Some zygomycetes are regularly isolated by veterinarians from domesticated animals in tropical and subtropical regions of the world, including the US gulf states.
Zygomycota, like all true fungi, produce cell walls containing chitin. They grow primarily as mycelia, or filaments of long cells called hyphae. Unlike the so-called 'higher fungi' comprising the Ascomycota and Basidiomycota which produce regularly septate mycelia, most Zygomycota form hyphae which are generally coenocytic because they lack cross walls or septa.
There are, however, several exceptions and septa may form at irregular intervals throughout the older parts of the mycelium or are regularly spaced in two sister orders of Zygomycota, the Kickxellales and Harpellales.
The unique character synapomorphy of the Zygomycota is the zygospore. Zygospores are formed within a zygosporangium after the fusion of specialized hyphae called gametangia during the sexual cycle Figure 2A. A single zygospore is formed per zygosporangium. Because of this one-to-one relationship, the terms are often used interchangably. The mature zygospore is often thick-walled Figure 2B , and undergoes an obligatory dormant period before germination.
Most Zygomycota are thought to have a zygotic or haplontic life cycle Figure 4. Thus, the only diploid phase takes place within the zygospore. Nuclei within the zygospore are believed to undergo meiosis during germination, but this has only been demonstrated genetically within the model eukaryote Phycomyces blakesleeanus Eslava et al. Figure 4. Generalized life cycle of Zygomycota.
Asexual reproduction occurs primarily by sporangiospores produced by mitosis and cell division. The only diploid 2N phase in the life cycle is the zygospore, produced through the conjugation of compatible gametangia during the sexual cycle see Figure 2A, B. Zygomycota typically undergo prolific asexual reproduction through the formation of sporangia and sporangiospores. Sporangiospores are distinguished from other types of asexual spores, such as conidia of the Ascomycota and Basidiomycota, by their development.
Walled sporangiospores are formed by the internal cleavage of the sporangial cytoplasm. At maturity, the sporangial wall typically disintegrates or dehisces Figure 3B , thereby freeing the spores that are usually dispersed by wind or water. Sporangia are formed at the ends of specialized hyphae called sporangiophores. In the model organism, Phycomyces blakesleeanus , sporangial development has been studied extensively to understand the genetic basis for various trophisms, including the strong phototrophic responses to blue light.
A unique spore dispersal strategy for the Mucorales is exhibited by the dung fungus Pilobolus , whose name literally means 'the hat thrower' see far left Title illustration. The entire black sporangium is explosively shot off of the top of the sporangiophore up to distances of several meters.
Phototrophic growth of the sporangiophore facilitates dispersal away from the dung onto a fresh blade of grass where it may be consumed by an herbivore, thereby completing the asexual cycle after the spores pass through the digestive system.
Some members of the Entomophthorales e. Interestingly, species of Basidiobolus , Conidiobolus and several other genera produce a second kind of spore on a long stalk that appears to have certain morphological adaptations for efficient insect dispersal. Figure 5. Dichotomously branching sporangiophore of Thamnidium elegans Mucorales. Barron Two variant types of sporangia include sporangiola and merosporangia. Sporangiola are simply uni-to-few spored sporangia containing between 1-to spores Figures 3A and 5.
Merosporangia are elongated sporangiola with uniseriate spores usually produced from a vesicle or stalk Figure 6. Figure 6. Scanning electron micrograph of uniseriate merosporangia produced on a vesicle hidden beneath merosporangia of Syncephalastrum racemosum Mucorales. Merosporangiferous members of the Zygomycota, however, do not form a clade, indicating that this sporangial type has evolved independently more than once within the phylum e.
A unique sporangiolum type is the trichospore, a one-spored sporangiolum, produced by members of the Harpellales Figure 7 and far right Title slide , which are endocommensals living within the gut of arthropods, including terrestrial beetles and millipedes, fiddler crabs, and the larvae of many aquatic insects Lichtwardt Trichospores possess one to several basal hair-like filaments that likely aid in the attachment of the spores to debris and plants in aquatic ecosystems before they reenter the arthropod gut.
Figure 7. Photo of thallus of Genistellospora homothallica Harpellales bearing trichospores attached to the hindgut cuticle of a Chilean blackfly. Like other Fungi, Zygomycota are heterotrophic and typically grow inside their food, dissolving the substrate with extracellular enzymes, and taking up nutrients by absorption rather than by phagocytosis, as observed in many protists.
The most common members of the Zygomycota are the fast growing members of the Mucorales. They function as decomposers in soil and dung, thereby playing a significant role in the carbon cycle.
Zygomycota also participate in a number of interesting symbioses. As mentioned above, the Harpellales inhabit arthropods particularly freshwater aquatic insect larvae; Figure 7 where they are attached to the chitinous lining of the hindgut. Harpellids presumably feed on nutrients that are not utilized by the arthropod. Because they are generally assumed to neither harm nor benefit the host animal, this association is considered commensalistic.
In contrast, the Entomophthorales include many insect pathogens that can cause huge disease outbreaks see center Title slide showing infected maggot fly.
Some of this pathogenicity is being tapped for use in the biocontrol of specific insect pests, including periodical cicadas Bidochka et al. A number of other Zygomycota are mycoparasitic, or parasites of other fungi. All members of the Dimargaritales only 15 species and many Zoopagales are typically obligate parasites of mucoralean hosts. Other mycoparasites in the Mucorales e. Figure 8. Sporangia of Spinellus fusiger Mucorales parasitic on fruitbodies of the mushroom Mycena pura. Certain species of Zoopagales parasitize non-fungal hosts, such as nematodes, rotifers, and amoebae Figure 9.
The Endogonales are a unique group in the Zygomycota because some members can form ectomycorrhizal associations with pine roots, while others appear to be saprobic.
Figure 9. The parasite Amoebophilus simplex Zoopagales and its amoeba host. Nutrient transfer occurs through a specialized hypha called a haustorium that enters the amoeba. The Zygomycota are thought to have diverged from the remaining fungi before the colonization of land by plants , million years ago Berbee and Taylor ; Heckman et al. Molecular phylogenetic studies place the Zygomycota near the base of the kingdom Fungi, diverging after the Chytridiomycota, the most basal fungal lineage James et al.
However, as presently circumscribed, it is uncertain whether the Zygomycota represent a monophyletic group. Prior to the use of molecular phylogenetics, the Zygomycota were classified into two classes, the Zygomycetes and Trichomycetes Alexopoulos et al. A morphological synapomorphy for this clade is the possession of a uniperforate septum with a lenticular cavity Figure 10; Benny et al. A large-scale phylogeny of the Mucorales, using three genes and at least one member of each recognized genus, suggests that several of the largest families and the two largest genera Mucor and Absidia are polyphyletic O'Donnell et al.
Figure Transmission electron micrograph of vegetative hypha of Kickxella alabastrina Kickxellales. White line separating the upper from the lower cell is a section of the cross wall or septum. Note the lens shaped plug that lies within the septal pore lenticular cavity. From Tanabe et al. The Entomophthorales appears to be one of the most distinctive and problematical lineages of Zygomycota for two reasons: 1 SSU rDNA analyses suggest that it may be more closely related to the Blastocladiales Chytridiomycota James et al.
Phylogenetic placement of one of the most problematic species, Basidiobolus ranarum , is uncertain Jensen et al. However, this species appears to be distinct from the Entomophthorales with which it has been classified traditionally.
Although B. Basidiobolus spp. Though controversial, congruent evidence from alpha- and beta-tubulin gene phylogenies support a zygomycete origin of the microsporidia, a group of highly reduced obligate intracellular parasites of a wide variety of animals including humans Keeling et al. Because several microsporidian species have emerged as major pathogens of immuno-compromised patients over the past two decades, this enigmatic group has received considerable attention recently by the scientific community.
Placement of the microsporidia, however, remains controversial. The common names 'pin' or 'sugar' molds are not formal taxonomic names for this group of fungi but refer to their morphological appearance or to one of the most common substrates upon which some members of the Mucorales Zygomycota grow.
Many members of the Mucorales produce unbranched sporangiophores with a sporangium for a 'head,' a structure that superficially resembles a pin, hence the common name 'pin' molds. Many species commonly cause economically destructive rots of fruits in storage. These fruits, including strawberries and nectarines, are high in simple sugars such as glucose, thereby explaining the origin of this common name.
The vast majority of Zygomycota, however, are never encountered by humans and therefore do not have common names. Alexopoulos, C. Mims and M. Introductory mycology. John Wiley and Sons, New York.
Benny, G. Humber and J. Zygomycota: Zygomycetes. Systematics and Evolution. Part A. McLaughlin, D. Springer-Verlag, New York. Benny G. Amoebidium parasiticum is a protozoan, not a Trichomycete. Mycologia Berbee, M.
Fungal molecular evolution: gene trees and geologic time. Part B. Dating the evolutionary radiations of the true fungi. Bidochka, M. Walsh, M. Ramos, R. Leger, J. Silver and D. Fate of biological control introductions: monitoring an Australian fungal pathogen of grasshoppers in North America.
USA Blackwell, M. Similarity of Amphoromorpha and secondary capilliconidia of Basidiobolus. Bruns, T. Vilgalys, S. Barns, D. Gonzalez, D. Hibbett, D. Lane, L. Simon, S. Stickel, T. Szaro, W. Weisburg and M. Evolutionary relationships within the fungi: analyses of nuclear small subunit RNA sequences.
Cavalier-Smith, T. A revised six-kingdom system of life. Cole, G. Patterns of development in conidial fungi. Pitman, London. Guarro, J. Gene and M. Atlas of clinical fungi, second addition. Eslava, A. Alvarez, and M. Meiosis in Phycomyces. Hajek, A. Pathology and epizootiology of Entomophaga maimaga infections in forest Lepidoptera. Heckman, D. Geiser, B. Eidell, R. Stauffer, N. Kardos and S. Molecular evidence for the early colonization of land by fungi and plants.
Science Hesseltine, C. Zygomycetes in food fermentations. The Mycologist 5: Binder, J. Bischoff, M. The terms ' anamorph ' and ' teleomorph ' are used to convey the asexual and sexual reproduction morphological types, respectively, in a particular fungus. The concept of anamorph and teleomorph is a confusing one for many students, as we are not accustomed to thinking about organisms with such reproductive flexibility. For a more thorough discussion of anamorph and teleomorph, refer to Alexopoulos et al.
Examples of meiospores—spores that are the products of meiosis—include ascospores see Ascomycota and basidiospores see Basidiomycota. Ascospores are formed inside a sac-like structure called an ascus Fig. An ascus starts out as a sac of cytoplasm and nuclei, and by a process called "free cell formation" Kirk et al. Ascospores vary in size, shape, color, septation, and ornamentation among taxa.
Basidiospores are formed on a basidium Fig. Basidiospores vary in size, color and ornamentation depending upon the taxonomic group. More information on dispersal of ascospores and basidiospores can be found below. Examples of mitospores are conidia sing.
Another type of asexual propagule produced by fungi in several different phyla is the chlamydospore. Conidia are formed from a modified hypha or a differentiated conidiogenous cell of the fungus. Conidiogenous cells can be formed singly on hyphae, on the surface of aggregated hyphal structures, or within different types of fruiting bodies. Fruiting bodies inside which conidia are formed are pycnidia and acervuli. Sporodochia and synnemata are examples of fruiting bodies on which conidia are formed.
Conidia are produced primarily by Ascomycota, although some Basidiomycota are capable of producing them as well. Sporangiospores are asexual propagules formed inside a globose or cylindrical sporangium by a process involving cleavage of the cytoplasm. Sporangiospores are thin-walled, one-celled, hyaline or pale-colored, and are usually globose or ellipsoid in shape. One to 50, sporangiospores may be formed in a single sporangium.
When mature, sporangiospores are released by breakdown of the sporangial wall, or the entire sporangium may be dispersed as a unit. Sporangiospores are produced by fungi in phyla Chytridiomycota and Zygomycota, as well fungal-like Oomycetes see section "Fungal-like Organisms Studied by Plant Pathologists and Mycologists". A zoospore is a microscopic, motile propagule, approx. Zoospores are produced by one group of true Fungi Chytridiomycota , and by fungal-like organisms in Kingdom Straminipila and some slime molds see section "Fungal-like Organisms Studied by Plant Pathologists and Mycologists".
Two types of flagella are known—the whiplash flagellum, which is directed backward, and the tinsel flagellum, which is directed forward. The tinsel flagellum is only present in members of Kingdom Straminipila and does not occur in true fungi. The length of time zoospores are able to swim is determined by their endogenous energy reserves—zoospores cannot obtain food from external sources—and environmental conditions.
Zoospores may exhibit chemotaxis—movement in response to a chemical gradient, e. At the end of its motile phase, the zoospore undergoes a process called encystment in which it either sheds or retracts the flagella and produces a cell wall.
The encysted zoospore, called a cyst, may germinate directly by the formation of a germ tube , or indirectly by the emergence of another zoospore. Zoospores are formed inside a sac-like structure called a zoosporangium by a process involving mitosis and cytoplasmic cleavage—similar to the formation of sporangiospores in sporangia.
Depending upon the taxonomic group, zoospores emerge from the zoosporangium through breakdown of the zoosporangial wall, through a preformed opening in the wall covered with a cap called an operculum that flips back, or by a gelatinous plug that dissolves.
Chamydospores are survival propagules formed from an existing hyphal cell or a conidium that develops a thickened wall and cytoplasm packed with lipid reserves. The thickened cell walls may be pigmented or hyaline, and chlamydospores develop singly or in clusters, depending upon the fungus. Chlamydospores are passively dispersed, in most instances when the mycelium breaks down. Chlamydospores are formed by many different groups of fungi and are often found in aging cultures.
Sclerotia sing. Sclerotia contain food reserves, and are a type of survival propagule produced by a number of fungi in phyla Ascomycota and Basidiomycota; in some fungi, such as Rhizoctonia solani , they are the only type of propagule produced, whereas in fungi such as Claviceps purpurea and Sclerotinia sclerotiorum , they are overwintering structures that can germinate directly, or give rise to structures in which the meiospores are formed.
The characteristics and diversity of the major phyla of true Fungi will be briefly described. Selected representatives of the different phyla are introduced and, in many instances, illustrated. A generalized life cycle also is presented for each phylum that illustrates when plasmogamy cell fusion , karyogamy nuclear fusion and meiosis occur relative to each other, and the types of structures involved in these events. For more detailed information on members of Kingdom Fungi, recommended reading is provided at the end of this article.
Phylum Ascomycota is the largest group of fungi, with approximately 33, described species in three subphyla—Taphrinomycotina, Saccharomycotina, and Pezizomycotina. Members of this phylum reproduce sexually or meiotically Fig. Many species of Ascomycota also or exclusively produce spores through an asexual or mitotic process; these spores, called conidia , exhibit a wide range of size, shape, color and septation among the different fungi in which they are formed. Conidia and ascospores are usually produced at different times of year, if ascospores are formed in the lifecycle.
The existence of many Ascomycota having sexual and asexual states that are separated in time and space has long confused those new to mycology and plant pathology. The asexual states of Ascomycota are especially important to the plant pathologist because they are more commonly encountered than the sexual state, and must be identified for control, quarantine, or other purposes.
Fungi that reproduce only via asexual means have been given various designations including deuteromycetes, fungi imperfecti, mitosporic fungi, conidial fungi, and anamorphic fungi. Subphylum Taphrinomycotina includes fungi that, with one known exception, do not form fruiting bodies—as examples, the fission yeast Schizosaccharomyces Fig. Subphylum Saccharomycotina contains approximately species of yeasts, most of which live as saprotrophs in association with plants and animals, but also including a small number of plant and animal pathogens Suh et al.
Asci are formed naked Fig. Yeasts traditionally have been important in the production of beer, wine, single cell protein and baker's yeast, but their role in industry has expanded to the production of citric acid, fuel alcohol, and riboflavin Kurtzman and Sugiyama Saccharomyces cerevisiae Fig.
In , S. Subphylum Pezizomycotina is the largest group in the phylum, with more than 32, identified species that occupy a wide range of ecological niches, occurring as saprotrophs, parasites and mutualists with plants, animals and other fungi.
Three different types of asci occur in this subphylum, prototunicate, unitunicate and bitunicate. Prototunicate asci release ascospores by breakdown of the ascus wall, whereas in the unitunicate and bitunicate asci, the ascospores are forcibly discharged. Bitunicate asci have an inner wall that balloons out from the outer wall prior to ascospore discharge, and in unitunicate asci the wall layers do not separate from each other. A wide range of fruiting bodies are formed by members of subphylum Pezizomycotina, including cleistothecia , chasmothecia , apothecia , perithecia and pseudothecia.
Stromata , hardened masses of hyphae on or in which perithecia or pseudothecia are formed, occur in some members of this subphylum. Cleistothecia sing. Common fungi that produce cleistothecia include the teleomorphic sexual states of Aspergillus and Penicillium Fig.
Species of Aspergillus are important in the production of fermented foods and beverages, including soy sauce, miso and rice wine sake. Some species of Aspergillus infect animals, causing a disease known as aspergillosis, and others produce mycotoxins. Aflatoxin is a potent carcinogen produced by A. The U. Food and Drug Administration established a strict limit of 20 parts per billion on aflatoxin levels in food, and the U. Penicillium species are also used in food production.
For example, the blue veins in Roquefort and Gorgonzola cheeses are due to the growth and sporulation of particular species of Penicillium Fig. The antibiotic penicillin, the "wonder drug" of the 20 th century, is produced by strains of P. Other species of Penicillium, such as P. Chasmothecia sing. The term is now used to refer to the fruiting bodies of the powdery mildew fungi Fig. Apothecia sing. Apothecia-forming fungi are also called "cup fungi" or discomycetes.
Some important groups of plant pathogens that form apothecia include species of Monilinia brown rot of peach; Figs. Perithecia sing. Most fungi producing perithecia also have unitunicate asci and are classified in Sordariomycetes , one of the largest classes of Ascomycota with more than 3, described species Zhang et al.
These fungi have also been called pyrenomycetes. Members of this group are common in nearly all ecosystems, where they occur as saprotrophs, endophytes of plants, or pathogens of plants, animals and other fungi. A large number of economically important plant pathogens belong to Sordariomycetes, including those that cause anthracnose diseases Glomerella cingulata , blasts Magnaporthe oryzae , rice blast pathogen , blights Cryphonectria parasitica , chestnut blight , ergot Claviceps purpurea , and Fusarium head blight scab of small grains Gibberella zeae.
Pseudothecia sing. Asci form in locules openings inside vegetative fungal tissue called ascostroma ; this group has been called loculoascomycetes, but is now placed in class Dothideomycetes. Other characteristics of Dothideomycetes include the formation of bitunicate asci, and many members of this group produce darkly pigmented, multiseptate asospores or conidia. Similar to the Sordariomycetes, members of Dothideomycetes occur in a wide range of habitats as saprotrophs and associate with plants as pathogens, endophytes and growing on the surface of plants as epiphytes Schoch et al.
Examples of well-known plant pathogens belonging to this group include Venturia inaequalis apple scab; Fig. Most of the lichen-forming members of Ascomycota belong in class Lecanoromycetes. This is the largest class of fungi, with over 13, described species Miadlikowska et al. Most of the members of this class produce apothecial fruiting bodies Figs. The lichen thallus produces a wide range of secondary metabolites that are of biological and ecological importance Miadlikowska et al.
The lichen thallus is able to grow under a range of adverse conditions and some can survive for hundreds of years. Lichens are found in a wide range of habitats from the Arctic to Antarctic, including some species that can grow in aquatic and marine environments Webster and Weber Phylum Basidiomycota represents the second largest phylum of fungi, with nearly 30, described species.
Members of phylum Basidiomycota produce basidiospores on a typically club-shaped structure called a basidium Fig. Characteristic of the mycelium of many members of Basidiomycota is the presence of clamp connections Figs. Three main lineages are recognized in phylum Basidiomycota: subphyla Ustilaginomycotina, Pucciniomycotina, and Agaricomycotina Blackwell et al. Ustilaginomycotina and Pucciniomycotina are composed mostly of plant parasitic species, known as smut and rust fungi, respectively, characterized by a state that produces thick-walled teliospores Figs.
The most extensively studied members of Ustilaginomycotina are species of Tilletia and Ustilago. Ustilago maydis , which causes corn smut Fig. These structures eventually become filled with dark teliospores and are considered a delicacy in Mexico called "cuitlacoche. Subphylum Pucciniomycotina include the group of plant parasites called rust fungi. The rust fungi are remarkable in having as many as five distinct types of spores in a single life cycle spermatia, aeciospores, urediniospores, teliospores, and basidiospores Fig.
Rust fungi that produce all five spore states are macrocyclic , those that do not form uredinospores are demicyclic , and those that do not form urediniospores and aeciospores are microcyclic. Rust fungi may complete the life cycle on one host autoecious rusts or require two unrelated alternate hosts for completion of the life cycle heteroecious rusts.
The most widely cited example of a macrocyclic, heteroecious rust is Puccinia graminis black stem rust , which forms two spore states uredinia and telia on cultivated wheat Fig. The fifth state, the probasidium producing basidiospores, is formed upon germination of the teliospores Fig. Subphylum Agaricomycotina, previously known as the Hymenomycetes, includes the morphologically diverse group of fungi that produce basidia in various types of fruiting bodies Fig.
This group includes the fungi commonly known as mushrooms Fig. Many species are saprotrophic, utilizing dead plant material including woody substrates. Some of these saprotrophic species are cultivated for food, for example, the common button mushroom Agaricus bisporus , oyster mushrooms Pleurotus ostreatus , and shiitake Lentinula edodes.
Other members of this group are important ectomycorrhizal fungi, forming mutualistic associations with the roots of a wide range of trees. Some fruiting bodies produced by ectomycorrhizae are considered choice edibles, for example, chanterelles Cantharellus cibarius and other species , porcini Boletus edulis , and the American matsutake Tricholoma magnivelare Fig. A few members of this group are economically important plant parasites, e. This is an ancient group of fungi, recognizable in the fossil record dating back at least million years.
Some AM fungi also produce storage structures inside plant roots called vesicles. Endomycorrhizal fungi produce an extensive network of hyphae outside the roots extraradical hyphae. The extraradical hyphae act like an extension of the plant roots, increasing the plant's access to water and soil minerals, particularly phosphorous and nitrogen.
The fungus is also able to access phosphate not otherwise available to plants, for example from organic matter by production of acid phosphatases.
Spores may be formed singly or in clusters, and the mycelium of AM fungi is coenocytic. Sexual reproduction is not known to occur in this phylum. The feature that is shared by all members of this phylum is the formation of zoospores with one posteriorly directed, whiplash flagellum. A few chytrids are economically important plant pathogens, e. As previously noted, the frog chytrid, Batrachochytrium dendrobatidis , has been implicated as a major factor in population declines of frogs and other amphibians around the world Berger et al.
This phylum contains approximately identified species divided amongst two ecologically distinct classes, Zygomycetes and Trichomycetes White et al. The most commonly encountered Zygomycetes are members of orders Mortierellales and Mucorales. Many members of these two orders are saprotrophs with rapidly growing, coenocytic mycelium.
The sexual reproductive state is the zygospore Fig. Members of order Mucorales, commonly called mucoraceous fungi, are common in soil, dung, plant material, and other types of organic matter. Some mucoraceous fungi are plant or animal pathogens, and others are used in the production of Asian foods such as tempeh. Species of Mucor and Rhizopu s Fig. Species of Pilobolus Figs.
Other Zygomycetes are associated with animals. For example, some species of Rhizopus and Mucor cause zygomycosis in immunocompromised humans. Entomophthorales, as the name suggests, include parasites of insects and other animals. Members of class Trichomycetes live in the guts of insects, millipedes, and crustaceans, but cause little or no harm to their hosts.
Oomycetes are fungal-like organisms that form zoospores with two flagella—a whiplash flagellum that is directed backwards and propels the zoospore, and a tinsel flagellum adorned with hairs that is directed forward, pulling the zoospore. The cell walls of Oomycetes contain cellulose, rather than chitin, and glucans. Kirk et al. Another characteristic of Oomycetes is the formation of an oospore , a thick-walled, resistant propagule which is the result of sexual reproduction.
Oomycetes belong in Kingdom Straminipila, also known as Chromista. In addition to Oomycetes, this kingdom includes diatoms, golden and brown algae, a type of algae called cryptomonads, and two other groups of organisms studied by mycologists in phyla Labyrinthulomycota and Hyphochytriomycota.
The tinsel type flagellum is a characteristic of all members of Kingdom Straminipila, hence it is also called a straminipilous flagellum. Oomycetes include some of our most devastating plant pathogens. These fungal-like organisms have changed the course of history. Consider 19 th century Ireland; life was hard for the millions of Irish in the s who relied almost entirely upon the "lumper" potatoes they grew on leased quarter-acre plots for food and rent.
It's said that the stomachs of these "cottiers" were distended from eating up to fourteen pounds of potatoes each day Large Then, in , the potatoes began to rot from a malady known as "Potato Murrain," what we now call late blight of potato.
Without potatoes, 4. Over the next 15 years, one million Irish died from the famine, and one and a half times that number fled Ireland. Late blight is caused by Phytophthora infestans , and this oomycete continues to be a major pathogen in potato production, although we now have the ability to control it through the application of fungicides. A number of other important plant pathogens are found among the Oomycetes, but only a few will be mentioned here.
Phytophthora ramorum causes sudden oak death and ramorum blight. Pythium species cause damping-off diseases under extremely wet conditions.
Seedlings are particularly vulnerable to attack by damping-off pathogens because their tissues are soft and easily invaded. Seedlings may be killed before or after they emerge from the soil.
Downy mildews Peronosporales are biotrophic Oomycetes that are characterized by the formation of white, downy sporangiophores on the surface of infected hosts. The white rusts Albugo spp. Albugo species parasitize crucifers and produce blister-like pustules filled with sporangia, which will germinate to produce motile zoospores.
White rusts can also cause infected stems to grow in a contorted or twisted manner. Members of Saprolegniales are called water molds.
Many of these produce fast-growing, robust hyphae on organic matter in aquatic environments, but some species of Saprolegnia are parasitic on fish and fish eggs. Labyrinthulids phylum Labyrinthulomycota , a small group of Straminipila, include organisms that cause rapid blight of turf grass and the wasting disease of eelgrass.
Labyrinthulids have a unique manner of movement—their microscopic, football-shaped cells produce an ectoplasmic net through which the cells glide. The slow, gliding movement of the cells within the ectoplasmic net can be observed under the microscope. Hyphochytrids phylum Hyphochytridomycota , Kingdom Straminipila are similar to chytrids in appearance, as their name suggests, and produce zoospores with a single anterior tinsel flagellum.
Hyphochytrids are one of the smallest groups of fungal-like organisms, both in size and in number of species with only 23 known.
Some hyphochytrids are known to parasitize algae, spores of AM fungi, and oospores of Oomycetes. Slime molds are organisms that have a trophic feeding stage in their life cycle that lacks a cell wall, either uninucleate amoeba or multinucleate plasmodium.
The lack of a cell wall facilitates engulfment of food, in contrast to true fungi that must absorb their nutrients through a cell wall. The slime molds are now included in the Amoebozoa Adl et al. Four groups of slime molds are recognized—plasmodial slime molds Myxomycota , cellular slime molds Dictyosteliomycota and Acrasiomycota and endoparasitic slime molds Plasmodiophoromycota.
We will briefly cover plasmodial slime molds and endoparasitic slime molds. For information on cellular slime molds, refer to one of the introductory mycology texts listed below Recommended Further Reading. The plasmodial slime molds are most commonly found in temperate forests, where they occur on plant litter, tree bark, and other types of plant material. The most conspicuous stage of the plasmodial slime mold is the fruiting structures, called sporophores Alexopoulos et al.
One of the most common slime molds in temperate regions is Fuligo septica. The sporophores of this slime mold are often found in ornamental bark and mulch, and look more like an animal's vomit than the fruiting structure of a living organism, thus earning the nickname: "dog vomit slime" Fig. The presence of slime molds in landscaping Fig. Members of phylum Plasmodiophoromycota are biotrophic parasites that produce their plasmodial stage inside the cells of plants, algae, diatoms, and Oomycetes.
Several members of this phylum are economically important plant parasites, including Plasmodiophora brassicae , which causes clubroot of crucifers Fig. Polymyxa graminis is a vector for soilborne wheat mosaic virus, an economically important disease of wheat. Members of this phylum produce cysts inside host cells; the cysts are released when the plant tissue breaks down, and germinate to release a zoospore that infects the host by injecting its cytoplasm into a host cell. Infected host tissue may become greatly swollen as in clubroot of crucifers.
As we have seen, a fungus is a eukaryotic organism that absorbs nutrients through its cell walls and generally reproduces by sporesTrue fungi belong to Kingdom Fungi, and other fungal-like organisms are placed in phyla outside the Kingdom Fungi.
Most fungi consist of a hyphal thallus that allows these organisms to colonize and exploit many different substrates and fill various ecological niches, as parasites, pathogens, mutualists, saprotrophs and decomposers. Fungi and fungal-like organisms survive and reproduce via a huge diversity of spore types, characteristic of each taxonomic group. This introduction has provided some basic information on reproduction, nutrient acquisition, and roles in the ecosystem, but much more information is available see Recommended Further Reading and the References.
Fungi are fascinating in and of themselves, but they are also critically important to humans in both detrimental and beneficial ways. For more information on fungi, popularized accounts by Hudler , Money , , and Moore are good sources of information written in an engaging manner.
Introductory mycology books by Alexopoulos et al. Tales of fungi, folklore, and human affairs can be found in Dugan and Findlay , and an engaging book on the impact of fungal plant pathogens by Money Gordon Wasson's book "Soma" is one of the first ethnomycological treatments of hallucinogenic mushrooms, and a recent book on the history of magic mushrooms by Letcher relays the story of their use from ancient Aztecs to contemporary society.
We thank a number of colleagues who have kindly provided us with images. PPNS No. This paper is Contribution No. Adl, S. Simpson, M. Farmer, R. Andersen, O. Anderson, J. Barta, J. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists.
Journal of Eukaryotic Microbiology, 52 5 , Alexopoulos, C. Mims, and M. Introductory Mycology. Fourth Edition. Berger, L. Speare, P. Daszak, D. Green, A. Cunningham, C. Goggin, R. Slocombe, M. Ragan, A. Hyatt, K. McDonald, H. Hines, K. Lips, G. Marantelli, and H. Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase.
The haploid phase is the predominant phase of the life cycle. Figure 5. The bright field light micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var.
Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores. Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains Figure 4. At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion. In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy.
During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus gives rise to haploid nuclei by meiosis. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia Figure 5.
Figure 6. The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia singular, basidium , which are the swollen terminal cell of a hypha.
The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn. The best-known fairy ring fungus has the scientific name Marasmius oreades. The body of this fungus, its mycelium, is underground and grows outward in a circle.
This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens, and toadstools. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.
The lifecycle of basidiomycetes includes alternation of generations Figure 7. Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse karyogamy , giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae.
The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and and it is the dominant stage.
Eventually, the secondary mycelium generates a basidiocarp , which is a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap. Figure 7. The lifecycle of a basidiomycete alternates generation with a prolonged stage in which two nuclei dikaryon are present in the hyphae.
The Glomeromycota is a newly established phylum which comprises about species that all live in close association with the roots of trees. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that all members of this family form arbuscular mycorrhizae : the hyphae interact with the root cells forming a mutually beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals from the soil to the plant.
The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots. Although they have coenocytic hyphae like the zygomycetes, they do not form zygospores. DNA analysis shows that all glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage. Imperfect fungi—those that do not display a sexual phase—are classified in the form phylum Deuteromycota.
Deuteromycota is a polyphyletic group where many species are more closely related to organisms in other phyla than to each other; hence it cannot be called a true phylum and must, instead, be given the name form phylum.
Since they do not possess the sexual structures that are used to classify other fungi, they are less well described in comparison to other divisions. Most members live on land, with a few aquatic exceptions.
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