Molds, mildews, & mushroomsAn introduction to fungi

Fungi• Large monophyletic Kingdom –

– Ca. 98,000 species described– Estimated to be 1.5 million species

• Significant group of organisms – both basic and applied biology

• Fungi play variety of roles – can be either destructive or beneficial

A number are plant pathogens• Fungi are major pathogens of

plants – cause great deal of damage to crops (losses of 20-50% of yield)

• Example: Wheat rust– Major losses in yields of wheat– Romans had a god of rust - Robigus– In Southeastern US

• Wheat rust severe, can’t grow wheat

• Corn replaced wheat – cornbread, bourbon

Coffee rust epidemic in Ceylon

Irish potato famine

• Potato imported to Europe in 1500’s, became a staple

• Several years of favorable weather lead to epidemic of late blight of potatoes

• Caused the Irish potato famine – mid 1800’s

Late blight of potato• Caused by

Phytophthora infestans

• No longer considered a true fungus

Fungal diseases of animals & humans

• Generally less widespread than diseases caused by bacteria and viruses (exception – athletes foot), but can be severe

• Certain diseases endemic to geographic areas – e.g. valley fever caused by Coccidioides sp. in the desert SW

• Incidence of human mycoses is on the increase with use of immunosuppresant drugs and AIDS

Fungi as symbionts• Mycorrhizal

association is very common among plants

• Lichens

Fungi as decomposers

• Important agents in the flow of energy and cycling of nutrients in biosphere

• Decomposition particularly of plant litter

• Food for higher trophic levels

Fungi as agents of deterioration

• Cause deterioration of organics –food, wood, clothing, leather

• Variety of antifungal products developed

Food and food supplements

• Yeast – bread and alcohol• Mushrooms• Fermentation of plant

products (soybeans) – soya sauce, tempeh, miso tofu,etc.

• Biotechnology – enzymes, antibiotics, statins, organic acid production

Mycotoxins• Produced by microfungal species on

different stored food products• Aflatoxin produced by Aspergillus flavus

Allergens

• Spores present in outdoor & indoor air

• Sick building syndrome• Stachybotrys

Myths & Religions

• Hallucigenic mushrooms

Used as model systems in biology

• Neurospora crassa – 1 gene – 1 enzyme

• Saccharomyces• Aspergillus

Major characteristics of fungi

• Eukaryotic• Heterotrophic• Osmotrophs• Modular organisms – indeterminate growth• Multinucleate• Generally surrounded by cell wall• Generally nonmotile (some produce motile cells)• Most form spores as reproductive units

Fungal biology

• Fungal body = thallus

• Vegetative (somatic) phase – absorbs and assimilates nutrients, grows

• Reproductive phase - all or part of thallus may differentiate to form reproductive structures – spores

• Asexual (mitosis) or sexual reproduction (meiosis, fertilization)

Vegetative thallus

• Variable – but most fungi form a branching network of multinucleate filaments

• Filament = hypha (pl. hyphae)

• Exceptions – yeasts, some lower fungi

Mycelium = filamentous fungal thallus

• All hyphae in a thallus form the mycelium

Largest mycelia • Armillaria gallica – fungus humongous

• 30 acres, 10 tons, 1500 years old – now larger Armillaria species have been found

Hyphae• Walled tubes that contain cytoplasm• Eukaryotic – nuclei, mitochondria, ER, etc• Grow at tip• Form lateral branches that grow at tip

Basic types of hyphae

• Aseptate hyphae – lack crosswalls, found in lower fungi

• Septate hyphae – crosswalls divide hyphae into compartments, are incomplete, found in higher fungi

For filamentous fungi• What is a cell?

– Not typical with one nucleus controlling a defined volume of cytoplasm, many nuclei occur together

– Have age transitions – tip is young with senescent cytoplasm away from the tip

• What is an individual?

Hyphae from two individuals may fuse • two or more one

Hyphal pieces may be separated from thallus • one two or more

Basic structure of hyphae

• Size – 2 μm to 1 mm in diameter, – 5-10 μm most common

• Surrounded by cell wall – gives hypha shape and prevents it from bursting from osmotic pressure– Cell walls composed primarily of polysaccharides

with less than 10% proteins and lipids– Microfibrillar polysaccharides embedded in an

amorphous matrix

Cell walls• Microfibrillar polysaccharides – in most fungi is

chitin – polymer of N-acetyl glucosamine ( 1,4) • Chitin accounts for 5-60% of cell wall• Amorphous matrix contains a variety of

polysaccharides – glucans, mannans

Fine structure of hyphae• Apical region of hyphal tip – typically 150-

500 μm long.• Dense cytoplasm, rich in organelles, few

vacuoles• Extreme tip (1-5 μm) contains many small

vesicles (the apical vesicle cluster, AVC) – with light microscope = spitzenkörper (in sepatate hyphae only); no other organelles

• Behind the apical region, vacuoles increase in number and size, lipid granules accumulate

Fungal hypha

Hyphal tip

Nuclei• Small (1-2 μm)• Nuclear envelope does not break down during

division, chromosomes not distinct• In apical region there are 1-50 nuclei• Fungal DNA less complex than other

eukaryotes - fewer repeated DNA segments compared to other eukaryotes (less than 10% compared to 35% in mammals)

Nuclei

Organelles

• Mitochondria – elongate with platelike cristae

• Endoplasmic reticulum – narrow membrane bound channels

• Golgi – ringed cisternae not flattened stacks

Hyphal growth

• Growth occurs at the tip

• Grow in pulse of radiolabelled wall precusor

• Vesicles also concentrated at tip – few other organelles in tip

Hyphal growth• Not the same as cellular growth • Filamentous fungi do not undergo cell division

after cell has doubled in size• Duplication cycle (analogous to cell cycle)• Apical compartment grows to a particular length,

nuclear division followed by formation of 1 to several septa

Hyphal branching

Hyphal branching

Carbon nutrition of fungi• All fungi are heterotrophic – obtain C from

organic sources

• In this regard, they are like animals, but

• Absorb nutrients from environment, do not ingest food

Three modes of heterotrophy• Saprotrophs (saprophytes)– obtain C from

non-living organic matter

• Biotrophs – obtain C from living organic matter– Parasites– Symbionts

• Necrotrophs – kill organisms and use C of dead bodies – Parasites

• Facultative or obligate

Fungi and Plants

• Originally fungi were classified with plants – nonmotile, have cell wall

• Are also similar to plants in that they are modular organisms not unitary like animals

• Modules in plants – buds and branch; in fungi – hyphal tips

• Comparison of characteristics of modular with unitary organisms

Unitary vs modular organismsCharacteristic Unitary Modular

Growth pattern determinate Indeterminate, iterative

Size Adults vary little Varies greatly

Mobility mobile immobile

Acquisition of resources

Use mobility Grow to resources

Reproductive capability

Increases with age, decreases

Can increase indefinitely

Unitary vs modular organisms

Clonal reproduction

Unusual Common

Internal age structure

Absent Present

Longevity Definite life span

Indefinite life span

Local damage Serious Unimportant

Vegetative phase Reproductive phase

• After period of growth, differentiation may occur

• Reproduction can be sexual or asexual

• Functions of reproduction

– Recombination of genetic information

– Propagation and dispersal of fungus

– Dormancy – dispersal through time

Asexual reproduction• Fragmentation of thallus

• Production of asexual spores (two main types)

– Sporangiospores – develop within a sporangium

• Lower fungi

– Conidium (pl. conidia) – formed externally at the tips of specialized hyphae

• Higher fungi

Sporangiospores

Conidia

Sexual reproduction

• Three events occur in sexual life cycle– Plasmogamy – union of two cytoplasms– Karyogamy – union of two nuclei– Meiosis – reduction division

• (In most organisms plasmogamy and karyogamy occur close together – called syngamy or fertilization

Sexual life cycle

• The three events lead to three phases based on nuclear condition

– Haploid (1n) – one set of chromosomes per nucleus

– Diploid (2n) – two sets of chromosomes per nucleus

– Dikaryotic (n+n) – two sets of chromosomes in separate nuclei

• Vegetative phase may be any of the above

Sexual life cycle

Sexual reproduction

• Spores are typically produced

• May be produced after plasmogamy & karyogamy – diploid (zygospores)

• May be produced after meiosis – haploid (meiospores)

Reproductive phases• Fungi may carry out sexual reproduction,

asexual reproduction or both types

• The sexual reproductive phase is the teleomorph (perfect phase)

• The teleomorph is used in phylogenetic classification system

• No problem for those species that reproduce sexually or both sexually and asexually – can identify and classify based on teleomorph

Reproductive phases• Asexual reproductive phase = anamorph

(imperfect phase)• For those fungi that only produce

anamorph, there is a problem – can’t classify in the phylogenetic classification system for fungi

• Sometimes difficult to connect anamorph and teleomorph - holomorph

• Separate artificial classification scheme set up for anamorphs

Phylogeny of fungi

• Traditionally fungi were classified with plants

• Theories that fungi evolved from algae

• Today, considered that fungi did not evolve from algae and are not very closely related to plants – more closely related to animals

Evidence for phylogeny of fungi

• A number of lines of evidence that fungi are not closely related to plants

• Most compelling evidence comes from recent work comparing base sequences of rDNA and other genes among organisms

Kindom Fungi• “lower fungi”

– Zoosporic fungi –• Chytridiomycota -706 spp,

Blastocladiomycota –179 spp, Neocallimastigomycota -20 spp

– “Zygomycota” – four subphyla– Glomeromycota – 169 spp.

• “higher fungi”– Ascomycota – 64,163

spp.– Basidiomycota – 31,515

spp.

Fungal phyla – lower fungi

• Zoosporic fungi• Chytridiomycota • Blastocladiomycota• Neocallimastigomycota

• “Zygomycota”

Fungal phyla – higher fungi

• Ascomycota • Basidiomycota

Classification

• Endings for major taxa of fungi

– -mycota = phylum (division)

– -mycotina = subphylum

– -mycetes = class

– -ales = order

– -aceae = family