differentiation for specialized cells. In many ways, the dynamic action of microtubules manifests one of the first, and most easily observable, indications in cells of what we would recognize as life - flagellar motility and/or the amoeboid exploration of the environment through process extension and retraction - all processes determined by the adaptive dynamics of microtubule formation and dissolution.
↑Multicellular lifecycle diagrams in terms of soma and germ line populations in A.) Plants, B.) Animals, C.) Fungi - (1) Meiosis, leading to the production of germ, (2) Mitosis, leading to production of soma and amplifying germ line populations (3) Sexual Recombination of germ lines within the species population
↑In the Evolution of Individuality, Buss spends considerable time examining Weismann's legacy, and the strengths and weaknesses of his Doctrine of the Continuity of the Germ Plasm.[6] As we will see, the transition to metazoans with a sequestered germ line has intermediates, and there is always a limit of validity for any given assumption; nonetheless, once the germ line is securely sequestered, Weismann's assumption appears to hold and yield insight.
↑The hallmarks, causes and effects of cellular senescence. (a) The key features of a senescent cell. (b) Senescence occurs in response to multiple contingencies of life (c) Senescence plays a dual role in development and tissue repair/regeneration.
↑This is the case for all metazoans that have sequestered the germ line. The monoblastic Porifera and the diploblastic Cnidaria may represent the intermediate stages between the transition to multicellularity and complete sequestration of the germline in triploblastic bilaterians.
↑Original description: Temporally alternating life cycles of unicellular holozoans. "Figure 3. Temporally alternating life cycles of unicellular holozoans. Each panel shows life stage transitions of two unicellular holozoan species representing each clade. Arrows indicate directionality of the transition. Loop arrows indicate cell division. Dotted arrows with question marks between stages indicate potential (unconfirmed) life-stage transitions. (a) Life stages of the colonial choanoflagellate Salpingoeca rosetta. The asexual life cycle (on the right) includes a single-celled sessile thecate stage (adhered to the substrate), slow and fast swimming single-celled stages, and two types of clonal colonial stages (chain and rosette colonies), in which neighbouring cells are linked by intercellular bridges. Starvation triggers the S. rosetta sexual cycle (on the left), in which diploid cells (slow swimmers) undergo meiosis and recombination, and the resulting haploid cells (which can also divide asexually) mate anisogamously. (b) Life stages of the colonial choanoflagellate Choanoeca flexa. Light-to-dark transitions induce C. flexa colonies to rapidly and reversibly invert their curvature while maintaining contacts among neighbouring cells between their collar microvilli, alternating between two colony conformations. In response to light, colonies exhibit a relaxed (flagella-in) feeding form. In the absence of light, colonies transition to an inverted (flagella-out) swimming form. (c) Life stages of the filasterean Capsaspora owczarzaki. In the trophic proliferative (filopodial) stage, cells are amoebae adhered to the substrate, extending several long, thin actin-based filopodia. These amoebas can detach from the substrate and actively aggregate in the aggregative or ‘multicellular’ stage, producing an extracellular matrix that presumably binds them together. In response to crowding or stress, cells from both the amoeba and the aggregative stages can encyst by retracting the filopodia into a cystic or resistance stage. (d) Putative life stages of the filasterean Pigoraptor vietnamica. Swimming flagellated cells can form long, thin, sometimes branching filopodia that can attach to the substrate. Flagellated cells can sometimes present wide lobopodia. Flagellated cells can retract the flagellum and become roundish, to either divide into two daughter flagellated cells or transition to a cystic stage. This can, in turn, produce two flagellated daughter cells. Cells can also form easily disintegrating aggregations of cells and feed jointly. The life stages of Pigoraptor chileana are very similar to the ones of P. vietnamica, but P. chileana shows a much reduced capability to produce filopodia and lobopodia (both stages are extremely rare in P. chileana). (e) Life stages of the ichthyosporean Creolimax fragrantissima. Single-nucleated amoebae disperse until they settle and encyst. The rounded cell undergoes multiple rounds of synchronous nuclear division (coenocytic division) without cytoplasmic division. Nuclei are later arranged at the periphery of the cell as a large central vacuole grows. Finally, the coenocyte cellularizes and new amoebas are released to start the cycle over again. (f) Life stages of the ichthyosporean Sphaeroforma arctica. Single-nucleated cells undergo multiple rounds of synchronous nuclear division (coenocytic division) without cytoplasmic division. Nuclei are later arranged at the periphery of the cell. Finally, the coenocyte cellularizes, releasing a number of daughter cells to start the cycle over again. (g) Life stages of the corallochytrean Corallochytrium limacisporum. Reproduction in C. limacisporum occurs mainly through binary fission (99% of the cases), during which a binucleated cell divides into two, symmetrical, uninucleate cells. Binucleate cells can form two lobes that can lead to cellular division (forming two monoucleate cells), or can reverse towards spherical cells. At this point (*), cells can transition to coenocytic growth (1% of the cases) and continue dividing their nuclei further forming quadrinucleated cells. Quadrinucleated cells can often form a clover-like shape (similar to bilobed cell), that generates either four mononucleate cells or returns to spherical shape and further divides to an eight, 12 and up to 32 nuclei coenocyte. Coenocytes can release dispersive amoebas to start the cycle over again. (h) Putative life stages of the pluriformean Syssomonas multiformis. A swimming flagellated cell can temporarily attach to the substrate through the anterior part of the cell body or move to the bottom and transform to an amoeboflagellate form by producing both wide lobopodia and thin short filopodia. Flagellated cells can lose the flagellum via different modes and transition into an amoeba stage, which produces thin, relatively short filopodia. Both amoeboflagellate and amoeba stages can transition back to the flagellate stage. Amoeboid cells can also encyst by retracting their filopodia and rounding the cell body. Palintomic divisions may occur in the cystic stage to release several flagellated daughter cells. Flagellated cells can partially merge and form temporary shapeless cell aggregates of both flagellated or non-flagellated cells and rosette-like colonies composed by only flagellated cells (showing outwards-directed flagella). In rich medium, solitary flagellated cells can sometimes actively merge and form a syncytium-like structure, which undergoes budding and releases flagellated daughter cells."
↑Reconstructed by the maximum likelihood method for 78 protein-coding genes. Numbers beside the internal nodes are maximum likelihood bootstrap values obtained from RaxML and Bayesian MCMC posterior probabilities. Black circles indicate 100% bootstrap support and 1.00 posterior probability values.
↑A: hedgehog and B: Notch homologues. The illustrated domains are some of those found by searches against the Conserved Domain Database. Numbers at the species names are accession numbers, protein IDs from the Joint Genome Institute (JGI) and references where annotation recently have been presented. Domain structure identified in Ministeria is compared with animals - Porifera (Amphimedon and Oscarella), Cnidaria (Nematostella) and Chordata (Homo) - and the choanoflagellate Monosiga. Abbreviations: Hh-signal domain, N-terminal hedgehog domain; Hint cleavage site, cleavage site of the C-terminal hedgehog domain; Hint domain, C-terminal hedgehog domain; Notch(DSL), Notch domain also called Delta Serrate Ligand; EGF, epidermal growth factor domain; NL, domain found in Notch and Lin-12; NOD, NOD region; NODP, NODP region; ANK, ankyrin reapeats; PTP, protein tyrosine phosphatase.
↑The five choanozoan classes (bold) form at least four distinct clades, one probably related to fungi and the others to animals. Innovations in pseudopod character and their multiple losses with the origin of cell walls during nutritional shifts from engulfing prey (phagotrophy) to saprotrophy or parasitism are indicated by bars. In the common ancestor of animals and choanoflagellates a subset of the filozoan actin-supportd tentacles aggregated as a collar around the cilium (flagellum) for filter feeding. Epithelia and connective tissue made the first animals: the filter-feeding sponges.
↑Cell structure divergence in phagotrophic non-amoeboid flagellates provided the basis for evolving animals, fungi, plants and chromists. Original description: "Cell structure divergence in phagotrophic non-amoeboid flagellates provided the basis for evolving animals, fungi, plants and chromists. Pseudopodia evolved secondarily, myosin II providing the basis for pseudopodia in animals, Amoebozoa (and Percolozoa) and muscles. Chloroplasts, originating when the plant ancestor enslaved and modified undigested cyanobacteria, were transferred laterally (red arrow) to make chromists (e.g. brown seaweeds, diatoms, dinoflagellates) whose ancestor modified an enslaved undigested red alga. The most basic eukaryote structural dichotomy contrasts Euglenozoa (parallel centrioles; cilia with paraxonemal rods; cytopharynx for feeding) and excavates (Percolozoa, Eolouka, Neolouka: orthogonal centrioles: no paraxonemal rods; feeding by phagocytosing prey drawn into a ventral groove by posterior ciliary currents). The pre-animal lineage lost excavate groove-feeding by evolving ventral ciliary gliding locomotion to generate Sulcozoa, protozoa with a dorsal proteinaceous pellicle (blue). Irrespective of whether the eukaryote tree is rooted within the protozoan subkingdom Eozoa as shown (most likely) or beside Eolouka-like Reclinomonas with the most primitive mitochondria, the immediate ancestors of animals (Choanozoa) arose by loss of the anterior cilium and sulcozoan dorsal pellicle to make opisthokonts (in red) with a radically simplified, more radially symmetric, microtubular cytoskeleton. Long actin-supported filodigits arose in the ancestor of Filosporidia and choanoflagellates and became a circlet of microvilli to make the choanoflagellate/sponge collar for catching bacteria. Filosporidia comprise Filasterea, Ichthyosporea, Corallochytrea. The four derived kingdoms (e.g. ANIMALIA, PLANTAE) are shown in upper case; all taxa in lower case belong to the basal eukaryotic kingdom Protozoa." - Of interest to us on our journey towards animals are: myosin, integrins, catenins, cadherins, epithelia, gametes (sperm and egg), and extracellular matrix (ECM).
![Multicellular lifecycle diagrams[g] Plants (A), Animals (B) and Fungi (C)](https://anichin-myid.netlify.app/host-https-en.wikiversity.org/wiki/File:Lifecycle_diagram.svg)
_Rstb20150476f01_-_CC(BY)4.0.jpg)
_Rstb20150476f02_-_CC(BY)4.0.jpg)
_Rstb20150476f03_-_CC(BY)4.0.jpg)
.png)
.png)