Fungi

Overview• Fungi are eukaryotes• Most are multicellular• Differ from other eukaryotess in nutritional mode, structural organization, growth & reproduction

• Molecular studies show they are more closely related to animals than to plants

Nutrition

• Absorptive nutrition enables fungi to live as decomposers and symbionts

• Heterotrophs• Acquire nutrition through absorption

• Digest food outside of their body by secreting hydrolytic enzymes – Exoenzymes– Decompose complex molecules so fungus can absorb them

Interface of Nutrition & Ecology

• Absorptive nutrition allows fungi to serve as decomposers (saprobes), parasites, or mutualistic symbionts

• Saprobic fungi absorb nutrients from non-living organic material (animal waste, dead plants & animals)

• Parasitic fungi absorb nutrients from cells of living hosts– Cause about 80% of plant diseases

• Mutualistic fungi absorb nutrients from the host but also benefit the host, such as aiding in uptake of nutrients

Structural Adaptations• Extensive surface area adapts fungi for absorptive nutrition

• Fungi are constructed of tiny filaments = hyphae– (yeast are an exception)– Hyphae have tubular walls which surround a membrane & cytoplasm

– Hyphae are divided into sepatarate cells by septa

• The hyphae form an interwoven mat = mycelium– Usually subterranean

• Fungi have cell walls, most made of chitin– Same material as exoskeleton of insects and arthropods

Figure 31.1 Fungal mycelia

Figure 31.2 Examples of fungal hyphae

Figure 31.2x Septate hyphae (left) and nonseptate hyphae (right)

Reproduction• Reproduce by releasing spores• Spores are produced either sexually or asexually

• Trillions of spores can be produced by a single organism

• Dispersed by wind and water over many miles

• If they land in a receptive spot, grow to form a mycelium

The Heterokaryotic Stage

• Some mycelia become genetically heterogeneous through fusion of 2 hyphae with genetically different nuclei

• Such a mycelium = heterokaryon

• Has some of the advantages of diploidy

Stages of The Sexual Life Cycle

• Two distinct stages in the union of partners during sexual reproduction

• Plasmogamy– The fusion of the parents’ cytoplasm when their mycelia come together

• Karyogamy – Fusion of the haploid nuclei of the 2 parents

• The two stages may be separated in time by hours, days, or years

• During the interim, the hybrid is a heterokaryon

Figure 31.3 Generalized life cycle of fungi (Layer 1)

Figure 31.3 Generalized life cycle of fungi (Layer 2)

Figure 31.3 Generalized life cycle of fungi (Layer 3)

Diversity of Fungi

• More than 100,000 species are known

• Four phyla– Chytridiomycota– Zygomycota– Ascomycota– Basidiomycota

Figure 31.4 Phylogeny of fungi

Chytridiomycota: The Chytrids

• Mainly aquatic• Form flagellated spores, so were once considered protists

• The most primitive fungi– Diverged first from protists

• Chitin cell walls• Absorptive nutrition

Figure 31.5 Chytridiomycota (chytrids)

Zygomycota: Zygote Fungi

• Live mostly in soil or on decaying plant or animal material

• One group forms mycorrhizae– mutualistic assiciation with the roots of plants

• Plasmogamy produces a resistant structure called a zygosporangium in which karyogamy, then meiosis occurs– The zygosporangium is multi-nucleated– Zygosporangium are resistant to freezing & drying and metabolically inactive

Figure 31.6 The common mold Rhizopus decomposing strawberries

Figure 31.7 The life cycle of the zygomycete Rhizopus (black bread mold)

Figure 31.7x1 Young zygosporangium

Figure 31.7x2 Mature zygosporangium

Figure 31.8 Pilobolus aiming its sporangia

Ascomycota: Sac Fungi• Over 60,000 species• Wide range of habitats, size, & complexity

• Many are important saprobes• Others cause devastating plant diseases• About half live in a mutualistic association with algae, forming lichens

• All produce sexual spore in sac-like asci

• The sexual stage is a fruiting body called an ascocarp

• Reproduce asexually by producing asexual spores: conidia

Figure 31.9 Ascomycetes (sac fungi): Scarlet cup (top left), truffles (bottom left), morel (right)

Figure 31.9x1 Carbon fungus

Figure 31.9x2 Aspergillus

Figure 31.10 The life cycle of an ascomycete

Figure 31.10x1 Life cycle of an ascomycete

Figure 31.10x2 Apothecium

Ascomycota: The Club Fungi

• Approximately 25,000 species• Includes mushrooms, shelf fungi, puffballs, & rusts

• Important plant decomposers• Also includes mycorrhiza-forming mutualists and plant parasites

• Reproduce sexually by producing complex fruiting bodies called basidiocarps

Figure 31.11 Basidiomycetes (club fungi): Greville's bolete (top left), turkey tail (bottom left), stinkhorn (right)

Figure 31.11x1 Coprinus comatus, Shaggy Mane

Figure 31.11x2 Geastrum triplex

Figure 31.11x3 Tremella messenterica, Witch’s Butter

Figure 31.11x4 Stinkhorn

Figure 31.11x5 Amanita

Figure 31.12 The life cycle of a mushroom-forming basidiomycete

Figure 31.12x Gills

Figure 31.13 A fairy ring

Table 31.1 Review of Fungal Phyla

Specialized Lifestyles

• Four types of fungi have developed highly specialized ways of life:

• Molds• Yeasts• Lichens• Mycorrhizae

Molds• A rapidly growing, asexually reproducing fungus

• Mold applies only to the asexual stage

• Many are destructive, but some are commercially important– penicillin

Figure 31.14 A moldy orange (left), Penicillium (right)

Figure 31.21 Fungal production of an antibiotic

Yeasts• Unicellular fungi • Inhabit liquid or moist habitats• Reproduce asexually by budding• Used commercially to raise bread and ferment alcohol

• One species is a normal inhabitant of moist human epithelial tissue

• May become pathogenic

Figure 31.15 Budding yeast

Lichens• A symbiotic association of millions of photosynthetic microorganisms held in a mesh of fungal hyphae

• The photosynthetic organisms are usually unicellular or filamentous green algae or cyanobacteria

• The lichen symbiosis is highly complex– The alga provides the fungus with food– The cyanobacteria in lichens fix nitrogen & provide organic nitrogen

– The fungus provides a physical structure for growth

– Hypahe reatin water & minerals and allow gas exchange

• Appear similar to mosses or simple plants

Figure 31.16 Lichens

Figure 31.17 Anatomy of a lichen

Figure 31.17x Anatomy of a lichen

Mycorrhizae• Mutualistic associations of plant roots and fungi

• Extensions of the fungal mycelium increase the absorptive surface of the plant roots

• The plant derives minerals absorbed from the soil by the fungus

• The fungus derives organic nutrients synthesized by the plant

• Almost all vascular plants have mycorrhizae

• Fungi are in permanent association with their plant host

Figure 31.18 Mycorrhizae

Figure 31.19 An experimental test of the benefits of mycorrhizae

Ecological Impacts

• Ecosystems depend on fungi as decomposers

• Provide ecosystems with inorganic nutrients essential to plant growth

• Recycle carbon, nitrogen, and other elements that otherwise would be tied in organic matter

• Structure suits function– Invasive hyphae enter tissues of dead organic matter

– Exoenzymes can hydrolyze polymers, including cellulose and lignin

Fungal Pathogens• About 30% of fungi are parasites, mostly of plants– Wheat rust– Dutch Elm disease

• Some secrete toxins harmful to humans– Aspergillus secretes carcinogenic aflatoxins on improperly stored grain or peanuts

– Claviceps purpurea secretes ergot on rye; can cause gangrene, hallucinations, etc (LSD; Salem witch trials)

• Human diseases– Skin diseases: athlete’s foot, rimg worm– Respiratory illnesses from inhaled spores: coccidiomycosis, histoplasmosis

Figure 31.20 Examples of fungal diseases of plants: Black stem rust on wheat (left), ergots on rye (right)

Figure 31.20x1 Strawberries with Botrytis mold, a plant parasitic fungus

Figure 31.20x2 Pink ear rot of corn

Evolution of Fungi• Fungi colonized the land with plants• Oldest fungi fossils are 460 million years old

• Fossils of the first vascular plants have mycorrhizae

• Plants probably moved onto land with fungi– The four phyla may have diverged from a common ancestor during the transition from water to land

• Fungi and animals evolved from a common protistan ancestor– Proteins & rRNA demonstrate that fungi are more closely related to animals than to plants