Chapter 31: Fungi

Unikonta= amoeba, fungi, animals, and some other protists 

• Unikonta divided into two main groups 

  • Amoebozoa 

    • Slime molds, other amoebas

  • Opisthokonta 

    • Animals 

    • Choanoflagellates

    • Fungi

    • Nucleariids

  • Unclear when fungi appeared

    • Molecular clock: 1 bya for animal-fungi split

    • Oldest fossil fungi: 460 mya

  • Even though fungi are often taught with plants, they have much more in common with animals 

Fungi are Opisthokonts

• Shared derived character (synapomorphy) of Opisthokonta: flagellate cells propelled by posterior flagellum, sometimes only ancestrally or at a specific point in their life cycle 

Synapomorphy for Fungi

• Synapomorphy (shared derived character) that defines fungi: cell wall made of glucans and chitin

• Both are complex chains of sugar monomers 

Fungi are (mostly) multicellular heterotrophs

• Absorb nutrients without ingestion (unlike animals) 

  • Secrete enzymes that break down many compounds: good decomposers

• single - vs multi-celled

  • Single celled: yeast

  • Multi-celled filaments: hyphae

  • Filaments form masses: mycelia 

• Cell structure

  • Cell wall with chitin

  • Hyphae may be divided by septa or not

  • Coenocyte = one giant cell with many nuclei 

Fungi have Complex Life Cycles

• Most life history stages of fungi are haploid 

• Reproduce asexually by producing spores

• Sexual reproduction: produces genetic variation 

  • Plasmogamy: fusion of cytoplasm of 2 different ‘mating types’ 

  • heterokaryon/dikaryon: mixture of haploid parental nuclei (n+n)

  • Karyogamy: fusion of nuclei (2n)

  • spores (n) produced by meiosis 

• Asexual reproduction

  • Spores (n) produced by mitosis

Important association with plants

• Parasitic/mutualistic relationships between fungi and plant roots usually involve haustoria 

  • Perforate plant cell wall but not plasma membrane

• Mycorrhizal fungi have a mutualism with plant roots 

  • Fungi provides phosphorus, other nutrients

  • Plant provides carbohydrates 

  • Two kinds

    • Arbuscular mycorrhizae: haustoria 

    • Ectomycorrhizae: form sheaths around roots

  • Extremely important to plants

    • Most vascular plants have mycorrhizae

100,000 fungal species in 5 major clades +2 

• Cryptomycetes & microsporidians 

  • Genomic studies indicate that these two groups form a sister group that are a basal fungal lineage

  • We need more data!

1. Chytrids 1000 spp. 

• Sister to other fungi; flagellated zoospores 

• Decomposers, parasites, and some mutualists

• Some causing major problems for frogs

2. Zoopagomycetes 900 spp. 

• parasites/commensals on animals 

3. Mucormycetes 750 spp. Decomposers 

• Fast growing molds causing foods such as bread, strawberries, peaches to rot 

  • Like rhizopus, black bread mold

• Some live as parasites or pathogens of plants (mycorrhizae) 

4. Ascomycetes 90,000 spp.: sac fungi

• Sexual spores formed in asci with ascocarps (fruiting bodies)

5. Basidiomycetes 50,000 spp.: mushrooms, puffballs, shelf fungi

• Sexual spores formed in basidia, with basidiocarps (fruiting bodies) 

Chytrids

• Sister taxon to a clade that makes up the rest of fungal diversity 

• Aquatic 

• Use zoospores for reproduction

  • Motile asexual spore that use a flagellum 

  • Unique in fungi 

• No documented sexual reproduction 

• Can cause chytridiomycosis in amphibians 

  • Easily spread by human activity 

Ascomycota 

• Very diverse

• Yeasts and multicellular species

• Some mycorrhizae 

• 40% of species are lichens with green algal or cyanobacterial symbionts 

• Life cycle

  • …

Ascomycota as Food

• Some of the more unusual fungi that humans eat (not typical mushrooms) 

• Truffles, morels, and ergot

• May looks gross but can be valuable ($$$) 

• Yeasts - bear, wine, breadmaking, vegemite, etc. 

Ascomycota as medicine 

• Free-living forms include Penicillium 

• Penicillin, like many fungi, produces antibiotics to help it compete with bacteria (penicillin)

Basidiomycota

• Decomposers, especially of wood (lignin)

  • Ex: shelf mushrooms

• Also part of lichens

  • Sometimes in na ascomycete-basidiomycete-algae trio 

• May produce basidiocarp (“mushroom”)

  • Gills of basidiocarp lined with basidia: dikaryotic cells at the end of hyphae 

  • Karyogamy (fusion of 2 nuclei) of basidia (2n) meiosis produces basidiospores (100s of millions) 

• Mushrooms, toadstools, puffballs, bracket fungi

• The more typical mushroom we eat are in this group 

• Only ~75 sp. Known to be poisonous 

• Common misconceptions:

  • Silver coins cooked with mushrooms, turn black with poisonous mushrooms

  • Easy to peel = edible 

  • Poisonous species only grown in spring and fall

  • Purple = poisonous 

  • If snails and beetles can eat them they are safe

Harmful Basidiomycetes

• About ⅓ of fungi parasites/pathogens

  • Especially on plants (80% of plant diseases) 

• Plant parasites

  • Cryphonectria parasitica kills american chestnuts 

  • Other parasites on crops 

    • Decrease yields

    • Sometimes toxic to humans 

      • Claviceps purpurea is a parasite of rye that causes ergotism 

  • Animal parasites

    • Skin (external) mycosis: athlete’s foot, jock itch, etc. (some animal parasites in other groups too)

    • Systemic (internal) mycosis: can be caused by inhaled spores 

Helpful Fungi

• Mycorrhizae and lichens

  • Phototroph and heterotroph exchanging products

• Ascomycete endophytes between leaf cells 

  • Release compounds that protect plant from insects 

• Food: mushrooms, truffles, cheeses, bread, beer, wine, etc. 

• Medicine: antibiotics and other drugs

• Bioengineering: Fungi can make eukaryotic products that bacteria can’t 

Lichens

• Symbiotic relationship between fungi and algae (green alga or cyanobacteria)

  • Alga provides food for both 

  • Fungus absorbs H₂O & minerals for both; protects alga from excess UV rays 

• Slow-growing

• Can live in harsh environments like bare rock that gets lot of UV rays but do not tolerate pollution well 

Cordyceps- “mind controlling” ascomycetes that parasitize insects 

Psilocybe- “magic mushrooms”