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