Chapter 31
Fungi
Intro to Fungi
• Eukaryotes
• Most are multicellular
• Were once thought to be plants
– Recent molecular studies show that they are more
closely related to animals than plants
Nutrition
• Heterotrophs that acquire nutrients by
absorption
– Small organic molecules are absorbed from
surrounding medium
• Digests food outside of body by secreting
enzymes on the food
– Exoenzymes: decompose complex molecules into
more simple compounds
Nutrition
• Saprobes (decomposers)
– Absorb nutrients from nonliving organic matter
• Parasites
– Absorb nutrients from living hosts
• Some infect lungs of humans
• Fungi cause 80% of plant disease
• Mutualistic symbionts
– Absorb nutrients from a host but provide
functions beneficial to their partners
• Aiding a plant in the uptake of minerals from the soil
Structure
• Vegetative bodies are usually hidden around
and within their food sources
• Constructed of hyphae
– Thin filaments that are composed of tubular walls
surrounding plasma membranes and cytoplasm
– Form an interwoven mat called mycelium
• Feeding network of a fungus
• Can be extremely large but hidden because it is
subterranean
• 2000: found the mycelium of one individual
fungus in Oregon that is 3.4 miles in diameter
and spreads through 2200 acres
– Equivalent to over 1600 football fields
• It is at least 2400 years old and hundreds of
ton in weight
– One of Earth’s oldest and largest organisms
Structure
• Hyphae are divided into cells by cross-walls, or
septa
– Have pores large enough to allow ribosomes,
mitochondria, and nuclei to flow from cell to cell
• Cell walls made of chitin
• Some don’t have septa
– Coenocytic fungi consist of a continuous
cytoplasmic mass with hundreds to thousands of
nuclei
• Resulted from the repeated division of nuclei without
cytoplasmic division
Structure
• Parasitic fungi have their hyphae modified as
haustoria, which are tips that penetrate the
tissues of the host plant
– There are even fungi with hyphae that are
adapted to feed on animals
Structure – Function Relationship
• Filamentous structure of the mycelium
provides an extensive surface area that assists
the absorptive function of fungi
– Mycelium can grow as much as a kilometer of
hyphae each day as it branches out throughout a
food source
Reproduction
• Release spores that are produces
either sexually or asexually
– Amount of spores is tremendous
• Puff balls can put out trillions of spores
– Spores function as dispersal agents
– Airborne spores of fungi have been
found more than 100 miles above Earth
Heterokaryotic Stages
• Nuclei of fungal hyphae and spores are
haploid, except for transient diploid stages
that form during sexual life cycles
• Some mycelia become genetically
heterogeneous through the fusion of two
hyphae that have genetically different nuclei =
heterokaryon
– Different nuclei may stay separate or join in a
“crossing over” occasion
Sexy Fungus
• Plasmogamy: the fusion of the two parents’
cytoplasm when their mycelia come together
• Karyogamy: fusion of the haploid nuclei
contributed by the two parents
– These stages may be separated by hours or even
centuries
– During interim, the hybrid exists as a heterokaryon
Diversity
• More than 100,000 species are known, and
there are estimated 1.5 million species
worldwide
• Fungi are broken down into four phyla:
– Chytridiomycota
– Zygomycota
– Ascomycota
– Basidiomycota
Chytridiomycota
• “Chytrids” are mainly aquatic
• Some are saprobes, others parasitize protists,
plants, and animals
– Contributing to the worldwide decline of
amphibians
• Form coenocytic hyphae
• Most primitive of fungi
– Diverged earliest in the lineage of fungi
Chytridiomycota
Zygomycota
• 600 different species
• Terrestrial and live in soil or on decaying
organic matter
• Mycorrhizae: mutualistic associations with the
roots of plants
• Hyphae are coenocytic
• Ex: Black Bread Mold
Mycorrhizae
Black Bread Mold
Ascomycota
•
•
•
•
60,000 different species
“Sac Fungi”
Marine, freshwater, and terrestrial
Include some of the most devastating plant
pathogens
• Live with algae to form lichens
• Produce sexual spores in a saclike asci
Ascomycota
Basidiomycota
• 25,000 species
– Include mushrooms
• “Club Fungus”
• Decomposers of wood and other plant
material
• Long-lived dikaryoric mycelium
– Reproduces sexually by producing basidiocarps,
which are the source of sexual spores
So…. When you eat mushrooms, you are eating Fungal Genitalia….
“Fairy Rings”
• A ring of mushrooms
• Grass in center is stunted because the
mycelium is using all the nutrients
Other Categories of Fungi
•
•
•
•
Molds
Yeasts
Lichens
Mycorrhizae
Molds
• Rapidly growing, asexually reproducing fungus
• Saprobes or parasites
• Mold is a mold if it is in the asexual stage
– Later will turn into zygo, asco, or basidiomycotas
• Imperfect Fungi
– Have no sexual stages whatsoever
Yeasts
• Unicellular fungi that inhabit liquid or moist
habitats, including plant sap and animal tissues
• “Bud” to reproduce
• Raise bread and ferment alcohol
– Metabolically active and release small CO2 bubbles or
ferment sugars to alcohol
• pH shift or immunodeficiency can lead to “yeast
infections” where normal yeast populations have
exponential growth
Yeast
Lichens
• Often mistaken for mosses or plants
• Symbiotic association of millions of
photosynthetic microorganisms held in a mesh of
fungal hyphae
– Algae provide fungus with food
– Bacteria in lichens fix nitrogen and provide organic
nitrogen
– Fungus provides alga with a suitable environment for
growth
• Absorb nutrients from dust in the air or from rain
Lichens
• Important pioneers on newly cleared rock and
soil surfaces
– Burned forests and volcanic flows
• Lichens make it possible for a succession of
plants to grow
• Can tolerate severe cold and desiccation
– Reindeer moss
– Can absorb more than ten times their weight in
water
Lichens
Mycorrhizae
• “Fungus Roots”
• Extensions greatly increase the absorptive
surface of the plant roots
• Exchange minerals accumulated from soil by
the fungus for organic nutrients synthesized
• Occur on almost all vascular plants
• Show up as mushrooms at the base of trees
Ecological Impact
• Principal decomposers
– Without them, carbon, nitrogen, and other
elements would become ties up in organic matter
• 10-50% of world’s fruit harvest is lost each
year due to fungus
• During the Revolutionary War, British lost
more ships to fungal rot than to enemy attack
Ecological Impact
• Pathogenic:
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30% of fungus is parasitic, mainly on plants
Dutch Elm: Killed ALL American Chestnut trees
Some release toxins that are carcinogenic: Aspergillus
Ergot poisoning: gangrene, nervous spasms,
hallucinations
• One epidemic killed more than 40,000 people in France in
944 A.D.
– One chemical from ergots is the raw material that LSD is made
• Helpful in treating high blood pressure and stopping
maternal bleeding after childbirth when given in small doses
Ecological Impact
• Mycosis: fungal infection
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Ringworm
Athletes foot
Histoplasmosis- causes tuberculosis-like symptoms
Opportunistic infections
• Only capable of occurring because body is not in homeostasis or the
immune response is lowered
– Yeast infections
• Food
– Cheeses
– Colas
– Truffles
• Use pigs to locate them because they give off a smell that mimics
female pig pheromones
Evolution of Fungi
• Oldest fossils of vascular plants have fungus
on them
– 460 million years ago
• Fungal ancestors were aquatic flagellated
organisms
• There is compelling evidence that animals and
fungi diverged from a common protistan
ancestor