Biol 3400 Tortora et al., Chapter 12 The Eukaryotes I. The Fungi Mycology = the study of fungi: Myco is Greek for “fungi”, ology = “the study of” A. What are fungi? • Primarily terrestrial organisms • 100,000 species have been described (Estimates suggest that there are up to 1.5 x 106 species) • Fungi can represent up to 50% of the biomass in agricultural soils and 90% of the biomass in forest soils • Diverse group of organisms • Essential to life on earth • Fungi are the Primary Decomposers!! Without fungi there would be an excess of biomass • Nearly all plants depend on symbiotic fungi – mycorrhizae. These fungi aid the plants in absorbing water and nutrients from soil • Major cause of disease – plants are particularly vulnerable • Important for a number of industrial involving fermentation – bread, wine, beer but also cheeses, soy sauce. Also used in organic acid production (citric acid) and antibiotics (penicillin). Saccharomyces cerevisiae is the best understood eukaryote. B. Characteristics of fungi 1. They are eukaryotes and contain: • a nucleus containing chromosomes • membrane bound organelles (mitochondria, ER, golgi, vacuoles, …) 2. Fungi are nonvascular 3. Fungi reproduce by spores • Spores are important for several reasons, including surviving environmental stress (desiccation, nutrient limitation) and dissemination (i.e., mass multiplication or r – strategy). • Spores are typically not motile • Both sexual and asexual spores are produced, depends on the fungus and conditions • There are several types of sexual spores that tend to be specific to fungal subdivisions. The sexual spores tend to result from the fusion of gametes that may come from the same thallus (i.e., self fertilizing or homothallic species) or from different but sexually compatible mycelia (i.e., heterothallic species) • There are several types of asexual spores o Sporangiospores – spores that form within a sac (sporangium) at the hyphal tip o Conidiospores – spores are not produced in a sac but rather at the tip or on the sides of the hyphae o Blastospores – a vegetative mother cell produces these spores by budding o Arthrospores – hypha fragments to form cells that behave like spores o Chlamydospores – cells surrounded by a thick wall before separation 1 Biol 3400 Tortora et al., Chapter 12 4. Fungal vegetative cells may be unicellular (yeasts) or comprised of a thread-like structure called a thallus → the vegetative structure from which they get nutrients 5. Fungi have well-developed cell walls comprised mainly of chitin (N-acetylglucosamine) 6. All fungi are chemoorganotrophs (heterotophic: must feed on preexisting organic matter) 7. Fungi utilize extracellular digestion (“digest then ingest”) 8. Most fungi store their food as glycogen (similar to animals), plants use starch and oils as compounds 9. Fungal membranes possess a unique sterol called ergosterol (replaces cholesterol found in animal membranes) 10. Fungi have a unique lysine biosynthesis pathway 11. Fungal tubulin proteins are unique 12. Most fungi have relatively small genomes with minimal repetitive DNA • Mitosis is generally accomplished without dissolution of the nuclear envelope C. The Fungal Thallus • Vegetative structure • plural - thalli • composed of hypha (pl. hyphae): releases enzymes that breakdown food around it, the fungus then absorbs the resulting products of hydrolysis o Thread-like structure o Evolution of terrestrial existence (allowed fungi to grow on land) • Mycelium = multiple hyphae. This is a collective term. The extensive root-like system found underground for many fungi and accounts for the bulk of the fungal biomass. • Hypha may have cross walls or septa (sing. = septum). In some species the hyphae are aseptate (ceonocytic) • Yeast Cell = single celled thallus (an adaptation to an aquatic environment) o Yeasts reproduce asexually via budding. This is a good characteristic for distinguishing between bacteria and yeasts o Some fungi are dimorphic and can change from a yeast form (Y) to a mycelial form (M) in response to environmental factors such as (nutrients, CO2 tension, temperature,…). This shift is called the YM shift. 2 Biol 3400 Tortora et al., Chapter 12 D. How do fungi grow? • Fungal growth occurs always at the tips of hypha. The spore stays stationary and hyphae grow out from it. E. Classification of fungi: • Originally fungi were grouped based on phenetic characteristics that did not represent their natural grouping • Molecular genetics have revolutionized how we look at the fungi The Fungi includes eight subdivisions: o Chytridiomycetes o Zygomycota o Ascomycota o Basidiomycota o Urediniomycetes o Ustilaginomycetes o Glomeromycota o Microsporidia • The first four subdivisions have distinct sexual spores and associated structures that can be used to identify the organism. They also often produce asexual spores that are responsible for the success of the organism. 1. Chytridiomycota “Chytrids” • The chytrids produce motile spores = zoospores • Zoospores are typically uniflagellate • Flagella are whiplash type and are attached posteriorly • Zoospores are typically formed in specialized structures = zoosporangia (zoospore sacs) • Zoosporangium holds thousands of the actual zoospores. • Cell walls composed of cellulose rather than chitin Ecological niches: • Decomposers in aquatic ecosystems • Pathogens in aquatic ecosystems (e.g., Frog decline caused by chytridiomycosis ”disease caused by chytrids”) • Mutualistic symbiotic relationships with animals o Rumens (such as cattle) o Intestines of hind-gut fermenting animals such as horses • e.g., Allomyces 3 Biol 3400 Tortora et al., Chapter 12 2. Zygomycota ”Zygosporic fungi” • Produce sexual spores called zygospores in a zygosporangium • “zygos” = yoke and “spora” = seed • Zygospores are typically thick walled and pigmented = a survival structure resistant to UV light and microbial factors • This group of fungi also typically produces non-motile asexual spores called sporangiospores • Asexual spores are often formed in specialized structures called a sporangium (spore sac; sporangia = pl.) • Thousand of spores are produced → contribute to the success of fungi. • The sporangia are produced on tall stalks in order to introduce the spores to more turbulent wind patterns above the boundary layer. This ensures better wind dispersion. • Coenocytic hyphae Ecological niches: • Decomposers in the soil - decaying plant and animal matter in the soil • A few are parasites of plants, insects, other animals and humans • e.g., Rhizopus, Mucor 3. Ascomycota ”Sac Fungi” • A very large group of fungi that produce sexual spores = ascospores • These are typically formed in “sac-like” structures termed asci (singular = ascus) • Ascospores are typically forcibly discharged (via pressure, such as being stepped on) • Asci are borne “naked” or in specialized fruiting structures called ascomata (or ascocarp) • Ascomata are often formed in “stroma”→ hyphae produced by a fungus organize into an interwoven tissue • Many Ascomycetes fungi are yeast Asexual reproduction • Many members of the ascomycota also reproduce asexual spores are termed conidiospores or “conidia” • This is an ecological adaptation that allow fungi to access and exploit new niches • Fungi as a rule are good examples of r-selection reproductive strategies: o Small size o Many offspring o Reduced energy o Early maturity o Most individuals die within a short period of time • There is tremendous diversity in asexual spores and the way they are formed • This is such a successful strategy, that some fungi have even lost the ability to reproduce asexually 4 Biol 3400 Tortora et al., Chapter 12 Ecological niches: • Important decomposers in freshwater, marine and terrestrial habitats because they degrade many recalcitrant materials, including cellulose, lignin and collagen • Many are parasites on higher plants - cause important plant diseases such as powdery mildew and Dutch elm disease • Many are edible – morels and truffles • e.g., Neurospora, Aspergillus, Saccharomyces cerevisiae 4. Basidiomycota “Basidiosporic fungi” • Commonly known as the club fungi, contains the mushrooms, boletes, bracket fungi, rusts and smuts • Have gills (underside of cap) that maximize surface area • Produce basidiospores on specialized structures called basidium. May be held in fruiting bodies known as basidiocarps Ecological niches: • Important terrestrial decomposers that decompose plant material (cellulose and lignin) • Many mushrooms are cultivated for food – Agaricus • Some mushrooms produce specific secondary metabolites – alkaloids that are either poisonous (Amanita phalloides – death angel) or hallucinogenic • e.g., Agaricus, Amanita 5. Urediniomycetes and Ustilaginomycetes • Often considered as Basidiomycota but they do not produce large basidiocarps but rather produce small basidia that arise at the tip of the hyphae at the surface of the host plant Ecological niches: • Important plant pathogens causing rust and smut • Some Urediniomycetes cause disease in humans • e.g., Ustilago 6. Glomeromycota • Often considered as zygomycetes by some • Only asexual reproduction is known to occur in this group – spores are produced and germinate when in contact with roots of a suitable host plant Ecological niches: • Important plant symbionts – most are endomycorrhizal symbionts of vascular plants. The fungus helps protect the host plant from stress and delivers soil nutrients to the plant which in turn provides carbohydrate to the fungus 5 Biol 3400 Tortora et al., Chapter 12 7. Microsporidia