Mammal
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
Cell (biology)
The cell is the basic structural and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the "building blocks of life"; the study of cells is called cellular biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as multicellular; the number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years ago. Cells are of two types: eukaryotic, which contain a nucleus, prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotes include two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling, they are simpler and smaller than eukaryotic cells, lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome, in direct contact with the cytoplasm; the nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: Enclosing the cell is the cell envelope – consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule.
Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma and Thermoplasma which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter; the cell wall consists of peptidoglycan in bacteria, acts as an additional barrier against exterior forces. It prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment; some eukaryotic cells have a cell wall. Inside the cell is the cytoplasmic region that contains the genome and various sorts of inclusions; the genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid.
Plasmids encode additional genes, such as antibiotic resistance genes. On the outside and pili project from the cell's surface; these are structures made of proteins that facilitate communication between cells. Plants, fungi, slime moulds and algae are all eukaryotic; these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles in which specific activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA; this nucleus gives the eukaryote its name, which means "true kernel". Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may not be present; the eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins.
All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation and thermosensation. Cilia may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." Motile eukaryotes can move using motile flagella. Motile cells are absent in flowering plants. Eukaryotic flagella are more complex than those of prokaryotes. All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out, maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells possess DNA, the hereditary material of genes, RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery.
There are other kinds of biomolecules in cells. This article lists these primary cellular components briefly
Plasmodesma
Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, species that have these structures include members of the Charophyceae, Charales and Phaeophyceae, as well as all embryophytes, better known as land plants. Unlike animal cells every plant cell is surrounded by a polysaccharide cell wall. Neighbouring plant cells are therefore separated by a pair of cell walls and the intervening middle lamella, forming an extracellular domain known as the apoplast. Although cell walls are permeable to small soluble proteins and other solutes, plasmodesmata enable direct, symplastic transport of substances between cells. There are two forms of plasmodesmata: primary plasmodesmata, which are formed during cell division, secondary plasmodesmata, which can form between mature cells. Similar structures, called gap junctions and membrane nanotubes, interconnect animal cells and stromules form between plastids in plant cells.
Primary plasmodesmata are formed when portions of the endoplasmic reticulum are trapped across the middle lamella as new cell wall is laid down between two newly divided plant cells and these become the cytoplasmic connections between cells. Here the wall is not thickened further, depressions or thin areas known as pits are formed in the walls. Pits pair up between adjacent cells. Plasmodesmata can be inserted into existing cell walls between non-dividing cells. A typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cells equating to between 1 and 10 per µm2. Plasmodesmata are 50–60 nm in diameter at the midpoint and are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, the desmotubule, they can transverse cell walls. The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or plasmalemma and has a similar phospholipid bilayer structure; the cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and is a continuous extension of the cytosol.
Trafficking of molecules and ions through plasmodesmata occurs through this space. Smaller molecules and ions can pass through plasmodesmata by diffusion without the need for additional chemical energy. Larger molecules, including proteins and RNA, can pass through the cytoplasmic sleeve diffusively. Plasmodesmatal transport of some larger molecules is facilitated by mechanisms that are unknown. One mechanism of regulation of the permeability of plasmodesmata is the accumulation of the polysaccharide callose around the neck region to form a collar, thereby reducing the diameter of the pore available for transport of substances; the desmotubule is a tube of appressed endoplasmic reticulum. Some molecules are known to be transported through this channel, but it is not thought to be the main route for plasmodesmatal transport. Around the desmotubule and the plasma membrane areas of an electron dense material have been seen joined together by spoke-like structures that seem to split the plasmodesma into smaller channels.
These structures may be composed of actin, which are part of the cell's cytoskeleton. If this is the case these proteins could be used in the selective transport of large molecules between the two cells. Plasmodesmata have been shown to transport proteins, short interfering RNA, messenger RNA, viral genomes from cell to cell. One example of a viral movement proteins is the tobacco mosaic virus MP-30. MP-30 is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata. Flowering Locus T protein moves from leaves to the shoot apical meristem through plasmodesmata to initiate flowering. Plasmodesmata are used by cells in phloem, symplastic transport is used to regulate the sieve-tube cells by the companion cells; the size of molecules that can pass through plasmodesmata is determined by the size exclusion limit. This limit is variable and is subject to active modification. For example, MP-30 is able to increase the size exclusion limit from 700 Daltons to 9400 Daltons thereby aiding its movement through a plant.
Increasing calcium concentrations in the cytoplasm, either by injection or by cold-induction, has been shown to constrict the opening of surrounding plasmodesmata and limit transport. Several models for possible active transport through plasmodesmata exist, it has been suggested that such transport is mediated by interactions with proteins localized on the desmotubule, and/or by chaperones unfolding proteins, allowing them to fit through the narrow passage. A similar mechanism may be involved in transporting viral nucleic acids through the plasmodesmata. Desmosome Gap junction Membrane nanotube
Mitosis
In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis is preceded by the S stage of interphase and is accompanied or followed by cytokinesis, which divides the cytoplasm and cell membrane into two new cells containing equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other; the process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, metaphase and telophase. During mitosis, the chromosomes, which have duplicated and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell; the result is two genetically identical daughter nuclei.
The rest of the cell may continue to divide by cytokinesis to produce two daughter cells. Producing three or more daughter cells instead of the normal two is a mitotic error called tripolar mitosis or multipolar mitosis. Other errors during mitosis can induce apoptosis or cause mutations. Certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Mitosis varies between organisms. For example, animal cells undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Most animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include the gametes -- egg cells -- which are produced by meiosis.
Numerous descriptions of cell division were made during 18th and 19th centuries, with various degrees of accuracy. In 1835, the German botanist Hugo von Mohl, described cell division in the green alga Cladophora glomerata, stating that multiplication of cells occurs through cell division. In 1838, Schleiden affirmed that the formation of new cells in their interior was a general law for cell multiplication in plants, a view rejected in favour of Mohl model, due to contributions of Robert Remak and others. In animal cells, cell division with mitosis was discovered in frog and cat cornea cells in 1873 and described for the first time by the Polish histologist Wacław Mayzel in 1875. Bütschli and Fol might have claimed the discovery of the process presently known as "mitosis". In 1873, the German zoologist Otto Bütschli published data from observations on nematodes. A few years he discovered and described mitosis based on those observations; the term "mitosis", coined by Walther Flemming in 1882, is derived from the Greek word μίτος.
There are some alternative names for the process, e.g. "karyokinesis", a term introduced by Schleicher in 1878, or "equational division", proposed by Weismann in 1887. However, the term "mitosis" is used in a broad sense by some authors to refer to karyokinesis and cytokinesis together. Presently, "equational division" is more used to refer to meiosis II, the part of meiosis most like mitosis; the primary result of mitosis and cytokinesis is the transfer of a parent cell's genome into two daughter cells. The genome is composed of a number of chromosomes—complexes of coiled DNA that contain genetic information vital for proper cell function; because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. Chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere; when mitosis begins, the chromosomes become visible.
In some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell disappears. Microtubules project from opposite ends of the cell, attach to the centromeres, align the chromosomes centrally within the cell; the microtubules contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes; as the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase. A new nuclear envelope forms around the separated daughter chromosomes, which decondense to form interphase nuclei. During mitotic progression after the anaphase onset, the cell may undergo cytokinesis. In animal cells, a cell membrane pinches inward between the two developing nuclei to produce two new cells. In plant cells, a cell plate forms between the two nuclei. Cytokinesis does not always occur.
The mitotic phase is a short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for the process of cell division. Interphase is divided into three phases: G1, S, G2. During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only durin
Cytoplasm
In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm; the main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, various cytoplasmic inclusions. The cytoplasm is about 80% water and colorless; the submicroscopic ground cell substance, or cytoplasmatic matrix which remains after exclusion the cell organelles and particles is groundplasm. It is the hyaloplasm of light microscopy, high complex, polyphasic system in which all of resolvable cytoplasmic elements of are suspended, including the larger organelles such as the ribosomes, the plant plastids, lipid droplets, vacuoles. Most cellular activities take place within the cytoplasm, such as many metabolic pathways including glycolysis, processes such as cell division; the concentrated inner area is called the endoplasm and the outer layer is called the cell cortex or the ectoplasm.
Movement of calcium ions in and out of the cytoplasm is a signaling activity for metabolic processes. In plants, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming; the term was introduced by Rudolf von Kölliker in 1863 as a synonym for protoplasm, but it has come to mean the cell substance and organelles outside the nucleus. There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles the vacuoles and sometimes the plastids; the physical properties of the cytoplasm have been contested in recent years. It remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelles while maintaining the cell’s structure; the flow of cytoplasmic components plays an important role in many cellular functions which are dependent on the permeability of the cytoplasm. An example of such function is cell signalling, a process, dependent on the manner in which signaling molecules are allowed to diffuse across the cell.
While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures require aid in moving through the cytoplasm. The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm. There has long been evidence, it is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal solution and at other times like an integrated network, forming a solid mass. This theory thus proposes that the cytoplasm exists in distinct fluid and solid phases depending on the level of interaction between cytoplasmic components, which may explain the differential dynamics of different particles observed moving through the cytoplasm, it has been proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. In this theory, the greater the concentration of cytoplasmic components, the less the cytoplasm behaves like a liquid and the more it behaves as a solid glass, freezing larger cytoplasmic components in place.
A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defence strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of small proteins and metabolites, helping to kickstart growth upon the cell's revival from dormancy. There has been research examining the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an alternative approach, the aggregate random forces within the cell caused by motor proteins explain the non-Brownian motion of cytoplasmic constituents; the three major elements of the cytoplasm are the cytosol and inclusions. The cytosol is the portion of the cytoplasm not contained within membrane-bound organelles. Cytosol makes up about 70% of the cell volume and is a complex mixture of cytoskeleton filaments, dissolved molecules, water; the cytosol's filaments include the protein filaments such as actin filaments and microtubules that make up the cytoskeleton, as well as soluble proteins and small structures such as ribosomes and the mysterious vault complexes.
The inner and more fluid portion of the cytoplasm is referred to as endoplasm. Due to this network of fibres and high concentrations of dissolved macromolecules, such as proteins, an effect called macromolecular crowding occurs and the cytosol does not act as an ideal solution; this crowding effect alters. Organelles, are membrane-bound structures inside the cell that have specific functions; some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, in plant cells, chloroplasts. The inclusions are small particles of insoluble substances suspended in the cytosol. A huge range of inclusions exist in different cell types, range from crystals of calcium oxalate or silicon dioxide in plants, to granules of energy-storage materials such as starch, glycogen, or polyhydroxybutyrate. A widespread example are lipid droplets, which are spherical droplets composed of lipids and proteins that are used in both prokaryotes and eukaryotes as a way of storing lipids such as fatty acids and sterols.
Lipid droplets make up much of the volume of adipocytes, which are specialized lipid-st
Phytomyxea
The Phytomyxea are a class of parasites of plants. They are divided into the orders Phagomyxida. A more common name for them is the plasmodiophorids, they develop within plant cells, causing the infected tissue to grow into a gall or scab. Important diseases caused by phytomyxeans include club root in cabbage and its relatives, powdery scab in potatoes; these are caused by species of Spongospora, respectively. The vegetative form is a multinucleate cell, called a plasmodium; this divides to form new spores, which are released when the host's cells burst. Both resting spores and motile zoospores, which have two smooth flagella, are produced at different stages. Within the plasmodium, dividing nuclei have a distinctive cross-like appearance. Plasmodiophorids are traditionally considered slime moulds, because of the plasmodial stage, thus they are classified as fungi, given names such as the Plasmodiophoromycota. However and ultrastructural studies indicate they belong to a diverse group of protists called the Cercozoa, or are related to them.
Class Phytomyxea Engler & Prantl 1897 em. Cavalier-Smith 1993 Genus? Pongomyxa Order Phagomyxida Cavalier-Smith 1993 Family Phagomyxidae Cavalier-Smith 1993 Genus Phagomyxa Karling 1944 Order Plasmodiophorida Cook 1928 em. Cavalier-Smith 1993 Family Endemosarcidae Olive & Erdos 1971 Genus Endemosarca Olive & Erdos 1971 Family Plasmodiophoridae Berl 1888 Genus Cystospora Elliott 1916 nomen dubium Genus Maullinia Maier et al. 2000 Genus Phytomyxa Schröter 1886 Genus Ligniera Maire & Tison 1911 Genus Membranosporus Ostenfeld & Petersen 1930 Genus Octomyxa Couch, Leitner & Whiffen 1939 Genus Plasmodiophora Woronin 1877 Genus Polymyxa Ledingham 1933 Genus Sorodiscus Lagerheim & Winge 1913 non Allman 1847 Genus Sorosphaera Schröter 1886 Genus Sorosphaerula Neuh. & Kirchm. 2011 Genus Spongospora Brunchorst 1887 Genus Sporomyxa Léger 1908 Genus Tetramyxa Goebel 1884 Genus Woronina Cornu 1872
Slime mold
Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms that can live as single cells, but can aggregate together to form multicellular reproductive structures. Slime molds were classified as fungi but are no longer considered part of that kingdom. Although not related to one another, they are still sometimes grouped for convenience within the paraphyletic group referred to as kingdom Protista. More than 900 species of slime mold occur all over the world, their common name refers to part of some of these organisms' life cycles where they can appear as gelatinous "slime". This is seen with the myxogastria, which are the only macroscopic slime molds. Most slime molds are smaller than a few centimeters, but some species may reach sizes of up to several square meters and masses of up to 30 grams. Many slime molds the "cellular" slime molds, do not spend most of their time in this state; as long as food is abundant, these slime molds exist as single-celled organisms.
When food is in short supply, many of these single-celled organisms will congregate and start moving as a single body. In this state they can detect food sources, they can change the shape and function of parts and may form stalks that produce fruiting bodies, releasing countless spores, light enough to be carried on the wind or hitch a ride on passing animals. They feed on microorganisms, they contribute to the decomposition of dead vegetation, feed on bacteria and fungi. For this reason, slime molds are found in soil, on the forest floor on deciduous logs. However, in tropical areas they are common on inflorescences and fruits, in aerial situations. In urban areas, they are found on mulch or in the leaf mold in rain gutters, grow in air conditioners when the drain is blocked. Slime molds, as a group, are polyphyletic, they were represented by the subkingdom Gymnomycota in the Fungi kingdom and included the defunct phyla Myxomycota and Labyrinthulomycota. Today, slime molds have been divided among several supergroups, none of, included in the kingdom Fungi.
Slime molds can be divided into two main groups. A plasmodial slime mold is one large cell; this "supercell" is a bag of cytoplasm containing thousands of individual nuclei. See heterokaryosis. By contrast, cellular slime molds spend most of their lives as individual unicellular protists, but when a chemical signal is secreted, they assemble into a cluster that acts as one organism. In more strict terms, slime molds comprise the mycetozoan group of the amoebozoa. Mycetozoa include the following three groups: Myxogastria or myxomycetes: syncytial, plasmodial, or acellular slime molds Dictyosteliida or dictyostelids: cellular slime molds ProtosteloidsEven at this level of classification there are conflicts to be resolved. Recent molecular evidence shows that, while the first two groups are to be monophyletic, the protosteloids are to be polyphyletic. For this reason, scientists are trying to understand the relationships among these three groups; the most encountered are the Myxogastria. A common slime mold that forms tiny brown tufts on rotting logs is Stemonitis.
Another form, which lives in rotting logs and is used in research, is Physarum polycephalum. In logs, it has the appearance of a slimy web-work of yellow threads, up to a few feet in size. Fuligo forms yellow crusts in mulch; the Dictyosteliida, cellular slime molds, are distantly related to the plasmodial slime molds and have a different lifestyle. Their amoebae do not form huge coenocytes, remain individual, they live in similar habitats and feed on microorganisms. When food runs out and they are ready to form sporangia, they do something radically different, they release signal molecules into their environment, by which they find each other and create swarms. These amoeba join up into a tiny multicellular slug-like coordinated creature, which crawls to an open lit place and grows into a fruiting body; some of the amoebae become spores to begin the next generation, but some of the amoebae sacrifice themselves to become a dead stalk, lifting the spores up into the air. The protosteloids have characters intermediate between the previous two groups, but they are much smaller, the fruiting bodies only forming one to a few spores.
Non-amoebozoan slime moulds include: Acrasids: slime molds which belong to the Heterolobosea within the super group Excavata. They have a similar life style to Dictyostelids, but their amoebae behave differently, having eruptive pseudopodia, they used to belong to the defunct phylum of Acrasiomycota. Plasmodiophorids: parasitic protists which belong to the super group Rhizaria, they can cause powdery scab tuber disease. The Plasmodiophorids form coenocytes, but are internal parasites of plants. Labyrinthulomycota: slime nets, which belong to the superphylum Heterokonta as the class Labyrinthulomycetes, they are marine and form labyrinthine networks of tubes in which amoeba without pseudopods can travel. Fonticula is a cellular slime mold. Fonticula is not related to either the Dictyosteliida or the Acrasidae. A 2009 paper finds it to be related to Nuclearia. Slime molds begin life as amoeba-like cells; these unicellular amoebae are haploid, feed on bacteria. These amoebae can mate if they encounter the correct mating type and form zyg
