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Chapter 31

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Fiona Rawle

Chapter 31 – Fungi - Eukaryotes that grow as single cells or as large, branching networks of multicellular filaments - Along with land plants and animals, fungi are one of 3 major lineages of large, multicellular euks that occupy terrestrial environment. - Fungi absorb nutrition from other dead or alive animals - Fungi that absorb nutrients from dead organisms are the world’s most important decomposers. Fungi and a handful of bacterial species are the only organisms capable of completely digesting both the lignin and cellulose that make up wood. - Act as parasites - Vast majority of fungi are mutualists (benefit host) - Because they recycle key elements such as carbon, nitrogen and phosphorous, fungi have a profound influence on productivity and biodiversity. 31.1 Why Do Biologists Study Fungi? Fungi Provide Nutrients for Land Plants - Fungi that live in close associated with plant roots are said to be mycorrhizal. - Fungi are critical to the productivity of forests, cropland and rangelands. Fungi Speed the Carbon Cycle on Land - Saprophytes: Fungi that digest dead plant material - Coal exists today because conditions were too acidic for fungi to do their job (decompose) - Amassive die-off in trees and shrubs produced gigantic quantities of rotting wood and led to the explosion of fungal abundance. - When trees die, fungi break down wood into sugars and other small organic compounds that they and other organisms can use as food. - Two basic components of carbon cycle on land: 1) the fixation of carbon by land plants – meaning that carbon in the atmospheric CO2 is converted to cellulose, lignin, and other complex organic compounds in the bodies of plants. 2) the release of CO2 from plants, animals, and fungi as the result of cellular respiration – meaning the oxidation of glucose and production of theATP that sustains life. - On land, fungi make the carbon cycle turn much faster. The nutrients they release feed a host of other organisms. Fungi Have Important Economic Impacts - Soil-dwelling fungi have been the source of many of the most important antibiotics currently being prescribed. - Some fungi, including saprophytes, damage crops. - Mushrooms, yeast. - Enzymes derived from fungi are used to improve the characteristics of foods ranging from fruit juice and candy to meat. FungiAre Key Model Organisms in Eukaryotic Genetics - In good condition, yeast cells grow and divide almost as rapidly as bacteria - Simple morphology provides an example of a pure eukaryotic cell - one that is suitable for experiments on how cell division occurs and how particular gens are turned on and off. - Biologists study fungi because they provide a window for understanding euk cells and because they affect a wide range of species in nature, including humans 31.2 How Do Biologists Study Fungi? Analyzing Morphological Traits - Only two growth forms: 1. Single-celled forms called yeasts 2. Multi-cellular, filamentous structures called mycelia The Nature of Fungal Mycelium - Some mycelia can grow to be extremely large (530 hectares in Oregon). Although most are much smaller and shorter lived, all mycelia are dynamic. Grow constantly in direction of food sources and die back in areas where food is running out. Body shape can change almost simultaneously throughout its life. - Hyphae: Individual filaments that make up a mycelium. Most are haploid, some are dikaryotic, meaning that each cell contains 2 haploid nuclei – one from eac parent. - Hyphae are long narrow filaments hat branch frequently. - Usually, each filament is broken into cell-like compartments by cross-walls called septa. Septa do not close off segments of hyphae completely. Instead, gaps called pores enable a wide variety of materials, even organelles and nuclei to flow from one compartment to the next. - Septa may have single large opening or series of small gaps that give them sieve-like appearance. - Because nutrients, mitochondria and genes can flow through mycelium, the fungal mycelium is intermediate between a multicellular land plant or animal and an enormous single-celled organism. - Some fungal species are coenocytic – meaning they lack septa. Have many nuclei scattered throughout the mycelium. - Fungal mycelia can penetrate tiny fissures in soil and absorb nutrients that are inaccessible to plant roots. - Because mycelia are composed of complex, branching networks of extremely thin hyphae, the body of a fungus has the highest surface-area-t-volume ration possible in a multicellular organism. Consider that hyphae found in any fist-sized ball of rich soil typically have a surface area equivalent to half page of this book. Surface area is important because it makes absorption efficient, and because fungi make their living on absorption. - The extraordinarily high surface area has a downside: Fungi are prone to drying out, because the amount of water that evaporates from an organism is a function of its surface area. Due to high surface area of mycelia, fungi are most abundant in moist habitats. Often endure conditions in the form of tough, watertight spores. - The mass of filaments on the inside of mushrooms is protected from drying by the densely packed hyphae forming the surface. Reproductive Structures - Most fungi fall into 4 major groups: 1. Chytridiomycota (chytrids) – water or wet soils. Only fungi with motile cells. The spores that chytrids produce during sexual reproduction have flagella, as do the gametesproduced by them. 2. Zygomycota – Hyphae are haploid and come in several mating types. Hyphae of different mating types look identical but will not combine unless the individuals have different alleles of one or more genes involved in mating. Individuals of different mating types may become yoked together. Cell from yoked hyphae fuse to form spore-producing structure called zygosporangium. Yoked hyphae that form a zygosporangium are the reproductive structure unique to this group. 3. Basidomycota (club fungi) - Mushrooms, bracket fungi, puffballs are among reproductive structures. Inside these structures, specialized cells called basidia form at the ends of hyphae and produce spores. Multiple mating types occur. 4. Ascomycota (sac fungi) – produce complex reproductive structures – the largest are often cup-shaped. The tips of hyphae inside these structures produce distinctive sac-like cells called asci.An ascus is a spore-producing structure found only inAscomycota. Evaluating Molecular Phylogenies - Fungi are much more closely related to animals than they are to land plants. - Most animals and fungi synthesize chitin. Chitin is a prominent component of the cell walls of fungi - The flagella that develop in chytrid spores and gametes re very similar to those of animals: single, located at the back of the body, move in a whiplash manner - Both store food by synthesizing starch as their storage product. - To understand the relationship between the four types of fungi, scientists have sequences series of genes and used the data to estimate the phylogeny. Several conclusions:  Chytrids include the most basal groups of fungi. Fungi evolved from aquatic ancestors. Also suggests that fungi made the transition to land early in their evolution.  Chytridiomycota and Zygomycota are paraphyletic. Neither represents a single common ancestor and all of its descendents. Means, either swimming gametes or yoked hyphae evolved more than once, or both were present in a common ancestor but then were lost in certain lineages  Microsporidians are within the monophyletic group of fungi. Based on the molecular phylogeny, researches are testing the hypothesis that fungicides – meaning molecules that are lethal to fungi – may prove to be effective in combating microspirdian infections.  Glomeromycota is monophyletic. Sexual reproduction has yet to be observed  Basidiomycota andAscomycota are monophyletic and are the most highly derived groups of fungi. Experimental Studies of Mutualism - Symbiotic- living together relationship. Land plants and fungi. - Biologists hypothesize that the evolution of symbiotic associations has played a large role in the diversification of fungi. - Mutualistic – benefit both species; parasitic – benefits one harms host; commensal – one benefits, other is unaffected. - Fungi that are typically symbiotic are usually unable to grow and reproduce if their regular
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