Vegetal On The World: fungi

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Ascomycetes

Aleuria aurantia. The Orange Peel Fungus is a widespread ascomycete fungus in the order Pezizales. The brilliant orange, cup-shaped ascocarps often resemble orange peels strewn on the ground, giving this species its common name.

Ascomycetes

The ascomycetes are fungi (phylum Ascomycota or Ascomycotina) that produce sexual spores in a specialized cell called an ascus. These diverse fungi, with more than thirty thousand species, can be found in almost every ecosystem worldwide. One of the most famous members of the ascomycetes is the truffle.

Ascomycetes, one of the four phyla of the fungus kingdom, by definition possess an ascus, a single cell inside of which sexual spores are produced.

The reproductive process has been well documented and occurs when the dikaryotic mycelium (the mass of hyphae forming the body) undergoes changes that precede the formation of the ascus. Dikaryotic is the genetic state in which two haploid nuclei are present in the cell. One nucleus is donated by each parent.

The first change occurs when the end cell of a hyphal strand begins to form a small bend. The cell divides into three cells; the outer two cells are haploid, and the middle cell is dikaryotic. The middle cell then elongates, and the nuclei migrate into its center.

The two haploid nuclei then fuse to form a single diploid nucleus, which undergoes mitosis and meiosis to form eight haploid nuclei. Cell walls form around the nuclei producing eight haploid ascospores. The ascospores are then liberated from the ascus.

The ascus wall determines the kind of dispersal of the spores. Some asci have a thin, single-layer wall, which breaks down to liberate spores. Unitunicate asci have a multilayer cell wall with a pore at the end of the ascus. Spore release is active through the pore.

beautifully colored ascomycete

Bitunicate asci have multilayer cell walls, and release of spores is by the separation of the layers of the cell wall, with the inner layer inflating to several times its normal size and then lifting off of the ascus, allowing the spores to be released.

The spores are released into the environment, where they germinate and produce haploid hyphae. The haploid hyphae fuse with compatible haploid hyphae, forming dikaryotic hyphae, and the process begins to repeat itself.

There are five different ways in which asci are formed in nature. First, asci can be produced by exposure to the environment, as are the asci of yeasts or the ascus of the peach leaf-curl pathogen Taphrina deformans. With these fungi, the ascospores are released by the breakdown of the ascus wall.

The other four ways of production of asci all take place inside structures made from mycelium, called ascocarps. These structures range from totally closed to open, like a cup. The totally closed ascocarps are called cleistothecia. Within these, the asci are scattered, and the spores are released by breakdown and decomposition of the fungal tissue.

Ascomycetes as Pathogens

Ascomycetes as Pathogens

Some members of the ascomycetes are very important plant and animal pathogens, causing serious diseases. One of the more impressive plant pathogens is the ergot fungus (Claviceps purpurea). This fungus colonizes the ovaries of grains, such as rye (Secale cereale).

It produces a mass of mycelium, called a sclerotium, which is hard and has a density similar to that of a seed. Because of this, the sclerotia are often found in the threshed grain. Sclerotia contain an accumulation of alkaloids and other secondary metabolites.

When the sclerotia are ground into flower and baked into bread, many of these secondary metabolites are passed into the bread. During the Middle Ages this fungus was responsible for a human disease called St. Anthony’s Fire. Today, this fungus is used for the natural production of a coagulant which is used in medicine.

Another group of plant pathogens are the powdery mildews. There are several hundred species of these fungi, which produce a powdery spore mass on the outer surfaces of plant leaves. If a leaf is infected before it has expanded, it will remain small and puckered andmay drop from the plant.

The powdery mildews are superficial and send hyphae through the leaf cuticle into the epidermis. The fungus then grows over the surface of leaf, giving it a powdery appearance. During the winter, the fungus produces cleistothecia on the surface of the leaf. Powdery mildew can occur on most plant species and can be very damaging to crop and ornamental plants.

Economic Uses

Truffle

Truffle is a generic term for fungi that form mycorrhizae (a symbiotic association) with the roots of various trees. The fungus grows into the roots and helps the plant tolerate stress, providing the plant with increased absorption of phosphorus from the soil.

In return, the plant gives the fungus metabolites that it needs for growth. These fungi then produce fruiting bodies, either in the soil or upon the surface of the soil. The truffles are produced in the soil up to a depth of 1 foot.

Truffles can be located in the soil using a trained sowor dog that is able to sniff out the volatile chemicals that are produced. However, it is important to note that edible fungi such as truffles can often be mistaken for highly toxic fungi and should never be gathered or eaten without expert identification.

Truffles are used as condiments and are able to impart unique aromas and sensations to food. They are shaped like small balls, varying in size from that of a pea to that of a golf ball. Truffles are only produced in nature and therefore fetch high prices.

The price of truffles depends on the species, size, and freshness. Prices of the famous French Black Périgord truffle (Tuber melanosporum) can reach into thousands of dollars per kilogram (2.2 pounds). In the United States, the Oregon white truffle is quite appealing and can be found in the Pacific North-west.

Morels (Morchella) are another choice edible ascomycete. Shaped like a little hat sitting upon a stalk, they are brown in color and have an appealing aroma. They add flavor to any food and are a favorite of many wild animals.

Scientific Uses

Some members of the ascomycetes are used for genetic studies. Such is the case for Neurospora, a common fungus found growing on soil and organic matter and one of the first organisms to be found in an area after a fire. A swith all ascomycetes, there are two compatible mating types, which makes for easy genetic study.

Using spore characteristics of shape, color, and texture, it is possible to see how mitosis and meiosis occur in the ascus by determining the placement of the spores. The fungus is readily mutated, which further enhances genetic study.

fungi , taxonomic groups

Basidiomycetes

Hinhvv

6:09 AM

Basidiomycetes

The Basidiomycetes constitute the largest of the three classes of the Basidiomycota (basidiosporic fungi), a very large class of about fourteen thousand species of the most diverse terrestrial fungi.

The largest fungi belong in the Basidiomycetes class, as do some of the most unusual. All members of Basidiomycetes produce a basidium from hyphal cells and not from spores. (The basidiumis a cell produced at the end of a dikaryotic hypha.)

The basidium will produce either two or four spores as the result of meiosis. The basidium may be either a nonseptate cell, with two or four sterigmata (the basidiospore is produced on the end of the sterigma) at the apex, or it may be septate.

The septa can be either horizontal or vertical. When observed from the apex, the vertical septa will produce a crosslike pattern. In either case, septate basidia will have one sterigma per cell. Basidiospores are thin-walled and may be released either actively or passively.

Basidiocarps

The basidiocarp is the fruiting body of the fungus. The fungus grows as a dikaryotic mycelium through the substrate. When the fungus has acquired sufficient energy, and environmental conditions are adequate, the fungus will produce a basidiocarp. The basidiocarp often appears overnight and may reach a meter in height.

Some basidiocarps are tiny, less than a centimeter in height. The basidiocarpmay look like a mushroom or may have the appearance of a golf ball or any variation in between. The basidiocarp may be edible or deadly poisonous. It often serves as food for wild animals and insects.

Classification

The Basidiomycetes are divided into two groups based on septa in the sterigma. Those that have a septate sterigma are classified in Phragmobasidiomycetidae, while those without septa in the sterigma are classified in Holobasidiomycetidae. Phragmobasidiomycetidae is a small group that includes some smaller fungi whose basidiocarps often have a gelatinous appearance.

Holobasidiomycetidae is a large group of fungi and is easily divided into two major groups based on the release of the basidiospore. The hymenomycetes release spores actively, while the gasteromycetes release spores passively.

Hymenomycetes

hymenomycetes

The hymenomycetes are the most familiar fleshy fungi. These are the ones that resemble the common mushroom and can be seen when the weather is warm and damp. These fungi may have gills or pores on the underside of the pileus (the cap).

The gills or pores are lined with a layer of basidia that produce spores. Some of these fungi are produced on the sides of trees or fallen wood and may be a centric. Many of them appear to arise from the ground. Colors of these fungi can be found anywhere in the rainbow, but most appear in earth tones.

Some of the major orders within the hymenomycetes are the Agaricales and the Boletales. The Agaricales contain the fungi that produce gills on the underside of the pileus. The Boletales contains the fungi that produce pores on the underside of the pileus.

For the most part, the Agaricales are fleshy fungi that are supple at maturity and last for no more than a week or two in nature. The Boletales are also fleshy and can be confused with other fungi that have pores and are hard. Some of these hard fungi are common parasites of trees; the "shelves" that they produce can grow for years.

Gasteromycetes

gasteromycetes

The gasteromycetes are a much more diverse group of fungi. The fungi in this group are often associated with soil or decomposing organic matter, although some may be mycorrhizal (in a symbiotic association with plant roots). There is tremendous diversity in this group, and many scientists believe that this group is artificial (not based on evolutionary relationships).

Some of themore interesting fungi of this group are shaped like balls that lie on the soil. Members of Lycoperdales and Sclerodermatales produce ball-like basidiocarps that form at the soil line. The size can range from that of a small marble up to that of a soccer ball.

These are called puffballs, as they release spores in small clouds when kicked. In nature, the upper layers of the basidiocarp crack, and spores are released as drops of water, hitting the outer layer of the basidiocarp. Many of these are edible when properly identified.

Other interesting fungi are found in the order Nidulariales. These are called the bird's nest fungi, as the basidiocarp resembles a small bowl containing two or three small "eggs", which contain the basidia. When a droplet of water lands in the "nest" the "eggs" are thrown upward and outward.

As they are released, a small thread is pulled behind, and the thread sticks onto some part of a plant, such as a blade of grass. The "egg" will then degrade and release the spores to be disseminated in the wind.

Among themost bizarre fungi are the stinkhorns. These fungi produce basidiocarps from structures that look like chicken eggs. The elongate structure produces the basidia on the end in a mass of slimy, smelly mucus. Flies are attracted to the smell, land on the mucus, and fly away. The basidiospores adhere to their feet and drop off, thereby disseminating the fungus.

Economic botany and plant uses , fungi , microorganisms , pests and pest control , taxonomic groups

Basidioporic Fungi

Hinhvv

2:52 AM

Basidioporic Fungi

Basidiosporic fungi (also known as the Basidiomycota or Basidiomycotina) are fungi that produce sexual spores on a specialized cell called a basidium.

The basidiosporic fungi are the most diverse phylum of the fungi world, with more than 22,300 species described. Some of the fungi in this phylum are microscopic, while the larger members of this group produce fruiting structures that are basketball-sized and weigh in excess of 10 pounds.

This phylum contains fungi that fall into three classes: mushroom, rusts, and smuts—and range widely in appearance, from the common mushroom to weblike fungi with an odor that can be detected at several feet.

Taxonomy

The basidiosporic fungi are divided into three classes: Basidiomycetes (mushrooms); Teliomycetes (rusts); and Ustomycetes (smuts). The Basidiomycetes are the higher basidiosporic fungi, which are normally fleshy. They produce true basidiocarps, and the only spore formed is the basidiospore.

The other two classes both have more than one spore form and do not have extensive mycelium. The Teliomycetes are commonly called rusts and are serious biotrophic parasites of plants. The rusts are able to complete their life cycle only in the presence of living plant host tissue.

The Ustomycetes are commonly called smuts and are mostly minor pathogens of plants, especially monocots. Some smuts have been cultured in axenic culture, where they form a "yeastlike" phase. The yeastlike phase has no true mycelium but rather individual cells.

Basidium

The basidium is a single cell on which basidiospores are produced externally. The basidium forms either as the terminal cell of a dikaryotic mycelium or from a resting spore that initially is dikaryotic.

basidium

The dikaryotic mycelium or spore contains two haploid nuclei, one donated by each of the parent strains. As the basidium begins to form, the two nuclei migrate into the center of the cell and fuse, forming a diploid nucleus.

This nucleus then undergoes meiosis, forming four haploidnuclei. As this is occurring, the cell wall of the basidium begins to produce little extensions called sterigmata, upon which the basidiospores will form. The tips of the sterigmata then inflate, and one nucleus migrates into each forming basidiospore.

The basidiospore is haploid and has a very thin cell wall. The spore is normally transmitted in air currents. Upon germination, the basidiospore produces a haploid mycelium which will fuse with a compatible hyphae, producing a dikaryotic mycelium.

Spore release from the basidiumcan be either active or passive. Passive release occurs when the junction of the sterigma and basidiospore separates, releasing the spore. Active release is more specialized. When the basidiospore is forming, a small segment of the spore wall at the junction with the sterigma loosens and fills with either gas or liquid.

At the time of release, the fluid or gas escapes, propelling the basidiospore away from the basidium. The distance traveled is not great, just enough to make sure that the basidiospore is able to enter into air currents for dissemination.

Hyphal Structure

The hyphae of the Basidiomycetes are septate and have special modifications at the septa. When a cell divides, a cross wall forms between the two daughter cells. With the dikaryotic hyphae of the Basidiomycetes, as the cell divides, the nuclei migrate toward the apex of the hyphae.

The nuclei then undergo mitosis, with one of the nuclei migrating into a small outgrowth of the hyphae and the other migrating backward. Septa form, creating a new dikaryotic cell near the apex and two haploid cells, one in line and the other as the outgrowth.

The outgrowth then turns and fuses with the haploid cell, and the nucleus migrates back to form a dikaryotic cell. The outgrowth remains visible with a microscope and is called a clamp connection.

The reproductive structure of the Ustomycetes is called a sorus. The sorus is a mass of dikaryotic spores that are normally dark brown or black in color. The sorus is formed in meristematic regions of the plants. The spores are called probasidia, because they form basidia when they germinate.

With the Teliomycetes, there are up to five distinct spore forms. The basidiospore lands on a susceptible plant and germinates, producing a haploid mycelium that infects the plant. The infection results in the formation of a haploid spermagonium that produces both spermatia and receptive hyphae.

When a compatible spermatia and receptive hypha combine, a dikaryotic hypha is produced, which initiates formation of an aecium. The aecium produces dikaryotic spores that are transmitted by air currents and infect another plant.

The resultant infection produces a subcuticular or subepidermalmass of thin-walled spores. These dikaryotic spores are called urediniospores and are formed in the uredinium. The urediniospores are blown by air currents and produce reinfection of the same species of plant.

At the end of the growing season, infections by urediniospores will result in the formation of a subcuticular or subepidermal mass of thick-walled spores called teliospores which are formed in the telium. These spores are initially dikaryotic but then become diploid and finally germinate by formation of the basidium.

Basidiocarps

basidiocarp

The basidiocarp is the fruiting body of the higher Basidiomycetes. This structure is multicellular and composed of hyphae. The basidiocarp resembles the familiar image of the mushroom. The mushroom consists of a stalk (stipe) which has a cap (pileus)on top.

The stipe can be as tall as a meter (40 inches), and the pileus as long as ameter in diameter. Alternatively, both parts could be less than a centimeter in size. The pileus has pores or gills on the underside, where the basidia are produced. The layer of basidia is called a hymenium or "fertile layer."

Other kinds of basidiocarpsmay be found in nature. Some are totally enclosed and remain on the ground, looking much like a golf ball. These are called puffballs. As the puffball matures, the other layers begin to crack at the apex.

When drops of rain fall, the force of the impact causes spores to puff out of the opening. Another kind of puffball is the earthstar. In these unique fungi, the outer layers pull away from central part of the puffball and form a starlike pattern on the ground.

Ecological Importance

The basidiosporic fungi all play important roles in ecosystems. The rusts and the smuts are impor- tant plant pathogens, capable of great destruction of crops. These fungi have been known for thou- sands of years and are some of the most devastating fungi around.

The mushrooms are part of the natural cycle of decay. They are found on the ground or on wood and are the later stages of decay of organic matter. Some mushrooms are found on living plants, where they can be serious pathogens. Others are edible and are excellent sources of digestible protein. Still others are toxic or poisonous and can be fatal when eaten.

Stinkhorns and the bird’s nest fungi are unique basidiosporic fungi. The stinkhorns are basidiocarps that form on the soil and produce the basidia in a mass of putrid cells.The stench from the cells draws flies, which walk over the spores and then disseminate them. These can be found in wooded areas and can be detected by smell at distances of up to several meters.

The bird’s nest fungi look like small birds’ nests. The outer part of the basidiocarp resembles a small nest, up to an inch in diameter. On the inside, several small puffball-like structures can be found, with basidia on the inside. These look like small eggs.

When a drop of water enters the nest, the force thrusts the “egg” upward and extends a small cord from the back. The small cord catches hold of a plant and suspends the egg in the air. As the egg dries, it turns into a powdery mass, which is blown about by the wind.

algae , cellular biology , fungi , physiology , water-related life

Bioluminesence

Hinhvv

8:17 PM

Bioluminesence

Bioluminesence is the production of light by living organisms, including algae and phytoplankton in the oceans and fungi on land.

Bioluminescence is a specific form of chemiluminescence in which the chemical energy that is produced in a chemical reaction is converted into radiant energy. In bioluminescence the reaction originates in a wide variety of living organisms, including a small number of plants. It should not be confused with fluorescence or phosphorescence, both of which do not involve a chemical reaction.

In either of the former cases the energy from a source of light, not from a chemical reaction, is basically absorbed and then re-emitted in some form of another photon. The chemical reactions that lead to bioluminescence release energy in the form of light.

Unlike the light bulb, in which electrical energy is converted into light, with some of this energy lost in the form of heat, a bioluminescent reaction is 100 percent efficient and converts all the emitted energy into light. Because there is no heat released, bioluminescence is also known as "cold light."

Species and Habitats

Bioluminescence is primarily marine in nature and is the only source of light in the deep ocean, which is the largest habitable biome of the earth. The phenomenon rarely occurs in any source of fresh water. Bioluminescent organisms include ctenophores, annelid worms, mollusks, insects, and fish.

Species and Habitats

The most common manifestation of this phenomenon on land is seen as a glowing fungus on wood or in the few families of luminous insects. This property can be used as ameans of species recognition in the darkness as well as for courtship, preying, and mating.

There are several bioluminescent fungi that are not marine in nature, occurring primarily in the tropics. These fungi appear in different colors. The most common is Panellus stiptucus,which is a small decay fungus that is mostly restricted to North America. The jack-o’-lantern mushroom (Omphalotus olearius) glows brightly, especially when fresh. A few Armillaria species are also reported to glow mildly. No luminous tree or plant is known, however.

Mechanisms of Bioluminescence

Bioluminescence occurs only when two different species are in contact and, almost exclusively, when oxygen is present. The two species are luciferin, which produces the light, and luciferase, a protein that triggers and catalyzes the reaction. The mechanism involves the loss of two electrons, also known as oxidation, by luciferin, a process achieved only through the intervention of luciferase to yield oxyluciferin.

Occasionally luciferin, luciferase, and a cofactor such as oxygen are bound together in a single moiety called photoprotein, which leads to light formation upon contact with a positively charged species, such as the calcium cation. The mechanism appears to involve a peroxide decomposition with free radical intervention.

Dinoflagellates

Dinoflagellates known as Pyrrhophyta, or fire plants, are the most common sources of bioluminescence at the surface of the ocean. They are a group of marine algae that produce light upon mechanical, chemical, or temperature changes. The phenomenon was first observed in the genus Noctiluca in the nineteenth century and has since been observed to occur within other species.

Generally, three types of stimuli can cause bioluminescence in dinoflagellates: mechanical, chemical, and temperature stimulation.

Dinoflagellates

Mechanical forms of stimulation, such as the stirring of water from a moving boat, a swimming fish, or a breaking wave, are prevalent in many Pyrrhophyta. The light appears to serve as a "burglar alarm" against grazing predators, which are then being seen through the flash by a larger second predator.

For example, as a copepod approaches the dinoflagellate, agitation of the seawater stimulates light flashes which a small fish, the secondary predator, uses to pinpoint the position of the copepod and eventually consume it. It appears that the mechanical stimulation deforms the cell membrane to create a short flash as little as one one-hundredth of a second.

Dinoflagellate luciferin is thought to derive from the similarly structured chlorophyll, which is found in most plants. The molecule is protected from luciferase at slightly basic medium by a luciferin-binding protein. However, once the acidity increases, the free luciferin reacts, and light is emitted.

The light produced by a single dinoflagellate is only six to eight photons in energy, and the flashing may last only one-tenth of a second. Larger organisms, such as jellyfish, provide flashes that may last up to tens of seconds. Temperature lowering in some dinoflagellate species also creates bioluminescence.

Purpose and Applications

The disappearance of the flash, once oxygen is consumed, has suggested that the bioluminescent reaction was originally used to remove toxic oxygen from primitive types of bacteria that developed at a time when oxygen was not available.

Bioluminescence has also played a crucial role in the direct studies of several cellular and biochemical processes, such as in the formation of ultimate carcinogens from benzoapyrene. The phenomenon has served scientists in many ways.

Calcium levels are monitored via the jellyfish biochemical system, adenosine triphosphate (ATP) measurements are achieved through the firefly, and the gene activity of organisms can be detected by splicing known bioluminescent proteins.

fungi , taxonomic groups , water-related life

Chytrids

Hinhvv

7:16 AM

Chytrids

Chytrids are fungi in the phylum Chytridiomycota. They have motile spores and are primarily aquatic organisms.

Like all fungi, chytrids live in their food and have an absorptive mode of nutrition in which they secrete digestive enzymes and absorb the breakdown products.

Chytrids also have cell walls made of chitin, make the amino acid lysine via the amino adipic acid (AAA) pathway, and possess a ribosomal DNA (deoxyribonucleic acid) sequence that places them more closely with other fungi than with any other group of organisms. The feature that sets the mapart from other fungi is the possession of a motile zoospore. All other fungi produce spores without flagella.

Characteristics

Aposteriorly oriented, whiplash-type flagellum is the feature that unites all the organisms in the division Chytridiomycota within the kingdom Fungi. As absorptive heterotrophs, they live either as saprophytes, growing on dead organic matter, or as parasites in living plants, other fungi, insects, or algae.

The vegetative organism may take the form of a spherical structure, with or without branching rhizoids, on the surface of substrate or host or may send mycelial threads through the material in which it is living.

Asexual reproduction occurs by a variety of means described below. Sexual reproduction is known to exist in several types of chytrids and in some species involves the alternation between a gamete-producing phase and a spore-producing phase.

The sporangia that produce the motile zoospores develop in a variety of ways. Two features are used to characterize development: the fate of the nucleus upon encystment of the zoospore and the number of zoosporangia produced from a single zoospore.

The three most common types of thallus development are endogenous-monocentric, exogenous-monocentric, and exogenous-polycentric. Endogenous-monocentric development occurswhen the zoospore nucleus stays within the encysted zoospore wall, undergoes mitosis, and produces a single zoosporangium.

Exogenous-monocentric development occurs when the zoospore nucleus migrates into the germ tube, undergoes mitosis, and produces a single zoosporangium. Exogenous-polycentric development occurs when the zoospore nucleus migrates into the germ tube, undergoes mitosis, and spreads to many locations for zoosporangium production.

The phylum-defining zoospore may be one of four basic morphological types. Though the types are determined by electron microscope, the morphological type can be recognized using light microscopy with experience. The four morphological types are the basis of classification at the ordinal level as described below.

Ecology and Habitats

Because chytrids are absorptive heterotrophs, they grow in their food, digesting complex food molecules and absorbing the simpler breakdown products. When growing in dead material, these fungi are saprophytes and are decomposing organisms in ecosystems. Because the zoospore requires water for dispersal, these fungi are found in aquatic environments.

However, they also can be found in soils that are wet with soil water. Chytrids also can live within living organisms as parasites, causing major declines in populations. The gut chytrids, Neocallimastigales, live in the rumina (stomachal cavities) of herbivorous mammals.

Taxonomy

Taxonomy

There are approximately eight hundred species of chytrids, arranged in five orders. Taxonomy of the different orders is based on the ultrastructure of the zoospore.

Ultrastructure features used in taxonomy include the presence or absence of a connection between the nucleus and the kinetosome by microtubules; whether ribosomes are dispersed or collected into a mass surrounded by membranes; the degree of organization of the microbody-lipid complex (MLC); the location and number of lipid globules; and presence or absence of a rumposome—a honeycomblike organelle of unknown function. The main characteristics of the five orders are described below.

Chytridiales. During examination of the main features of the zoospore—lipid globule, microbody, mitochondria, and nucleus—the nucleus seems to occupy whatever space is left over within the zoospore.

Rootlet microtubules are located within the plasma membrane connecting the kinetosome to the rumposome. Ribosomes are gathered in the center of the cell, enclosed within membranes. In the MLC, the posteriorly located lipid globules are in close association with the microbody, mitochondrian, and rumposome.

Spizellomycetales. The nucleus of the zoospore is close to the kinetosome or, if separated, is connected to it viamicrotubules or a rhizoplast. Rootlet morphology is variable, and ribosomes are scattered throughout the cytoplasm.

The MLC has a loose association of the microbody and lipid at the anterior end of the zoospore with the mitochondria located toward the rear. There is no rumposome. Ribosomes are dispersed throughout the zoospore.

Neocallimastigales. Neocallimastix and other genera of the order are uniflagellate or multiflagellate and live in the rumen of herbivorous mammals. Because they live in this unique environment, rumen chytrids are obligate anaerobes.

The zoospores lack any of the MLC organelles and the rumposome. All these anaerobic fungi are cellulolytic and digest plant cell walls of the food upon which sheep and cattle feed.

Monoblepharidales. The zoospores have a centrally located nucleus that is not connected to the kinetosome. Microtubules extend randomly into the cytoplasm from the kinetosome.

The MLC has a rumposome in close association with a microbody and anteriorly located lipid globules. The ribosomes are centrally located, surrounding the nucleus. These fungi have amycelial growth form and reproduce sexually by producing amotilemale cell and a nonmotile egg cell.

Blastocladiales. A nuclear cap consisting of ribosomes encased within a membrane located anteriorly to a cone-shaped nucleus and a single largemitochondrian with a side body complex are the two most distinctive features of these fungi. Some of these fungi produce mycelial growth forms, whereas others produce the saclike zoosporangium with rhizoids.

Evolutionary History

Evolutionary history of the chytrids can be traced back to the Pennsylvanian period through fossil evidence. Sequential analysis of the small subunit ribosomal DNA gene from fifty-four chytrids indicates that the Chytridiomycota are related to other fungi and that there are natural groups within the division: Blastocladiales, Monoblepharidales, and Neocallimastigales.

Despite the diversity of the data, the monophyletic nature of the Chytridiales and Spizellomycetales is not rejected. The DNAgroupings closely resemble groupings based on zoospore ultrastructure.

Representative Organisms

Allomyces is a mycelial member of the Blastocladiales, which is interesting because it has an alternation of generations between a gamete-producing thallus (gametothallus) and a spore-producing thallus (sporothallus).

In all organisms with alternation of generations, the gametothallus produces gametes by mitosis in gametangia. The gametes are distinguished by size, the male being smaller than the female.

The motile male gamete is chemotactically attracted to the hormone sirenin, which is produced by the female gametes and enables fertilization. Upon fertilization, the zygote nucleus undergoes mitosis as the germ tube develops into mycelia without cross walls.

The dichotomously branched mycelia of the sporothallus produce two types of sporangia. The thin-walled sporangia produce diploid spores by mitosis. These diploid zoospores are responsible for increasing numbers of Allomyces in its habitat.

The sporothallus also can produce a thick-walled sporangium capable of with standing harsh environmental conditions. Zoospores in this sporangium are produced by meiosis. When these haploid zoospores geminate, the nucleus divides by mitosis and spreads throughout the dichotomously branched mycelia. The life cycle of the fungus now is completed.

Batrachochytrium is interesting because it parasitizes frogs. Within the last decade, declines in populations of frogs around the world have been described. Batrachochytrium dendrobatidis is responsible for this chytridiomycosis in amphibians, including salamanders.

Blastocladiella is a developmental biology tool. The thallus has the exogenous, monocentric developmental pathway resulting in a rhizoidal system with a single thin-walled, colorless sporangium or a single thick-walled, resistant sporangium.

The chemical environment of the developing thallus determines which sporangium is produced. High carbon dioxide levels favor the development of the thick-walled sporangium.

This shift from a thin-walled sporangium pathway to a thick-walled sporangium pathway has been traced to a disruption of the Krebs cycle. This organism is one of a few nongreen organisms in which light promotes the growth of the organism.

Coelomomyces is a mycelial member of the Blastocladiales that parasitizes invertebrate animals. Coelomomyces alternates between a haploid gametothallus and a diploid sporothallus.

The unique feature of Coelomomyces is that each phase is specific for a different host. The diploid sporothallus parasitizes mosquitoes and grows as wall-less mycelia within the hemocoel of the mosquito larvae. Coelomomyces has been studied as a possible mycoinsecticide against mosquitoes.

Difficulty in using Coelomomyces as a mycoinsecticide occurred until the discovery of the fact that an alternate host was required to achieve completion of the life cycle. The zoospores produced by the thick-walled sporangium within the mosquito are produced by the process of meiosis and are haploid.

The haploid zoopore must infect a microcrustacean copopod or ostracod in order for the gametes to be produced. The haploid zoospore develops into the gametothallus, which produces the motile gametes. The resulting zygote will infect mosquito larvae, completing the life cycle.

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