Fungi

Chapter 31

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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

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Defining Fungi

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Defining Fungi (Cont.)

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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

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General Biology of the Fungi

Multicellular fungi consist of long, slender filaments called hyphae

-Some hyphae are continuous

-Others are divided by septa

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General Biology of the Fungi

A mass of connected hyphae is called a mycelium

-It grows through and digests its substrate

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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

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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

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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

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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

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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

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General Biology of the Fungi

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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

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General Biology of the Fungi

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General Biology of the Fungi (Cont.)

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Phylogenetic Relationships

There are five major fungal phyla

-Based on mode of sexual reproduction

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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

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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

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Chytridiomycetes

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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

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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

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Zygomycetes

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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

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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

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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

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Ascomycetes

Asexual reproduction is very common

-Occurs through conidia formed at the ends of modified hyphae called conidiophores

-Many conidia are multinucleate

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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

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Ascomycetes

Yeasts have become increasingly important in genetic research

-Yeast artificial chromosomes (YACs)

-Yeast two-hybrid system

-Fungal genome initiative

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Basidiomycetes

Basidiomycetes (phylum Basidiomycota) include some of the most familiar fungi

-Mushrooms, puffballs, jelly fungi

-Plant pathogens such as rusts and smuts

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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

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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

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Basidiomycetes

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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

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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

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Deuteromycetes

Include economically important molds

-Penicillium

-Penicillin

-Blue cheeses

-Aspergillus

-Soy sauce

-Soy paste

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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

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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

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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

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Ecology of Fungi

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Ecology of Fungi (Cont.)

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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

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Ecology of Fungi

Lichens have invaded the harshest habitats, where they are often the first colonists

Lichens have pigments

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Ecology of Fungi (Cont.)

Lichens are used as bioindicators of air quality

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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

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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

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Ecology of Fungi

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Ecology of Fungi (Cont.)

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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

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Fungal Parasites and Pathogens

Fungal species cause many diseases in plants

-Armillaria has damaged large regions of coniferous forests

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Fungal Parasites and Pathogens

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Fungal Parasites and Pathogens (Cont.)

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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

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Fungal Parasites and Pathogens

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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

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Fungal Parasites and Pathogens

Batrachochytrium dendrobatidis causes chitridiomycosis

-Responsible for the worldwide decline in amphibian populations