Fungi Chapter 31

Defining Fungi Mycologists believe there may be as many as 1.5 million fungal species Fungi are classified into six main groups -Chytrids -Zygomycetes -Glomeromycetes -Ascomycetes -Basidiomycetes -Deuteromycetes

Defining Fungi

Defining Fungi (Cont.)

Defining Fungi Fungi share the following characteristics 1. Are heterotrophs that absorb nutrients 2. Have a number of different cell types 3. Have cell walls that include chitin 4. Some have a dikaryon stage 5. Undergo nuclear mitosis

General Biology of the Fungi Multicellular fungi consist of long, slender filaments called hyphae -Some hyphae are continuous -Others are divided by septa

General Biology of the Fungi A mass of connected hyphae is called a mycelium -It grows through and digests its substrate

General Biology of the Fungi Cells walls of fungi are formed of polysaccharides including chitin -Cross-linked with proteins Chitin is also found in the hard shells (exoskeletons) of arthropods

General Biology of the Fungi Hyphae may have more than one nucleus -Monokaryotic – One nucleus -Dikaryotic – Two nuclei Sometimes many nuclei intermingle in the common cytoplasm of the fungal mycelium -Heterokaryotic – Nuclei from genetically distinct individuals -Homokaryotic – Nuclei are genetically similar to one another

General Biology of the Fungi Fungi have an unusual mitosis -Nucleus does not break down and reform Fungi lack centrioles -Instead, they have small, amorphous structures termed spindle plaques -These form the spindle apparatus within the nucleus

General Biology of the Fungi Fungi can reproduce sexually and asexually Sexual reproduction involves the fusion of two haploid hyphae of compatible mating types -In some fungi, fusion immediately results in a diploid (2n) cell -Others, have a dikaryotic stage (1n + 1n) before parental nuclei form diploid nucleus

General Biology of the Fungi Spores are the most common means of reproduction among fungi -May form from sexual or asexual processes -Most are dispersed by wind -Some by insects or small animals Chytrids are the only group to retain ancestral flagella and have motile zoospores

General Biology of the Fungi

General Biology of the Fungi Fungi obtain food by secreting digestive enzymes into their substrates -They then absorb the organic molecules produced by this external digestion Fungi can break down cellulose and lignin Some fungi are carnivorous

General Biology of the Fungi

General Biology of the Fungi (Cont.)

Phylogenetic Relationships There are five major fungal phyla -Based on mode of sexual reproduction

Phylogenetic Relationships Increasing molecular sequence data suggest that fungi are more closely related to animals than to plants The Glomeromycota, Ascomycota and Basidiomycota are monophyletic -Other two phyla are not The phylogeny of Deuteromycota is yet to be determined

Chytridiomycetes Chytridiomycetes or chytrids are members of the phylum Chytridiomycota -Most closely related to ancestral fungi -Only fungi with flagellated spores -Have chitin in their cell walls -Life cycle has both haploid and diploid multicellular stages

Chytridiomycetes

Zygomycetes Zygomycetes (phylum Zygomycota) are incredibly diverse -Include the common bread molds -And a few human pathogens -Lack septa in their hyphae except when they are reproducing

Zygomycetes Sexual reproduction begins with the fusion of gametangia, which contain numerous nuclei -A zygosporangium is produced, within which a zygospore develops Asexual reproduction occurs much more frequently -Hyphae produce clumps of erect stalks, called sporangiophores -Tips form sporangia with spores

Zygomycetes

Glomeromycetes Glomeromycetes (phylum Glomeromycota) are a tiny group of monophyletic fungi -They form intracellular associations with plant roots called arbuscular mycorrhizae -They show no evidence of sexual reproduction

Ascomycetes Ascomycetes (phylum Ascomycota) contain about 75% of the known fungi -Include bread yeasts, common molds and many serious plant pathogens -Also cup fungi and morels

Ascomycetes Ascomycetes are named for their reproductive structure: a microscopic, saclike ascus Asci form within the ascocarp Karyogamy occurs within asci Meiosis and mitosis follow, producing 8 haploid nuclei that become walled ascospores

Ascomycetes Asexual reproduction is very common -Occurs through conidia formed at the ends of modified hyphae called conidiophores -Many conidia are multinucleate

Ascomycetes Yeasts are unicellular ascomycetes -Most reproduce asexually by budding Yeasts can ferment carbohydrates -Break down glucose into ethanol and CO2 -Used to make bread, beer and wine -Saccharomyces cerevisiae

Ascomycetes Yeasts have become increasingly important in genetic research -Yeast artificial chromosomes (YACs) -Yeast two-hybrid system -Fungal genome initiative

Basidiomycetes Basidiomycetes (phylum Basidiomycota) include some of the most familiar fungi -Mushrooms, puffballs, jelly fungi -Plant pathogens such as rusts and smuts

Basidiomycetes Basidiomycetes are named for their sexual reproductive structure, the club-shaped basidium Karyogamy occurs within basidia Meiosis follows The four haploid products are incorporated into basidiospores, which are borne externally

Basidiomycetes Spore germination leads to the production of monokaryotic hyphae -This results in a monokaryotic mycelium, or primary mycelium Different mating types of monokaryotic hyphae may fuse -This results in a dikaryotic mycelium, or secondary mycelium The basidiocarps (mushrooms) are formed entirely of secondary mycelium

Basidiomycetes

Deuteromycetes Deuteromycetes, formerly called imperfect fungi, are a polyphyletic group -There are about 15,000 species -No sexual reproductive stage has been observed -Many appear to be related to the ascomycetes

Deuteromycetes Hyphae of different types fuse spontaneously to produce heterokaryotic hyphae -These exhibit a special kind of genetic recombination called parasexuality -Genetically distinct nuclei exchange portions of chromosomes Parasexuality also occurs in other fungi

Deuteromycetes Include economically important molds -Penicillium -Penicillin -Blue cheeses -Aspergillus -Soy sauce -Soy paste

Ecology of Fungi Fungi, together with bacteria are the principal decomposers in the biosphere Fungi are virtually the only organisms capable of breaking down cellulose and lignin Fungi have entered into fascinating symbioses with a variety of life forms -Obligate symbiosis – Essential for fungus survival -Facultative symbiosis – Nonessential

Ecology of Fungi Types of symbioses -Pathogens and parasites benefit at the expense of their host -Commensals benefit one partner but do not harm or benefit the other -Mutualistic relationships benefit both partners

Ecology of Fungi Endophytic fungi live in the intercellular spaces inside plants -Some fungi protect their hosts from herbivores by producing toxins -Italian rye grass is more resistant to aphid feeding in the presence of endophytes

Ecology of Fungi

Ecology of Fungi (Cont.)

Ecology of Fungi Lichens are symbiotic associations between a fungus and a photosynthetic partner -Ascomycetes are found in all but about 20 of the 15,000 lichen species

Ecology of Fungi Lichens have invaded the harshest habitats, where they are often the first colonists Lichens have pigments

Ecology of Fungi (Cont.) Lichens are used as bioindicators of air quality

Ecology of Fungi Mycorrhizae are mutualistic relationships between fungi and plants -Found on the roots of about 90% of all known vascular plant species -Two principal types

Ecology of Fungi Arbuscular mycorrhizae -Hyphae penetrate the root cell wall -By far the most common (70% of all species) -Fungal partners are Glomeromycetes Ectomycorrhizae -Hyphae surround but do not penetrate the root cells -Most hosts are forest trees (pines,oaks) -Fungal partners are mostly Basidiomycetes

Ecology of Fungi

Ecology of Fungi (Cont.)

Ecology of Fungi Fungi also form mutual symbioses with animals -Ruminant animals host fungi in their gut -Leaf-cutter Attini ants have domesticated fungi which they keep in underground garden

Fungal Parasites and Pathogens Fungal species cause many diseases in plants -Armillaria has damaged large regions of coniferous forests

Fungal Parasites and Pathogens

Fungal Parasites and Pathogens (Cont.)

Fungal Parasites and Pathogens Fungi can also spoil food products that have been harvested and stored Some fungi secrete toxins that make foods poisonous -Fusarium – Vomitoxin -Aspergillus flavus - Aflatoxin

Fungal Parasites and Pathogens

Fungal Parasites and Pathogens Fungi also cause human and animal diseases -Candida – Thrush; vaginal infections -Pneumocystis jiroveci – Pneumonia -Athlete’s foot and nail fungus Fungal diseases are difficult to treat because of the close phylogenetic relationship between fungi and animals

Fungal Parasites and Pathogens Batrachochytrium dendrobatidis causes chitridiomycosis -Responsible for the worldwide decline in amphibian populations