Presentation on theme: " Fungi are a huge and important component of the biosphere. Their diversity alone is staggering:although 100,000 species have been described, it is."— Presentation transcript:
Fungi are a huge and important component of the biosphere. Their diversity alone is staggering:although 100,000 species have been described, it is estimated that there are actually as many as 1.5 million species of fungi Fungi breakdown and recycle nutrients. Almost all plants depend on a symbiotic relationship with fungi that helps their roots absorb minerals from the soil.
Two species of fungi decomposing a log
Like animals,fungi are heterotrophs they cannot make their own food as plants and algae can. But unlike anials,fungi do not ingest their food the digest their food while it is still in the environment by secreting powerful hydrolytic enzymes,called exoenzymes,into their surrounding. Exoenzymes breakdown complex molecules to smaller organic compounds that the fungi can absorb into their bodies and use.
Among the fungi are species that live as decomposers (saprobes),parasites,and mutualstic symbionts. Saprobic fungi break down and absorb nutrients from nonliving organic material,such as fallen logs,animal corpses,and the wastes of living organisms. Parasites fungi are pathogenic,including species that infect human lungs and other species that are responsible for about 80% of plant diseases.
Mutualistic fungi also absorb nutrients from a host organism,but the mutualistic fungi reciprocate with functions beneficial to the host organism,but the mutualistic fungi reciprocate with functions beneficial to the host in some way,such as aiding a plant in the uptake of minerals from the soil.
The bodies of these fungi typically form a network of tiny filaments called hyphea. Hyphea are composed of tubular cell walls surrounding the plasma membrane and cytoplasm of the cells. Unlike the cellulose walls of plants,fungal cell walls contain chitin a strong but flexible nitrogencontaining polysaccharide that is also found in the external skeletons of insects and other arthropods.
Fungal hyphae form an interwoven mass called a mycelium that surrounds and infiltrates the material on which the fungus feeds.
In most fungi,the hyphea are divided into cells by crosswalls,or septa. Coenocytic fungi,these organisms consist of a continuous cytoplasmic mass contining hundreds or thousands of nuclei. Some species have specialized hyphea called haustoria that enable them to penetrate the tissue of their hosts.
Mutually beneficial relationships between such fungi and plant roots are called mycrrhizae. Ectomycorrhizal fungi form sheaths of hyphae over the surface of a root and also grow into the extracellular spaces of the root cortex. Endomycorrhizal fungi extends their hyphae through the root cell wall and into tubes formed by invagination of the root cell memebrane.
fungi propagate themselves by producing vast numbers of spores,ethier sexually or asexually. Spores can be carried long distances by wind or water,and if they land in a moist place where there is food, they germinate,producing new mycelia. Generalizes the many different life cycles that can produce fungal spores.
Generalized life cycle of fungi.
The nuclei of fungal hyphae and spores of most species are haploid,except for transient diploid stages that form during sexual life cycle. Generally,the sexual reproduction in fungi begins when hyphae from two distinct mycelia release sexual signaling molecule called pheromones. If the mycelia are of different mating types the pheromones from each partner bind to receptor on the surface of the other and the hyphea extends toward the source of the pheromones.
When the hyphea meet they fuse,this “compatibilty test”contributes to genetic variation by preventing hyphea from fusing with other hyphea from the same mycelium or another genetically identical mycelium. The union of the cytoplasm of the two parent mycelia is known as plasmogamy. In many fungi,the haploids nuclei contributed by each parent do not fuse right away. Instead parts of the mycelia contain coexisting,genetically different nuclei.
Such a mycelium is said to be a hetrokaryon(meaning different nuclei) In some species,heterokaryotic mycelia become mosaics,with different nuclei restricted to different parts of the network. In other species,the different nuclei mingle and may even exchange chromosomes and genes in a process similar to crossing over. In some fungi,the haploid nuclei pair off two to a cell,one from each parent.
Such a mycelium is said to be dikaroyotic(meaning two nuclei). As a dikaryotic mycelium grows,the two nuclei in each divide in tandem without fusing. Hours,days,or even centuries may pass between plasmogamy and the next stage in the sexual cycle,karyogamy. During karyogamy,the haploid nuclei contribute by the two parent fuse,producing diploid cells.
Zygotes and other transient structure form during karyogamy the only stage in which diploidy exists in most fungi. The heterokaryotic condition also offer some of the advantages of diploidy in that one haploid genome may be able to compensate for harmful mutation in the other.
In addition to sexual reproduction,many fungi can reproduce asexually. Clones are produced by mitotic production of spores which can be spread by air or water. Some species only reproduce asexually. Some fungi that can reproduce asexually grow as mold. Mold grow rapidly as mycelia and produce spores.
Many species that can grow as mold can also reproduce sexually if they come in contact with other mating types. Another asexual fungi is yeast. Yeast inhabit liquid or moist habitats,including plant sap and animal tissue. Instead of producing spores,yeast species can also grow as filamentous mycelia,depending on the availabilty of nutrients. Sometimes yeast can produce sexually.
Penicillium a mold commonly encountered as a saprobe on food.
Yeat saccharomyces cerevisiae in various stages of budding(SEM).
Many molds and yeasts have no known sexual stage. Mycologists,have traditionally called such fungi deuteromycetes(traditional classification for a fungus with no known sexual stage)or imperfect fungi. Whenever a sexual stage of a so-called deuteromycete is discovered,the species is classified in a particular phylum,depending on the type of sexual structure is formed.
Fungi and animalia are sister kingdom. While the majority of fungi lack flagella,the linages of fungi thought to be the earliest to diverge do have flagella. Most of the protist that share a close common ancestor with animals and fungi also have flagella. These three groups of eukaryotes-the fungi,animals,and their protistan relatives-are called opishokonts,members of the clade opisthokonta.
Phylogenetic evidence also suggests that the ancestor of fungi was uncellular. Based on molecular clock analysis,scientists have estimated that the ancestors of animals and fungi diverged into separate linages l.5 billion years ago. However the oldest undisputed fossils of fungi are only about 460 million years old. One possible explanation for this discrepancy is that the microscopic ancestors of terrestrial fungi fossilized poorly.
Fossil fungal hyphae and spores from the ordovician period (about 460 million years ago;LM).
Much of the fungal diversity we observe today may have had its phylogenetic origin in adaptive radiation when life began to colonize land. Fossils of the earliest known vascular plants from the late silurian period contain evidence of mycorrhizae,the symbiotic relationship between plants probably existed in this symbiotic relationship from the earliest periods of colonization of land.
Fungi classified in the phylum chytridiomycota,called chytrids,are ubiquitous in lake and soil. Some are saprobes;other parasitize protists,plants,or animals. Like other fungi,chytrides have cell walls made of chitin,and they also share certain key enzymes and metabolic pathways with other fungal group.
Phylogency of fungi
Chytrides are unique among fungi in having flagellated spores,called zoospores. Until recently,systematists thought that fungi lost flagella only once in their history,after chytrids had divergance from other linage. Molecular data indicate that somechytrides are actually more closly related to another fungal group the zygomycetes,if this is true then flagella were loston more than one occasion during the evolution of fungi.
The approximatly 1,000 known species of zygomycetes(fungi in the phylum zygomycota)exhibit a considerable diversity of life histories. This phylum includes fast growing molds responsible for rotting produce such as peaches,strawberries,and sweet potatos during storage. Horizontal hyphae spread out over the food,penetrate it and absorb nutrients.
The hyphae are coenocytic,with septa found only where reproductive cells form. The parents in a sexual union are mycelia of different mating types that possess different chemical markers,although they may appear identical. Plasmogamy produces a sturdy structure called a zygosporangium,in which karyogamy and the meiosis occur.
Zygosporangosporangia are resistant to freezing and dying and are metabolically inactive. When condition improve,a zygosporangium undergoes meiosis,germinates into a sporangium,and releases genetically diverse haploid spores that may colonize a new substrate.
Microsporidia are unicellular parasites of animals and protists. They are often used in the bilogical control of insect pests. While microsporidia do not normally infect humans,they do pose a risk to people with AIDS and other immune-compromised conditions.
The glomeromycetes,fungi assigned to the phylum glomeromycota,were formally thought to be zygomycetes. In spite of their small numbers-only 160 species have been identified to date-the glomeromycets are an ecoligically significant group. All glomeromycetes form a distinct type of endomycorrhizae called arbuscular mycorrhizae. The tips of the hyphae that puch into plant root cells branch into tiny treelike structure known as arbusculares.
Mycologists have described more than 32,000 of ascomycetes(fungi in the phylum ascomycota)from a variety of marine,freashwater,and terrestrial habitats. The defining feature of ascomycetes is the production of sexual spores in saclike asci(singular,ascus);thus they are commonly called sac fungi. Unlike zygomycetes,most ascomycetes bear their sexual stages in fruiting bodies,or ascocarps,which rang in size from microscopic to macroscopic.
Unlike zygomycetes,most ascomycetes bear their sexual stages in fruiting bodies,or ascocarps,which range in size from microscopic to macroscopic. Ascomycetes vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels. They include some of the most devastating plant pathogens.
Morchella esculenta, e tasty morel Tuber melanosporum, a truffle
Altough the life cycle of various ascomycete group differ in details of their reproductive structure and processes,there are some common element. Ascomycetes reproduce asexually by producing enormous number of asexual spores called condia. Condia are formed inside sporangia,as are the asexual spores of most zygomycetes.
Incontrast to the life cycle of zygomycetes,extended dikaryotic stage of ascomycetes(and also basidiomycetes) provides increased opportunity for genetic recombination.
Approximtely 30,000 fungi,including mushrooms and shelf fungi,are called basidiomycetes and are classified in the phylum basidiomycota. The name of the phylum derives from the basidium(latin for “little pedestal”),a cell in which a transient diploid stages occurs during the fungal life cycle. The club-like shape of the basidium also gives rise to the common name club fungus.
Puffballs emitting spores ( Dic )
The life cycle of a basidiomyete usually include a long-lived dikaryotic mycelium. Periodically,in response to enviormental stimuli the mycelium reproduces sexually by producing elaborate fruiting bodies called basidiocarps(a mushroom is a fimulare example). The numerous basidia in a basidiocarp are the sources of sexual spores called basidiospores. The mushroom’s cap supports and protects a large surface area of basidiaon gills.
Fungi are well adapted as decomposers of organic material, including the cellulose and lignin of plant cell walls. Fungi and bacteria are primarily responsible for keeping ecosystems stocked with the inorganic nutrients essential for plant growth. Without these decomposers,carbon,nitrogen,and other elements would become tied up in organic matter.
Mycorrhizae are enormously important in natural ecosystem and agriculture. It is relatively easy to demonstrate the significance of mycorrhzae by comparing the growth of plants with and without mycorrhizae. Foresters commonly inoculate pine seedlings with ectomycorrhizal fungi to promote vigorous growth in the tree.
Some fungi share their digestive service with animals,helping breakdown plant material in the guts of cattle and other grazing mammals. Many species of ants and termites take advantage of the digestive power of fungi by raising them in farms. The insect scour tropical forests in search of leaves,which they carry back to their nest and feed it to the fungi. The fungi have been so dependent on their caretakers that in many cases they can no longer survive without the insect.
Ant taking a stem of a plant to take it to the fungus.
Lichens are a symbiotic association of millios of photosynthetic microorganisms held in a mass of fungal hyphae. They form a surface-hugging carpet that can be found growing on rocks,rotting logs,trees,and roofs in various shrub-like,leaf-like,or encrusting forms. The merger of fungus and alga or cyanobacterium is so comeplete that lichens are actually given scientific names as though they were single organism.
More than 13,500 species have been described,accounting for a fifth of all known fungi. In most lichens that have been studied,each parter provides something the other cannot obtain on its own. The fungi of many lichens reproduce sexually by forming ascocarps or basidiocarps. Soredia are small cluster of fungal hyphea with embedded algea.
Free-living fungus that provides the antibiotic penicillin,for example,is thought to have descended from a lichen fungus. As tough as lichens are,many do not stand up very well to air population. Their passive mode of mineral uptake from rain and moist air makes them particularly sesitive lichens and an increase in the number of hardier species in an area can be an early warning that air quality is deteriorating.
Of the 100,000 known species of fungi,about 30% make their living as parasites,mostly on or in plants. For example,ophiostoma ulmi,the ascomycete that causes dutch elm disease,has drastically changed the landscape of the northeastern united states on logs that were sent from europe to help pay world war 1 debts,the fungus is carried from tree to tree by bark beetles. Another ascomycete,cryphonectria has killed 4 billion native american chestnut trees in the eastern united states.
Some of the fungi that attack food crops are toxic to humans. Certain species of the ascomycete mold aspergillus contaminate improperly stored grain and peanuts by secreting carcinogenic compounds called aflatoxins. Of diseased rye is inadvertently milled into flour and then consumed,poisons from the ergots can cause ergotism, acondition charcterize by gangrene,nervous spasms,burning sensation,hallucination,and tempory insanity.
Animals are much less susceptible to parasitic fungi then are plants. Only about 50 species of fungi are known to parsitize humans and other animals,but these relativity few species do considerable damage. The general term for such a fungal infection is mycosis. Skin mycoses include the disease ringworm,so named because it appears as circular red areas in the skin.
Some mycoses are opportunistic,occuring only when a change in the body’s microbiology,chemistry,or immunology allows fungi to grow unchecked. Under certain circumstances,candida can grow too rapidly and become pathogenic, leading to so-called yeast infection. Many other opportunities mycoses in human have become more common in recent decades,due in part to AIDS which compromiss the immune system.
The danger posed by fungi should not overshadow the immense benefits we derive from these remarkable eukaryotes. many yeast and fungi are used for acholic beverages medical supplies and food. Fungi is used for reasearhing in biotechnology.