Ug 0 Home Read Signin ‘Usvensr o Moyes Search in book... Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices. © 83. Classifications of Fungi By the end of this section, you will be able to do the following * Identify fungi and place them into the five major phyla ac- cording to current classification * Describe each phylum in terms of major representative species and patterns of reproduction The kingdom Fungi contains five m: hyla that were established accord- ing to their mode of sexual reproduced or using molecular data. Previous: Characteristics of Fungi Next: Ecology of Fungi“form phylum," because superficially they appeared to be similar. However, most mycologists have discontinued this practice. Rapid advances in molec ular biology and the sequencing of 18S rRNA (ribosomal RNA) continue to show new and different relationships among the various categories of fungi The five true phyla of fungi are the Chytridiomycota (Chytrids), the Zygomycota (conjugated fungi), the Ascomycota (sac fungi), the Basidiomycota (club fungi) and the recently described Phylum Glomeromycota (Figure 1). Shr Ue. BasdoryeoiaAscomyecta Glomeremjeota ZyoomyectaChytidamyeota Animate Figure 1: Fungal phyla. Note: “mycota” is used to designate a phylum while "-mycetes” formally denotes a class or is used informally to refer to all members of the phylum. Chytridiomycota: The Chytrids The only class in the Phylum Chytridiomycota is the Chytridiomycetes. The chytrids are the simplest and most primitive Eumycota, or true fungi. The evolutionary record shows that the first recognizable chytrids appeared dur ing the late pre-Cambrian period, more than S00 million years ago. Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular; how- ever, a few form multicellular organisms and hyphae, which have no septa hatwiaan calle lenannniticl The Chitride ara tha any funni that have ra. Previous: Characteristics of Fungi Next: Ecology of Fungiswim with the help of a single flagellum. An unusual feature of the chytrids is that both male and female gametes are flagellated. The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects, or amphibians (Figure 2), while others are saprobes. The chytrid species Allomyces is well characterized as an experimental organism. Its reproduc- tive cycle includes both asexual and sexual phases. A/liomyces produces diploid or haploid flagellated zoospores in a sporangium. Figure 2: Chytrids. The chytrid Batrachochytrium dendrobatidis is seen in these light micrographs as transparent spheres growing on (a) a freshwater arthropod (water mite) and (b) algae. This chytrid causes skin diseases in many species of amphibians, resulting in species decline and extinction. (credit: modification of work by Johnson ML, Speare R,, cc) Previous: Characteristics of Fungi Next: Ecology of FungiZygomycota: The Conjugated Fungi The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota. They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. Most species are saprobes, living off decaying organic material: < few are parasites, particularly of insects. Zygomycetes play a considerable commercial role. For example, the metabolic products of some species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones. Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage. The fungi usually re- produce asexually by producing sporangiospores (Figure 3). The black tips o bread mold are the swollen sporangia packed with black spores (Figure 4) When spores land on a suitable substrate, they germinate and produce a new mycelium. Sexual reproduction starts when environmental conditions become unfavorable. Two opposing mating strains (type + and type -) must be in close proximity for gametangia from the hyphae to be produced and fuse, leading to karyogamy. Each zygospore can contain several diploid nu- clei. The developing diploid zygospores have thick coats that protect them from desiccation and other hazards. They may remain dormant until envi- ronmental conditions are favorable. When the zygospore germinates, it un- dergoes meiosis and produces haploid spores, which will, in turn, grow into. anew organism. This form of sexual reproduction in fungi is called conjuga- tion (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi". Previous: Characteristics of Fungi Next: Ecology of FungiZygomycete Life Cycle _Myeelia Asexual Germination |{ reproduction} } mitosis ‘Germination: Nyce frm Ifthe to mating ‘ypes (+ and) are in close e9ge Proxim, extensions calee S80 Sametanga frm Germination Seoveen nem ee + Mat cametangia ge - Haplad spores are joms ack fa, formed inaice. Goave Cy ~~ fat ay” kayoaim Cay ‘The rue tise to Zygote forma zygote with a) ‘multiple aiploid nucle Figure 3: Zygomycete life cycle, Zygomycetes have asexual and sexual phases in their life cycles. In the asexual phase, spores are produced from haploid sporangia by mitosis (not shown). In the sexual phase, plus and minus haploid mating types conjugate to form @ heterokaryotic zygosporangium. Karyogamy then produces a diploid zygote. Diploid cells in the zygote undergo meiosis and germinate to form a haploid sporangium, which releases the next generation of haploid spores. Previous: Characteristics of Fungi Next: Ecology of Fungi% The photo shows a thick layer of green mold growing on bread. Fuzzy white projections grow from the mold. Figure 4: Rhizopus spores. Asexual sporangia grow at the end of stalks, which appear as (a) white fuzz seen on this bread mold, Rhizopus stolonifer. The black tips (b) of bread mold are the spore-containing sporangia. (credit b: modification of work by “polandeze’/Flickr) Ascomycota: The Sac Fungi The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Filamentous ascomycetes produce hy- phae divided by perforated septa, allowing streaming of cytoplasm from ‘one cell to another. Conidia and asci, which are used respectively for asexual and sexual reproduction, are usually separated from the vegetative hyphae by blocked (non-perforated) septa. Many ascomycetes are of commercial importance. Some play a beneficial role for humanity, such as the yeasts used in baking, brewing, and wine fermentation, and directly as food delica- cies such as truffles and morels. Aspergillus oryzae is used in the fermenta- tion of rice to produce sake. Other ascomycetes parasitize plants and ani- mals, including humans. For example, fungal pneumonia poses a significant Previous: Characteristics of Fungi Next: Ecology of FungiAscomycetes not only infest and destroy crops directly; they also produce poisonous secondary metabolites that make crops unfit for consumption Asexual reproduction is frequent and involves the production of conidio- phores that release haploid conidiospores (Figure 5). Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains (Figure 5). The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antherid- ium and the ascogonium combine in plasmogamy, without nuclear fusion. Special dikaryotic ascogenous (ascus-producing) hyphae arise from this dikaryon, in which each cell has pairs of nuclei: one from the “male” strain and one from the “female” strain. In each ascus, two haploid nuclei fuse in karyogamy. Thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus in each ascus gives rise to haploid nuclei by meiosis, and spore walls form around each nucleus. The spores in each ascus contain the meiotic products of a single diploid nucleus. The ascospores are then re- leased, germinate, and form hyphae that are disseminated in the environ- ment and start new mycelia (Figure 6). VISUAL CONNECTION Previous: Characteristics of Fungi Next: Ecology of Fungi[2 Ascomycetes have both sexual and asexual life cycles. In the asexual life cycle, the haploid (In) mycelium branches into a chain of cells called the conidiophore. Spores bud from the end of the conidiophore and germinate to form more mycelia. In the sexual life cycle, a round structure called an antheridium buds from the male strain, and a similar structure called the ascogonium buds from the fe- male strain. In a process called plasmogamy, the ascogo- nium and antheridium fuse to form a cell with multiple haploid nuclei. Mitosis and cell division result in the growth of many hyphae, which form a fruiting body called the ascocarp. The hyphae are dikaryotic, meaning they have two haploid nuclei. Asci form at the tips of these hy- phae. Ina process called karyogamy, the nuclei in the asci fuse to form a diploid (2n) zygote. The zygote undergoes meiosis without cell division, resulting in an ascus with four In nuclei arranged in a row. Each nucleus undergoes mitosis, resulting in eight ascospores, which are also ar- ranged in a rowat the tip of the hyphae. Dispersal and germination results in the growth of new mycelia Previous: Characteristics of Fungi Next: Ecology of FungiFigure 5: Ascomycete life cycle. The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase. In each ascus, the four nuclei produced by meiosis divide once mitotically for a total of eight haploid ascospores. The haploid phase is the predominant phase of the life cycle in Ascomycetes. Which of the following statements is true? Previous: Characteristics of Fungi Next: Ecology of Fungib. A diploid ascus that forms in the ascocarp undergoes karyo- gamy, meiosis, and mitosis to form eight ascospores. c. Ahaploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores. d. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores. Answer: Statement “a" is true. Sum ¥ Figure 6: Ascospores. The bright field light micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var. flavus. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell) Previous: Characteristics of Fungi Next: Ecology of FungiBasidiomycota: The Club Fungi The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, ba- sidium), which are the swollen terminal cells of hyphae. 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 (Figure 7). These mushroom-producing basidiomycetes are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The gills are ac- tually compacted hyphae on which the basidia are borne. This group also includes shelf fungi, which cling to the bark of trees like small shelves. In ad- dition, the basidiomycota include smuts and rusts, which are important plant pathogens. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycota are inedible and produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness. The in farnous death cap mushroom (Amanita phalloides) is related to the fly agaric seen at the beginning of the previous section. Previous: Characteristics of Fungi Next: Ecology of FungiFigure 7: Fairy ring. The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring." 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. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen. (Credit: “Cropcircles'/Wikipedia Commons] The lifecycle of basidiomycetes includes alternation of generations (Figure 8). Most fungi are haploid through most of their life cycles, but the basid- iomycetes produce both haploid and dikaryotic mycelia, with the dikaryotic phase being dominant. (Note: The dikaryotic phase is technically not diploid since the nuclei remain unfused until shortly before spore production.) In the basidiomycetes, sexual spores are more common than asexual spores. The sexual spores form in the club-shaped basidium and are called basid- iospores. In the basidium, nuclei of two different mating strains fuse (karyo- gamy), giving rise to a diploid zygote that then undergoes meiosis. The hap- loid nuclei migrate into four different chambers appended to the basidium, and then become basidiospores. Each basidiospore germinates and generates monokaryotic haploid hy- Previous: Characteristics of Fungi Next: Ecology of Fungicontains haploid nuclei of two different mating strains. This is the dominant dikaryotic stage of the basidiomycete life cycle. Thus, each cell in this mycelium has two haploid nuclei, which will not fuse until formation of the basidium. Eventually, the secondary mycelium generates a basidiocarp, 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. VISUAL CONNECTION Previous: Characteristics of Fungi Next: Ecology of FungiBasidiomycete Life Cycle Basidios| Mycelia form. There are two mating types (+ and -) + Mating type Tei Ce Q 2? Plasmogamy: Fusion between + and — mating types results in formation of a dikaryotic mycelium. Sexual Reproduction Basidiocarp © basidium, Mitosis: Under the right Craracte environmental (2n) conditions, a basidiocarp forms. Gills of the basidiocarp contain cells called basidia, Karyogamy: Basidia form Basidia diploid nuclei pores (n) Dispersal and germination | QP Meiosis: Four haploid nuclei are formed in the Cell division: Four basidiospores are formed. Basidium with four nuclei (1) Figure 8: Basidiomycete life cycle. The lifecycle of a basidiomycete has alternate generations with haploid and dikaryo primary mycelia fuse to form a dikaryotic secon tic mycelia. Haploid dary mycelium, which is the dominant stage of the life cycle, and produces the basidiocarp. Which of the following statements is true? a. A basidium is the fruiting body of a m fungus, and it forms four basidiocarps. ushroom-producing b. The result of the plasmogamy step is four basidiospores. Previous: Characteristics of Fungi Next: Ecology of Fungid. A basidiocarp is the fruiting body of a mushroom-producing fungus. Answer: Statement "d" is true. Asexual Ascomycota and Basidiomycota Imperfect fungi—those that do not display a sexual phase—were formerly classified in the form phylum Deuteromycota, an invalid taxon no longer used in the present, ever-developing classification of organisms. While Deuteromycota was once a classification taxon, recent molecular analysis has shown that some of the members classified in this group belong to the Ascomycota (Figure 9) or the Basidiomycota. Because some members of this group have not yet been appropriately classified, they are less well de- scribed in comparison to members of other fungal taxa. Most imperfect fungi live on land, with a few aquatic exceptions. They form visible mycelia with a fuzzy appearance and are commonly known as mold. Previous: Characteristics of Fungi Next: Ecology of FungioH / Zs J My SO um_ Figure 9: Aspergillus. Aspergillus niger is an asexually reproducing fungus (phylum Ascomycota) commonly found as a food contaminant. The spherical structure in this light micrograph is an asexual conidiophore. Molecular studies have placed Aspergillus with the ascomycetes and sexual cycles have been identified in some species. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell) The fungi in this group have a large impact on everyday human life. The food industry relies on them for ripening some cheeses. The blue veins in Roquefort cheese and the white crust on Camembert are the result of fun- gal growth. The antibiotic penicillin was originally discovered on an over- grown Petri plate, on which a colony of Penicillium fungi had killed the bac- terial growth surrounding it. Other fungi in this group cause serious dis- eases, either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds, as seen in the aflatoxins released by fungi of the genus Aspergillus. Glomeromycota The Glomeromycota is a newly established phylum that comprises about 230 species, all of which are involved in close associations with the roots of Previous: Characteristics of Fungi Next: Ecology of Fungiarbuscular mycorrhizae: the hyphae interact with the root cells forming a mutually beneficial association in which the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fun- gus supplies essential minerals from the soil to the plant. The exception is Geosiphon pyriformis, which hosts the cyanobacterium Nostoc as an endosymbiont, 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. Arbuscular mycorrhizae mycorrhizae commonly involving Glomeromycetes in which the fun- gal hyphae penetrate the cell walls of the plant root cells (but not the cell membranes) ascocarp fruiting body of ascomycetes Ascomycota (also, sac fungi) phylum of fungi that store spores in a sac called ascus basidiocarp fruiting body that protrudes from the ground and bears the basidia Basidiomycota (also, club fungi) phylum of fungi that produce club-shaped struc- Previous: Characteristics of Fungi Next: Ecology of Fungiclub-shaped fruiting body of basidiomycetes Chytridiomycota (also, chytrids) primitive phylum of fungi that live in water and pro- duce gametes with flagella Deuteromycota former form phylum of fungi that do not have a known sexual repro- ductive cycle (presently members of two phyla: Ascomycota and Basidiomycota) Ectomycorrhizae mycorrhizae in which the fungal hyphae do not penetrate the root cells of the plant Glomeromycota phylum of fungi that form symbiotic relationships with the roots of trees mold tangle of visible mycelia with a fuzzy appearance Zygomycota (also, conjugated fungi) phylum of fungi that form a zygote con- tained in a zygospore zygospore structure with thick cell wall that contains the zygote in zygomycetes LICENSE SHARE THIS BOOK Classifications of Fungi Copyright © vw by Various Authors - See Each Chapter Attribution. 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