Introduction to Mycology

Mycology

• Study of fungi

• Out of 100,000 species of fungi, only about 200 are pathogenic to humans and animals

• But they are also beneficial

• also important in food chain because they can decompose dead plant matter

fungi (kingdom)

• Chemoheterotrophs

• Acquire food by absorption

• Except for yeasts, fungi are multicellular

• Reproduction: sexual and asexual spores

fungi

• Eukaryotic, non-photosynthetic heterotrophs which produce exoenzymes and obtain nutrients by absorption

• Parasitic (e.g. dermatophytes causing ringworms)

• Saprophytic

• Mutualistic

• Overgrowth of yeasts (often commensals) → cause localized lesions

• Can tolerate high osmotic pressures and acidic environments as low as pH 5.0 (optimum pH is about 6)

• All are chemoheterotrophs (requiring organic compounds for energy and carbon)

• They are aerobic or facultatively anaerobic; few anaerobic

• Optimum temp. for growth of 20-30C; pathogenic fungi causing systemic mycoses can tolerate 37C

• comparatively slow-growing on lab media

• Reproduction by spore formation may be either sexual or asexual

saprophytic

decomposition of organic matter, causes sporadic opportunistic infections in animals

mutualistic

obligatory association with other microorganisms; non-pathogenic

• molds

• yeast

2 main morphological fungal forms

molds

• multicellular

• vegetative and reproductive phases are two distinct phases

• can grow as branching filaments called hyphae

hyphae

• branching filaments

• (singular: hypha)

• a mass of threadlike filaments which combine to make up the fungal mycelium

yeast

• Unicellular, non-filamentous

• the same cell function in vegetative growth (asexual) as well as in sexual reproduction

• oval or spherical appearance

• Frequently found as a white powdery coating on fruits and leaves

• budding

• Capable of facultative anaerobic growth

• If oxygen is present, yeasts perform aerobic respiration to metabolize carbohydrates into carbon dioxide and water

• If oxygen is absent, they ferment carbohydrates and produce ethanol and carbon dioxide.

• This fermentation is used in the brewing, wine-making and baking

• E.g. Saccharomyces species

septate hyphae

In most molds, the hyphae contain cross-walls called septa (singular: septum), which divide them into uninucleate cell-like units

coenocytic hyphae

in few class of fungi, the hyphae contain no septa and appear as long, continuous cells with many nuclei

Hyphal cell wall

• Impart rigidity and osmotic stability

• Mainly composed of carbohydrate components including chitin micromolecules with cellulose cross linkages

• In yeasts, contain protein complexed with polysaccharides

• Bilayer cell membrane, the predominant sterol is ergosterol in contrast to cholesterol

vegetative hypha

portion of hypha that obtains nutrients

reproductive/aerial hypha

portion concerned with reproduction; projects above the surface of the medium on which the fungus is growing; often bear reproductive spores

budding

• Parent cells forms a protuberance (bud) on its outer surface

• Bud elongates and parent cell’s nucleus divides

• One nucleus migrates into the bud

• Cell wall material laid down between the bud and parent cell

• But eventually breaks away

pseudohypha

formed when some yeasts produce buds that fail to detach themselves, forming a short chain of cells.

Saccharomyces species

produce ethanol in brewed beverages and carbon dioxide for leavening bread dough

dimorphic fungi

• Some pathogenic species of fungi can exhibit dimorphism (two forms)

• Moldlike form

• Yeastlike form

• Dimorphism is temperature dependent

moldlike form

produce vegetative and aerial hyphae

Yeastlike form

reproduce by budding

37C

at this temperature, the fungus is yeastlike

25C

at this temperature, the fungus is moldlike

Zygomycota

Sporangiospores are formed only by fungi in this phylum

microconidia

In dermatophytes, these multicellular structures are produced

Arthroconidia

formed from the disintegration of hyphae within keratinized structures

1. Ascomycota

2. Basidiomycota

3. Zygomycota

deuteromycota (Fungi imperfecti)

3 phyla; teleomorphs

teleomorphs

produce both sexual and asexual forms

zygomycetes

include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables; other food spoilage

Rhizopus stolonifer

common black bread mold

zygomycota

• “conjugation fungi”

• Saprophytic molds that have coenocytic hyphae

• E.g. Rhizopus stolonifer

• Asexual spores: sporangiospores

• Sexual spores: zygospores

ascomycetes

• Also called sac fungi

• Septate hyphae mostly but unicellular yeast may form pseudomycelium

• the septa possess central pores called septal pores

• Spores are produced in asci

• Reproduction happens asexually by budding or by spores

• Fertilization occurs in two steps: plasmogamy and karyogamy

• summary: sac fungi; include fungi that cause Dutch elm disease and rye smut (ergot)

basidiomycetes

• Also called club fungi

• Most advanced and most commonly seen

• Reproduce asexually by basidium

• most common decomposers of wood

• Decompose cellulose and lignin

• summary: club fungi; include common mushroom

Deuteromycetes

• Also known as fungi imperfecti

• Either saprophytic or parasitic

• Some members belong to both ascomycetes and basidiomycetes

• Reproduce only by asexual spores (e.g. conidia formation) produce a variety of toxins in foods, feeds, and grains in storage

• serious pathogens and are responsible for causing a variety of plant and animal diseases

• summary: also known as Fungi Imperfecti, include most of the disease-causing fungi such as those causing ring worms (dermatophytes)

fragmentation of hyphae

Filamentous fungi can reproduce asexually by __

spore formation

this reproduction may be either sexual or asexual and both may occur in some species

1. zoospores

2. sporangiospores

3. chlamydospores

4. blastoconidia

5. oidia

6. conidia

7. arthroconidia

8. macroconidia and microconidia

types of asexual spores in fungi

zoospores

motile spores; presence of flagella which help in swimming

sporangiospores

singles cell spores formed within sacs (known as sporangium) formed at the end of a special hypha (known as sporangiophore); non-motile; dispersed by air.

chlamydospores

thick-walled which contain storage products, single cell spores formed from cells of the vegetative hyphae by accumulation of protoplast and secretion of thick wall

blastoconidia

conidia which are produced by budding as in Candida albicans

oidia

formed under adverse conditions; individual cells separated from hyphae; small yeast-like cells; thin walled spores

conidia

• small or large, single cell spores formed at the tips or sides of a fungal hyphae conidiophores.

• Ex. Penicillin, Aspergillus

arthroconidia

spores which are formed and subsequently released during the process of hyphal fragmentation. Spores may be formed successfully as in dermatophytes. (A) or with intervening empty cells as in Coccidioides immitis (B)

Macroconidia and microconidia

large and small multi-celled conidia which are produced by dermatophytes in culture; lack mitochondria

fungal reproduction

formation of spores

asexual

• sexual organs are not involved •

• Methods of asexual reproduction

  • Budding

  • Single fission

  • Fragmentation

  • Spore formation

sexual

involves fusion of compatible nuclei from 2 parent cells

1. antheridium

2. oogonium

sex organelles of fungi

antheridium

Male gametangia

oogonium

Female gametangia

sexual reproduction

involves 3 phases:

1. plasmogamy

2. karyogamy

3. meiosis

plasmogamy

• fusion of protoplasm

• Anastomosis of two cells or gametes and fusion of their protoplasts takes place

• In the process, the two haploid nuclei of opposite sexes (compatible) nuclei) are brought together but the nuclei will not fuse

• Various methods by which compatible nuclei are brought together in plasmogamy. Some are:

  • Gametic copulation

  • Gamete-gametangial copulation

  • Gametangial copulation

  • Somatic copulation

  • Spermatization

karyogamy

fusion of nucleus

meiosis

reductional nuclear division; Diploid nucleus reduced to 4 haploid nucleus

planogametic copulation or conjugation

• motile gametes are called planogametes. The fusion of two gametes, one or both of which are motile is called_____

• There are three different types:

  • isogamy

  • anisogamy

  • oogamy

isogamy

• morphologically similar but compatible type of gametes unite to form a motile zygote

• e.g. Synchytrium

anisogamy

• union of one larger gamete with another smaller gamete

• the resultant zygote is motile

• the zygote resulting from isogamous or anisogamous planogametic copulation forms a ‘resting sporangium’

• e.g. blastocladiales

heterogamy/oogamy

• a non-motile female gamete (oosphere) is fertilized by a motile male gamete.

• this results in the formation of oospores

• e.g. Chytridiomycetes

oospores

• a resistant structure and resting spore

• germinate and produce mycelium directly

gametangial contact

• in this method, the male gamete and the female gamete come in contact and one or more nuclei from the male gamete enter the female gamete. oogonium dissolved in the intervening wall through a pore or through a fertilization tube

• in no case the gametangia actually fuse or lose their identity during the sexual act

• e.g. fungi in peronosporales and common in some ascomycota where antheridia and female organs (ascogonia) may or may not be well define

gametangial copulation

• a process of fusion of entire contents of the two mating gametangi

• 2 types:

  • anisogametangial copulation

  • isogametangial copulation

anisogametangial copulation

• fusion between unequal gametangia. the zygote forms a resting sporangium

• e.g. aquatic fungi (chytridiomycetes)

isogamous copulation

• 2 morphologically similar gametangial hyphae come in contact, the wall at the point of contact dissolves and the contents mix in the cell thus formed

• this results in the formation of zygospore

• e.g. Mucor, Rhizopus, Phycomyces

spermatization

• some fungi like rusts bear numerous minute, non-motile uninucleate, male cells called spermatia (sing. spermatium)

• spermatia are produced in spiral receptacles called spermagonia (sing. spermagonium); or pycnia (sing. pycnium)

• spermatia carried by insects, wing, or water to the female gametangium, which is usually a special receptive hypha (or trichogyne) to which they become attached

• a pore develops at the point of contact and the contents of spermatium pass into the particular respective hyphae

somatogamy

• no sex organs are produced and somatic cells function as gametes

• common in ascomycota and basidiomycota fungi

1. ascospores

2. basidiospores

3. zygospore

4. oospores

types of sexual spores

ascospores

non meiotic spores produced inside specials sacs called asci; produced by members of ascomycota

basidiospores

non-motile meiosporeformed on short outgrowths of club-shaped structures called basidium

zygospore

large, thick-walled spores formed at the tips of 2 sexually compatible hyphae; produced by members of Zygomycota

oospores

spores formed within a specialized female structure (oogonium) after fertilization of eggs by male gametes formed in an antheridium