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What Fungi are Like
- Most fungi are filamentous
- Most fungi produce spores
How Fungi are Classified
- Zygospore fungi form zygospores
- Sac fungi form ascospores
- Yeasts are single cells
- Club fungi have basidiospores
* Rusts & smuts are parasites
- Imperfect fungi reproduce asexually only
Fungi Form Symbiotic Relationships
- Lichens are soil farmers
- Mycorrhizae are “fungus roots”
FUNGI
- Eukaryotic
- Heterotrophs- Acquire nutrition thru absorption
- Digest food outside body by secreting hydrolytic
enzymes, called exoenzymes
- This decomposes complex molecules into simpler
molecules.
- Small organic molecules are absorbed from
surrounding medium
- Fungi can be decomposers (saprobes), parasites, or
mutualistic symbionts
Structure of Fungi
- Body is composed of tiny filaments, called hyphae
(singular; hypha)
- Hyphae form an interwoven mass called a
mycelium (singular; mycelia)
- Can be huge, up to 3.4 miles in diameter (1600
football fields!)
- Most are multicellular- long hyphae are divided
into cells by cross-walls, called septa (singular;
septum)
- Cell walls differ from CW of plants- They don’t
contain cellulose, they contain chitin
- Some fungi do not contain septa; Coenocytic fungi;
hyphae are a continuous cytoplasmic mass w/
100’s or 1000’s of nuclei.
- Filamentous structure of mycelium gives high
surface area for maximized absorption of nutrients
- Fungus constantly puts its energy into increasing
hyphal length.
Reproduction of Fungi
- Release spores that are produced sexually or
asexually
- Fungi can release trillions of spores that are
carried by wind or water
- Spores will germinate into new mycelia if they
land in a moist place that has food.
Nuclei & Spores of Fungi
- Haploid in most species
- Sometimes have diploid stages during sexual life
cycles
- But, sometimes 2 hyphae fuse that have 2
genetically different nuclei- Heterokaryon
- These different nuclei may stay in different
parts of the mycelium
- Or, they may mingle & exchange genetic info.
in a process similar to crossing over
- In many fungi w/ sexual life cycles, union of
partners occurs in 2 stages
1) Plasmogamy- 2 parents’ cytoplasm fuse when
their mycelia come together.
2) Karyogamy- Haploid nuclei of parents fuse
- Plasmogamy & karyogamy may be separated by
hours, days, or even years
- In the meantime, the haploid nuclei of both
parents stay separate
- Sometimes, the haploid nuclei pair off, 2 to a cell,
one from each parent- Dikaryotic
Diversity of Fungi
- More than 100,000 species known, up to 1.5 million
- 4 Phyla
1) Chytridomycota- Mainly aquatic
- Some saprobes
- Others parasitize protists, plants, & animals
- Chytrids used to be classified as protists, b/c
they have flagellated spores called zoospores
- Form coenocytic hyphae
- The most primitive fungi
2) Zygomycota- Mostly terrestrial & live in soil or
on decaying plant & animal material
- One group forms mycorrhizae- mutualistic
association w/ plant roots
- Hyphae only contain septa where reproductive
cells are found
- Example- Rhizopus stolonifer
- Horizontal hyphae spread over food,
penetrate it, & absorb nutrients
- In asexual phase, bulbous black sporangia
form at tips of hyphae.
- Each sporangium hold 100s of haploid spores
- Spores are dispersed into air & land on food
for growth of more mycelia
- If conditions get worse, species reproduces
sexually
- Parents have opposite mating types.
- Plasmogamy results in a resistant
structure called zygosporangium
- It is resistant to freezing & drying,
waits until conditions improve, then it
releases genetically diverse haploid spores
3) Ascomycota- Also known as sac fungi
- Marine, freshwater, & terrestrial
- Range from simple unicellular yeasts to
elaborate cup fungi & morels
- ½ of them live mutualistically with algae, called
lichens
- Some form mycorrhizae w/ plants
- Produce sexual spores in saclike asci (singular;
ascus)
- Unlike zygomycetes, most sac fungi show their
sexual stages in macroscopic fruiting bodies,
called ascocarps
- Asexually, they form spores externally.
- Zygomycetes form their spores inside
sporangia
4) Basidiomycota- Club fungi
- Examples- mushrooms, puffballs, & rusts
- Decompose wood & other plant material, b/c
they are best at decomposing complex polymer
called lignin, an abundant component of wood
- Some also form mycorrhizae or parasitize
plants
Life Cycle- Long-lived dikaryotic mycelium
- In response to environmental stimuli, the
mycelium reproduces sexually by producing
elaborate fruiting bodies called basidiocarps
- A mushroom is an example of a basidiocarp
- It may release a billion basidiospores
Different way of classification
1) Molds- Rapidly growing, asexually reproducing
fungus
Ex- Rhizopus
Later in life, this fungus may reproduce sexually,
producing zygosporangia, ascocarps, or
basidiocarps, depending on the type of fungus
2) Yeasts- Unicellular fungi that inhabit liquid or
moist habitats
- Reproduce asexually, by simple cell division
3) Lichens- Symbiotic association of millions of
photosynthetic microorganisms held together by
a mesh of fungal hyphae
- Microoranisms are usually green algae or
cyanobacteria
- Fungi is usually an ascomycete or
basidiomycete
4) Mycorrhizae- Mutualistic association of plant
roots and fungi
- They exchange minerals that the fungi picks
up for organic nutrients that the plant makes
- Zygo-, Asco-, and Basidio-, all have members
that form mycorrhizae