Heterokont
The heterokonts or stramenopiles are a major line of eukaryotes containing more than 25,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton. Other notable members of the Stramenopiles include the parasitic oomycetes, including Phytophthora of Irish potato famine infamy and Pythium which causes seed rot and damping off; the name "heterokont" refers to the type of motile life cycle stage, in which the flagellated cells possess two differently shaped flagella. In 1899, Luther created "Heterokontae" for some algae with unequal flagella, today called Xanthophyceae; some authors would include other groups in Heterokonta, expanding its sense. The origin of the other name of the group, "stramenopile", is explained by David and Adl et al.: Regarding the spelling of stramenopile, it was spelled stramenopile. The Latin word for straw is strāmĭnĕus, -a, -um, adj. made of straw—thus, it should have been spelled straminopile or straminipile.
However, Patterson stated that this is a common name and, as a common name, it can be spelled as Patterson chooses. If he had stipulated that the name was a formal name, governed by rules of nomenclature his spelling would have been an orthogonal mutation and one would correct the spelling in subsequent publications. But, it was not Patterson’s desire to use the term in a formal sense. Thus, if we use it in a formal sense, it must be formally described. However, here is the strange part of this, many people liked the name, but wanted it to be used formally. So they capitalized the first letter, made it Stramenopiles. Many heterokonts are unicellular flagellates, most others produce flagellated cells at some point in their lifecycles, for instance as gametes or zoospores; the name heterokont refers to the characteristic form of these cells, which have two unequal flagella. The anterior straminipilous flagellum is covered with one or two rows of lateral bristles or mastigonemes, which are tripartite, while the posterior flagellum is whiplike and shorter, or sometimes reduced to a basal body.
The flagella are inserted subapically or laterally, are supported by four microtubule roots in a distinctive pattern. Mastigonemes are manufactured from glycoproteins in the cell's endoplasmic reticulum before being transported to the anterior flagella's surface; when the straminipilous flagellum moves, the mastigonemes create a retrograde current, pulling the cell through the water or bringing in food. The mastigonemes have a peculiar tripartite structure, which may be taken as the defining characteristic of the heterokonts, thereby including a few protists that do not produce cells with the typical heterokont form. Mastigonemes have been lost in a few heterkont lines, most notably the diatoms. Many heterokonts are algae with chloroplasts surrounded by four membranes, which are counted from the outermost to the innermost membrane; the first membrane is continuous with the host's chloroplast endoplasmic reticulum, or cER. The second membrane presents a barrier between the lumen of the cER and the primary endosymbiont or chloroplast, which represents the next two membranes, within which the thylakoid membranes are found.
This arrangement of membranes suggests that heterokont chloroplasts were obtained from the reduction of a symbiotic red algal eukaryote, which had arisen by evolutionary divergence from the monophyletic primary endosymbiotic ancestor, thought to have given rise to all eukaryotic photoautotrophs. The chloroplasts characteristically contain chlorophyll a and chlorophyll c, the accessory pigment fucoxanthin, giving them a golden-brown or brownish-green color. Most basal heterokonts are colorless; this suggests. However, fucoxanthin-containing chloroplasts are found among the haptophytes; these two groups may have a common ancestry, also a common phylogenetic history with cryptomonads, being grouped by some authors in the Chromista. This may be interpreted as suggesting that the ancestral heterokont was an alga, all colorless groups arose through loss of the secondary endosymbiont and its chloroplast; as noted above, classification varies considerably. The heterokont algae were treated as two divisions, first within the kingdom Plantae and the Protista: Division Chrysophyta Class Chrysophyceae Class Bacillariophyceae Division Phaeophyta In this scheme, the Chrysophyceae is paraphyletic to both other groups.
As a result, various members have been given their own classes and divisions. Recent systems treat these as classes within a single division, called the Heterokontophyta, Chromophyta, or Ochrophyta; this is not universal, however. The discovery that oomycetes and hyphochytrids are related to these algae, rather than fungi, as thought, has led many authors to include these two groups among the heterokonts. Should it turn out that they evolved from colored ancestors, the heterokont group would be paraphyletic in their absence. Once again, howe
Contractile vacuole
A contractile vacuole is a sub-cellular structure involved in osmoregulation. It is found predominantly in unicellular algae, it was known as pulsatile or pulsating vacuole The contractile vacuole is a specialized type of vacuole that regulates the quantity of water inside a cell. In freshwater environments, the concentration of solutes is hypotonic, higher inside than outside the cell. Under these conditions, water osmosis causes water to accumulate in the cell from the external environment; the contractile vacuole acts as part of a protective mechanism that prevents the cell from absorbing too much water and lysing through excessive internal pressure. The contractile vacuole, as its name suggests, expels water out of the cell by contracting; the growth and contraction of the contractile vacuole are periodical. One cycle takes several seconds, depending on the environment's osmolarity; the stage in which water flows into the CV is called diastole. The contraction of the contractile vacuole and the expulsion of water out of the cell is called systole.
Water always flows first from outside the cell into the cytoplasm, is only moved from the cytoplasm into the contractile vacuole for expulsion. Species that possess a contractile vacuole always use the organelle at hypertonic environments, since the cell tends to adjust its cytoplasm to become more hyperosmotic than the environment; the amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments, less water will be expelled and the contraction cycle will be longer; the best understood contractile vacuoles belong to the protists Paramecium, Amoeba and Trypanosoma, to a lesser extent the green alga Chlamydomonas. Not all species that possess a contractile vacuole are freshwater organisms; the contractile vacuole is predominant in species that do not have a cell wall, but there are exceptions which do possess a cell wall. Through Evolution, the contractile vacuole has been lost in multicellular organisms, but it still exists in the unicellular stage of several multicellular fungi, as well as in several types of cells in sponges.
The number of contractile vacuoles per cell varies, depending on the species. Amoeba have one, Dictyostelium discoideum, Paramecium aurelia and Chlamydomonas reinhardtii have two, giant amoeba, such as Chaos carolinensis, have many; the number of contractile vacuoles in each species is constant and is therefore used for species characterization in systematics. The contractile vacuole has several structures attached to it in most cells, such as membrane folds, water tracts and small vesicles; these structures have been termed the spongiome. The spongiome serves several functions in water transport into the contractile vacuole and in localization and docking of the contractile vacuole within the cell. Paramecium and Amoeba possess large contractile vacuoles, which are comfortable to isolate and assay; the smallest known contractile vacuoles belong with a diameter of 1.5 µm. In Paramecium, which has one of the most complex contractile vacuoles, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm.
After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm which can be opened and closed. Other protists, such as Amoeba, have CVs that move to the surface of the cell when full and undergo exocytosis. In Amoeba contractile vacuoles collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport; the way in which water enters the CV had been a mystery for many years, but several discoveries since the 1990s have improved understanding of this issue. Water could theoretically cross the CV membrane by osmosis, but only if the inside of the CV is hyperosmotic to the cytoplasm; the discovery of proton pumps in the CV membrane and the direct measurement of ion concentrations inside the CV using microelectrodes led to the following model: the pumping of protons either into or out of the CV causes different ions to enter the CV. For example, some proton pumps work as cation exchangers, whereby a proton is pumped out of the CV and a cation is pumped at the same time into the CV.
In other cases, protons pumped into the CV drag anions with them, to balance the pH. This ion flux into the CV causes an increase in CV osmolarity and as a result water enters the CV by osmosis. Water has been shown in at least some species to enter the CV through aquaporins. Acidocalcisomes have been implied to work alongside the contractile vacuole in responding to osmotic stress, they were detected in the vicinity of the vacuole in Trypanosoma cruzi and were shown to fuse with the vacuole when the cells were exposed to osmotic stress. The acidocalcisomes empty their ion contents into the contractile vacuole, thereby increasing the vacuole's osmolarity; the CV indeed does not exist in higher organisms, but some of its unique characteristics are used by the former in their own osmoregulatory mechanisms. Research on the CV can therefore help us understand. Many issues regarding the CV remain, unsolved: Contraction. I
Eukaryote
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to Eukarya, their name comes from the Greek εὖ and κάρυον. Eukaryotic cells contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells; these act as sex cells. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, makes up one of the domains of life in the three-domain system. The two other domains and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved 1.6–2.1 billion years ago, during the Proterozoic eon. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton; the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1937 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes; however he mentioned this in only one paragraph, the idea was ignored until Chatton's statement was rediscovered by Stanier and van Niel.
In 1905 and 1910, the Russian biologist Konstantin Mereschkowski argued that plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, On the origin of mitosing cells. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA; this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles and chloroplasts. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's nucleus came from the ability of Gram-positive bacteria to form endospores. In 1987 and papers, Thomas Cavalier-Smith proposed instead that the membranes of the nucleus and endoplasmic reticulum first formed by infolding a prokaryote's plasma membrane.
In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus reviving Mereschkowski's theory. Eukaryotic cells are much larger than those of prokaryotes having a volume of around 10,000 times greater than the prokaryotic cell, they have a variety of internal membrane-bound structures, called organelles, a cytoskeleton composed of microtubules and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and pinches off to form a vesicle, it is probable that most other membrane-bound organelles are derived from such vesicles.
Alternatively some products produced by the cell can leave in a vesicle through exocytosis. The nucleus is surrounded with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, involved in protein transport and maturation, it includes the rough endoplasmic reticulum where ribosomes are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, the Golgi apparatus. Vesicles may be specialized for various purposes. For instance, lysosomes contain digestive enzymes that break down most biomolecules in the cytoplasm. Peroxisomes are used to break down peroxide, otherwise toxic. Many protozoans have contractile vacuoles, which collect and expel excess water, extrusomes, which expel material used to deflect predators or capture prey.
In higher plants, most of a cell's volume is taken up by a central vacuole, whi
Pseudopodia
A pseudopod or pseudopodium is a temporary arm-like projection of a eukaryotic cell membrane. Filled with cytoplasm, pseudopodia consist of actin filaments and may contain microtubules and intermediate filaments. Pseudopods are used for ingestion. Different types of pseudopodia can be classified by their distinct appearances. Lamellipodia are thin. Filopodia are slender, thread-like, are supported by microfilaments. Lobopodia are amoebic. Reticulopodia are complex structures bearing individual pseudopodia. Axopodia are the phagocytosis type with long, thin pseudopods supported by complex microtubule arrays enveloped with cytoplasm; however some pseudopodial cells are able to use multiple types of pseudopodia depending on the situation: Most of them use a combination of lamellipodia and filopodia to migrate. The human foreskin fibroblasts can either use lamellipodia- or lobopodia-based migration in a 3D matrix depending on the matrix elasticity. Several pseudopodia arise from the surface of the body, or, a single pseudopod may form on the surface of the body.
Cells which make pseudopods are referred to as amoeboids. To move towards a target, the cell uses chemotaxis, it senses extracellular signalling molecules, chemoattractants, to extend pseudopodia at the membrane area facing the source of these molecules. The chemoattractants bind to G protein-coupled receptors, which activate GTPases of the Rho family via G-proteins. Rho GTPases are able to activate WASp which in turn activate Arp2/3 complex which serve as nucleation sites for actin polymerization; the actin polymers push the membrane as they grow, forming the pseudopod. The pseudopodium can adhere to a surface via its adhesion proteins, pull the cell's body forward via contraction of an actin-myosin complex in the pseudopod; this type of locomotion is called Amoeboid movement. Rho GTPases can activate phosphatidylinositol 3-kinase which recruit PIP3 to the membrane at the leading edge and detach the PIP3-degrading enzyme PTEN from the same area of the membrane. PIP3 activate GTPases back via GEF stimulation.
This serves as a feedback loop to amplify and maintain the presence of local GTPase at the leading edge. Otherwise, pseudopodia can't grow on other sides of the membrane than the leading edge because myosin filaments prevent them to extend; these myosin filaments are induced by cyclic GMP in D. discoideum or Rho kinase in neutrophils for example. In the case there is no extracellular cue, all moving cells navigate in random directions, but they can keep the same direction for some time before turning; this feature allows cells to explore large areas for colonization or searching for a new extracellular cue. In Dictyostelium cells, a pseudopodium can form either de novo as normal, or from an existing pseudopod, forming a Y-shaped pseudopodium; the Y-shaped pseudopods are used by Dictyostelium to advance straight forward by alternating between retraction of the left or right branch of the pseudopod. The de novo pseudopodia form at different sides than pre-existing ones, they are used by the cells to turn.
Y-shaped pseudopods are more frequent than de novo ones, which explain the preference of the cell to keep moving to the same direction. This persistence is modulated by PLA2 and cGMP signalling pathways; the functions of pseudopodia include locomotion and ingestion: Pseudopodia are critical in sensing targets which can be engulfed. A common example of this type of amoeboid cell is the macrophage, they are essential to amoeboid-like locomotion. Human mesenchymal stem cells are a good example of this function: these migratory cells are responsible for in-utero remodeling. Pseudopods can be classified into several varieties according to the number of projections, according to their appearance: Lamellipodia are broad and flat pseudopodia used in locomotion, they are supported by microfilaments which form at the leading edge, creating a mesh-like internal network. Filopodia are slender and filiform with pointed ends, consisting of ectoplasm; these formations are supported by microfilaments which, unlike the filaments of lamellipodia with their net-like actin, form loose bundles by cross-linking.
This formation is due to bundling proteins such as fimbrins and fascins. Filopodia are observed in some animal cells: in part of Filosa, in "Testaceafilosia", in Vampyrellidae and Pseudosporida and in Nucleariida. Lobopodia are bulbous and blunt in form; these finger-like, tubular pseudopodia contain both endoplasm. They can be found in different kind of cells, notably in Lobosa and other Amoebozoa and in some Heterolobosea. High-pressure lobopodia can be found in human fibroblasts travelling through a complex network of 3D matrix. Contrarily to other pseudopodia using the pressure exerted by actin polymerization on the membrane to extend, fibroblast lobopods use the nuclear piston mechanism consisting in pulling the nucleus via actomyosin contractility to push the cytoplasm that in turn push the membrane, leading to pseudopod formation. To occur, this lobopodia-based fibroblast migration needs nesprin 3, RhoA
Ochrophyta
Ochrophyta is a group of photosynthetic heterokonts. The classification of the group is still being worked out; some authors divide it into two subphyla, Phaeista Cavalier-Smith 1995 and Khakista Cavalier-Smith, 2000. Others prefer listing only lower taxa. Based on the following works of Ruggiero et al. 2015 & Silar 2016
Amoeba
An amoeba called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape by extending and retracting pseudopods. Amoebas do not form a single taxonomic group. Amoeboid cells occur not only among the protozoa, but in fungi and animals. Microbiologists use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement. In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Amoeboid organisms are no longer classified together in one group; the best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, the multicellular "social amoeba" or slime mould Dictyostelium discoideum.
Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba have bulbous pseudopods, rounded at the ends and tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have thread-like pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like structures; some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia supported from within by bundles of microtubules. Free-living amoebae may be "testate", or "naked"; the shells of testate amoebae may be composed of various substances, including calcium, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms. To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.
This organelle is necessary because freshwater has a lower concentration of solutes than the amoeba's own internal fluids. Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and burst. Marine amoebae do not possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water; the food sources of amoebae vary. Some amoebae are live by consuming bacteria and other protists; some eat dead organic material. Amoebae ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, there is no fixed place on the cell at which phagocytosis occurs; some amoebae feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
The size of amoeboid cells and species is variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter, within the size range of many bacteria. At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter. Most of the free-living freshwater amoebae found in pond water and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye; some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, engulf them by phagocytosis. Amoeboid stages occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds classified in the class Myxogastria, the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage.
The amoeboid cells of the former combine to form a giant multinucleate organism, while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism. Other organisms may present amoeboid cells during certain life-cycle stages, e.g. the gametes of some green algae and pennate diatoms, the spores of some Mesomycetozoea, the sporoplasm stage of Myxozoa and of Ascetosporea. The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus". Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus; the term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid. In 1822, the genus Amiba was erected by the Frenc
Actinophryid
The actinophryids are an order of heliozoa. They are the most common heliozoa in fresh water and can be found in marine and soil habitats. Actinophryids are unicellular and spherical in shape, with many axopodia that radiate outward from the cell body. Axopodia are a type of pseudopodia that are supported by hundreds of microtubules arranged in a needle-like internal structure; these axopods adhere to passing prey and assist with cell movement, as well as playing a part in cell division and cell fusion. Actinophryids are aquatic protozoa with a spherical cell body and many needle-like axopodia, they resemble the shape of a sun due to this structure, the inspiration for their common name: heliozoa, or "sun-animalcules". They range in size from a few micrometers to a full millimeter across; the cell body is vacuolated, with the ectoplasm consisting entirely of these structures. The endoplasm of actinophryids is darker and denser than the outer layer, can sometimes be seen as a sharp boundary under a light microscope.
The organisms can be either mononucleate, with a single, well defined nucleus in the center of the cell body, or multinucleate, with 10 or more nuclei dispersed throughout the organism. The cytoplasm of actinophryids is granular, similar to that of Amoeba. Contractile vacuoles are common in these organisms, who use them to maintain homeostasis and control buoyancy; these are visible as clear bulges from the surface of the cell body that fill rapidly deflate, expelling the contents into the environment. The most distinctive characteristic of the actinophryids is their axopodia; these axopodia consist of a central, rigid rod, coated in a thin layer of ectoplasm. These axonemes are terminate there, sometimes close to a nucleus; the axonemes are composed microtubules arranged in a double spiral pattern characteristic of the order. Due to their long, parallel construction these microtubules demonstrate strong birefringence; these axopodia are used for prey capture and cell fusion and division. They can be flexible when the organisms are starved, are dynamic, undergoing frequent construction and destruction.
When used to collect prey items, two methods of capture have been noted, termed axopodial flow and rapid axopodial contraction. Axopodial flow involves the slow movement of a prey item along the surface of the axopod as the ectoplasm itself moves, while rapid axopodial contraction involves the collapse of the axoneme's microtubule structure; this behavior has been documented in many species, including Actinosphaerium nucleofilum, Actinophrys sol, Raphidiophrys contractilis. The rapid axopodial contraction occurs at high speed in excess of 5mm/s or tens of body lengths per second; the axopodial contractions have been shown to be sensitive to environmental factors such as temperature and pressure as well as chemical signals like Ca2+ and colchine. They may be triggered by mechanical or electrical stimulation. Reproduction in actinophryids takes place via fission, where one parent cell divides into two or more daughter cells. For multinucleate heliozoa, this process is plasmotomic as the nuclei are not duplicated prior to division.
It has been observed that reproduction appears to be a response to food scarcity, with an increased number of divisions following the removal of food and larger organisms during times of food excess. Actinophryids undergo autogamy during times of food scarcity; this is better described as genetic reorganization than reproduction, as the number of individuals produced is the same as the initial number. Nonetheless, it serves as a way to increase genetic diversity within an individual which may improve the likelihood of expressing favorable genetic traits. Plastogamy has been extensively documented in actinophryids in multinucleate ones. Actinosphaerium were observed to combine without the combination of nuclei, this process sometimes resulted in more or less individuals than combined; this process is not caused by contact between two individuals but can be caused by damage to the cell body. Under unfavourable conditions, some species will form a cyst; this is the product of autogamy, in which case the cysts produced are zygotes.
Cells undergoing this process withdraw their axopodia, adhere to the substrate, take on an opaque and grayish appearance. This cyst divides until only uninucleate cells remain; the cyst wall is thickly layered 7-8 times and includes gelatinous layers, layers of silica plates, iron. Placed in Heliozoa, the group's current location within the larger tree of life is debated, it may belong to Raphidomonadea. There are several genera included within this classification. Actinophrys have a single, central nucleus. Most have a cell body 40-50 micrometer in diameter with axopods around 100 μm in length, though this varies significantly. Actinosphaerium are several times larger, from 200-1000 μm in diameter, with many nuclei and are found in fresh water. A third genus, was named as a junior subjective synonym of Actinosphaerium by Mikrjukov & Patterson in 2001, but Cavalier-Smith & Scoble preserve the genus. Heliorapha was added to this classification by Cavalier & Smith, the genus Ciliophrys. Classification based on Cavalier-Smith and Scoble 2013.
Cavalier-Smith 2013 In subclass Raphopoda Cavalier-Smith 2013 Order Actinophyrida Hartmann 1913 Family Actinosphaeriidae Cavalier-Smith 2013 Genus Actinosphaeri
