Fungus-like protists Fungi and animals share a common ancestor with amoebozoans (amoebae + slime...
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Transcript of Fungus-like protists Fungi and animals share a common ancestor with amoebozoans (amoebae + slime...
Fungus-like protists
Fungi and animals share a common ancestor with amoebozoans (amoebae + slime molds)
Fungus-like protists
1) Plasmodial slime molds (Physarum, below)
2) Cellular slime molds Dictyostelium…
Plasmodial slime mold life cycle
spores hatch into2 haploid forms
that switch back + forth,fuse to form a zygote
Cellular slime mold life cycle
fruiting body forms; some cells become non- reproductive stalk
thousands of unrelatedhaploid cells aggregate to form a single “slug”
2 cells may fuse into a“giant cell” (2N) that eats other amoebae as it grows
Fungi
Heterotrophic decomposers - feed by absorbing nutrients
Single-celled (yeasts) or multicellular
May be free-living, parasitic, or mutualists (= symbionts)
Bodies composed of threadlike structures called hyphae
Cell walls contain chitin
Key Terms:
Symbiosis – two organisms (or species) living in close association (e.g. mycorrhizae)
mutualism – both benefit
parasitism – parasite benefits, host suffers
commensalism – one benefits, the other is unaffected
Endosymbiosis – one organism living inside another (host)
Examples: Plastids, mitochondria evolved through endosymbiosis
Rhizobium bacteria living in root nodules – fix nitrogen
Lichens – algal symbionts within a fungal host
Heterotrophs – ingest or absorb an external source of organic carbon (molecules with CH’s)
- Fungi are heterotrophs that absorb organic carbon from their surroundings
Decomposers (saprobes) – break down complex organic molecules into simpler organic molecules
- Fungi and bacteria are important decomposers, recycling carbon and other nutrients
Biogeochemistry – the (re-)cycling of key elements O, C, N, P, S
Basic fungal structure: hyphae and mycelia (singular: mycelium)
Hyphae release digestive exo-enzymes into their surrounding
Hyphae form a web called the mycelium that greatly increases surface area, maximizing uptake of dissolved nutrients from the substrate (= whatever the fungus is growing on)
Hyphae release digestive exoenzymes into their surrounding
- break down organic matter into small molecules that can be absorbed
Produce enzyme that can break down..
1- lignin, a complex polymer that makes wood tough
2- cellulose, a polymer of glucose (sugar) that animals can’t break down (hence why termites + cows need gut symbionts)
Fungi grow by extending the tips of their hyphae through cytoplasmic streaming
Hyphae can be divided into individual cells by partitions called septa
Some fungi grow by repeated mitotic divisions of nuclei without cell division coenocytic condition
- giant multi-nucleated cells, similar to slime molds
Some fungi form mutualistic or parasitic associations with plants
Use special hyphae called haustoria to penetrate cell wall of plants
- push into cell surrounded by plant plasma membrane
Fungal Life Cycle
Fungi spread by producing huge #’s of spores
- structures that resist harsh environmental conditions
- can disperse long distances by wind
Produced during both sexual and asexual phases of life cycle
2N
N
N+N
Stage 1: PlasmogamyMost hyphae contain haploid nuclei
2 hyphae of different mating types can grow together + fuse = plasmogamy
2N
N
N+N
Stage 2: KaryogamyEventually, 2 haploid nuclei from different parents fuse into diploid nuclei
Zygote quickly undergoes meiosis, producing haploid spores
2N
N+N
Heterokaryotic stage = separate haploid nuclei from different parents, in the same hyphae
Karyogamy = the 2 haploid nuclei fuse into 1 diploid nucleus
(like syngamy)
Fungal Evolution
Fungi evolved from an ancestor that was an aquatic protist with a flagellum (like sperm cells of animals)
Molecular evidence indicates this ancestor was also single-celled
- thus, animals & fungi independently evolved multicellularity
- only primitive fungi have flagellated spores
Campbell & Reece 2002
phylogeny assuming flagellae were lost once
new phylogeny indicates flagellae were lost often
Chytrids
May be single-celled or form multi-cellular hyphae
Only fungi w/ flagellated spores, called zoospores
Relationship to Zygomycetes is still controversial
Aquatic
Chytrids
Branching hyphae increase surface area for uptake of nutrients from surrounding aquatic medium
zoospore
Zygomycetes life cycleHaploid
1) hyphae of opposite mating type fuse to form heterokaryotic (N+N) zoosporangium
- contains many haploid nuclei from each parent
- resists bad conditions
1
Haploid
2) when conditions get better, karyogamy occurs: haploid nuclei fuse into diploid nuclei
- zygote then undergoes meiosis, producing genetically diverse spores
- regular sporangia form
1
Zygomycetes life cycle
Haploid
3) Regular sporangia may form and produce spores by mitosis (asexual reproduction)
1
3
Zygomycetes life cycle
Pilobolus sp. – dung fungusMicrosporidia - highly modified parasites (Encephalitozoon intestinalis)
Phylum Zygomycota (Zygomycetes)
Phylum Glomeromycota - Glomeromycetes
Formerly put in zygomycetes; now their own phylum
Only 160 known species, but ecologically critical
Form endomycorrhizae, mutualistic associations inside of plant roots
>90% of plants have endomycorrhizae associated with roots
Fungal partner takes up minerals like phosphate from soil, transfers them to root tissue of host plant
Endomycorrhizae
Hyphae penetrate cell walls, but do not puncture plasma membrane of plant cells
Instead, push inside host cell surrounded by plant membrane like fingers in a glove
Taiz & Zeiger 2002
Phylum Glomeromycota - Glomeromycetes
Formerly put in zygomycetes; now their own phylum
Only 160 known species, but ecologically critical
Phylum Ascomycota (ascomycetes)
“Sac fungi” produce sexual spores in saclike asci
Hyphae fuse into N+N heterokaryotic stage
Septa form cells with 2 haploid nuclei each
- one of these grows into an ascus
Developing asci are housed in the ascocarp, the fruiting body that will later eject the spores
Karyogamy in the ascus combines both parental genomes meiosis produces 4 haploid cells
Each undergoes mitosis 8 haploid ascospores
Hyphae fuse into N+N heterokaryotic stage
Septa form cells with 2 haploid nuclei each
- one of these grows into an ascus
Penicillium sp. – an ascoymycete (formerly called a deuteromycete)
- source of antibiotic penicillin
Campbell & Reece 2005
Saccharomyces sp. -- “yeast”
Common disease organisms
Used by humans to ferment sugar in dough or grains, for baking or production of adult beverages
- under anaerobic conditions, metabolize sugar to ethanol and CO2 (makes dough rise)
S. cerevisiae is model eukaryotic cell for molecular biologists to study
- 1st fully sequenced eukaryotic genome
Saccharomyces sp. - “yeast”
Many yeasts have no known sexual stage
Yeast cells budding (asexual reproduction)
Phylum Basidiomycota - basidiomycetes
Campbell & Reece 2005
includes common mushrooms, toadstools, shelf fungi
important decomposers of wood
long-lived heterokaryotic stage, giving rise to basidiocarp (“mushroom”) in bad conditions
A single mushroom cap produces a billion basidiospores
heterokaryoticN+N
Phylum Basidiomycota – sexual stages
Basidiocarps of a basidiomycete form a “fairy ring” overnight
Heterokaryotic mycelium connects mushrooms underground
- expands outward, digesting organic matter in soil
Ectomycorrhizae Freeman 2005
Many basidiomycetes form ectomycorrhizae with plant roots
- roots are completely covered in a layer of hyphae
Exoenzymes release nitrogen from decaying matter transferred to hyphae extending in between outer-most root cells
Self-quiz: Know the 4 different types of reproductive structures that are characteristic of the different fungal phyla
Fungal SymbiosesLichens – fungus & alga mutualism
- Endosymbiont is usually a chlorophyte (green alga)- Fungus is the host
Mycorrhizae – fungus & plant root mutualism
Endomycorrhizae - glomeromycete symbiont(inner)
Ectomycorrhizae - basidiomycete symbiont(outer)
Parasitic fungi cause many plants & animal diseases
Lichens – mutualism between fungus (Ascomycete) & green alga
3 growth forms of lichens: - Foliose (leaf-like) - Crustose - Fruticose (shrub-like)
This relationship evolved 3 separate times
Lichens – mutualism between fungus (Ascomycete) & green alga
This relationship evolved 3 separate times
Enable plants to eventually grow on what was bare rock by eroding the rock surface, trapping soil
Are very sensitive to air pollution, acid rain
Ecto-mycorrhizae Endo-mycorrhizae
Taiz & Zeiger 2002
Fungi are responsible for many plant diseases; destroy 10-50% of crops worldwide
Eating fungus-infected grains is a severe human health issue in much of the world
- contributes to high rate of liver cancer in areas that consume peanut meal, which supports fungi that produce liver toxins
Historically, eating ergot-infected grain caused outbreaks of madness and death, once thought to be demon possession
- ergot fungus produces lysergic acid, similar to LSD