Chapter 22

Fungi Evolution

And Diversity

• Although fungi were once classified as plants, they are more like animals

• Fungi are heterotrophs, but digest their food externally and then absorb it

• Largest living organism is a honey mushroom which covers 2200 acres underground in Blue mountains of Oregon

• Fungi release digestive enzymes into the environment then absorb the resulting nutrients

• Chytrids are different from all other fungi because they are aquatic and have flagellated spores and gametes

• Fungi trace their ancestry to protists and share common ancestor with animals after plants split away

• Most likely ancestor of fungi and animals was a flagellated unicellular protist

• Very early plant fossils have mycorrhizae associations

• Some fungi including yeasts are unicellular, but vast majority are multicellular

• Thallus or body of most fungi is a multicellular structure called a mycelium

• A mycelium is a network of filaments called hyphae allowing a large surface area to volume ratio

• Fungal cells are quite different from plant cells because they lack chloroplasts and have cell walls that contain chitin not cellulose

• Chitin is a polymer of glucose molecules with a nitrogen-containing amino acid group attached to it

• Fungi store energy in glycogen like animals

• Terrestrial fungi lack mobility and move toward food by growing the hyphae

• Fungi that have cross walls are called septate

• Nonseptate fungi are multinucleated with many nuclei in hyphae cytoplasm

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• Terrestrial fungi sexual reproduction involves these steps

• During sexual reproduction, hyphae from two different mating types make contact and fuse

• Paired haploid nuclei n + n is called dikaryotic

• They eventually fuse

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meiosis

haploidhyphae

dikaryoticstage

diploidzygote

• As adaptations to life on land, fungi produce nonmotile, but windblown sexual and asexual spores

• Spore is a reproductive cell that develops into new organism without need to fuse with another reproductive cell

• Large mushrooms may produce billions in a few days

• When a spore lands on an appropriate food source, it germinates and begins to grow

• R.H. Whittaker separated fungi from the other eukaryotic kingdoms because they are the only multicellular saprotrophs

• Today some place fungi in supergroup Opisthokonts with animals and certain heterotrophic protists

• Fungi differentiated by life cycle and type of structure used to produce spores:

• Chytrids are the simplest and resemble first fungi

• They are aquatic or live on moist soil with flagellated cells

• Most reproduce asexually, but some have an alternation of generations life cycle like green plants and some algae

• Zygospore fungi are saprotrophic living on plant and animal remains or bakery goods

• Black bread mold, Rhyopus stolonifer, is often found on bread

• Stolons are horizontal hyphae on the surface of the bread and rhizoids grow into the bread to carry out digestion

• A sporangium is a capsule that produces spores called sporangiospores

• Hyphae of opposite mating types, termed plus and minus, are chemically attracted, touch, and form gametangia

• These merge and result in a large nucleate cell where mating types pair and fuse

• It becomes a zygospore that undergoes meiosis and germination

• AM fungi stands for arbuscular mycorrhizal fungi

• Arbuscules are branching invaginations the fungus makes as it invades plant roots

• Mycorrhizae are a mutualistic association that benefits both fungus and plant

• Sac fungi are divided into two main groups: the sexual sac fungi in which sexual reproduction has been known and

• Asexual sac fungi in which sexual reproduction has not been observed

• Examples of sexual sac fungi include:

• Yeast, Saccharmyces, that are used in baking and brewing industries

• Neurospora, the experimental material for one-gene-one-enzyme studies

• Red bread molds, morels, and truffles

• Asexual sac fungi were in the phylum Deuteromycota, the imperfect fungi

• They include yeast Candida and molds Aspergillus and Penicillium which is now named Talaromyces

• Asexual reproduction in sac fungi is the norm

• Yeasts reproduce by budding where a small cell forms and pinches off as it grows to full size

• Others produce spores called conidiospores

• The conidia develop at the tips of specialized aerial hyphae called conidiophores

• Sexual reproduction in sac fungi includes an ascus or sac that develops during sexual reproduction

• A fruiting body is a reproductive structure where spores are produced and released

• Because mitosis follows meiosis, each ascus contains eight haploid nuclei and produces eight spores

• Sac fungi are essential in decomposing materials containing cellulose, lignin, or collagen

• Some are symbiotic with algae to form lichens while others with plant roots to form mycorrhizae. Some are pathogens causing powdery mildews, leaf curl, chestnut blight, and Dutch elm disease

• Ergot infects rye and other grains

• Produces the drug penicillin used against infections and cyclosporine that suppresses immune response in transplantation operations

• Others used to make foods like blue cheese

• Some diseases include ringworm, rose gardener’s disease, Chicago disease, and basketweaver’s disease

• Yeasts are both beneficial and harmful

• Used to make wines and beer and cause bread to rise in baking

• When normal balance of microbes is distrubed in organs like the vagina, yeast like Candida albicans causes inflammation

• Molds like Aspergillus used to make soy sauce, citric acid, inks, medicines, dyes, plastics, toothpaste, soap, and chewing gum

• Others like Aspergillus flavus grows on moist seeds and is the most potent natural carcinogen known

• Mold Stachybotrys chartarum or black mold causes “sick-building” syndrome

Fig. 22.8

• Athlete’s foot and ringworm are termed tineas

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b.

a.

a: © Dr. P. Marazzi/SPL/Photo Researchers, Inc.; b: © John Hadfield/SPL/Photo Researchers, Inc.

• Club fungi are the mushrooms, toadstools, puff balls, shelf fungi, jelly fungi, bird’s nest fungi, stinkhorns, smuts, and rusts

• At least 75 species are poisonous

• Most are saprotrophs with some parasitic species

• Possess a basidium or club-shaped structure in which spores called basidospores form

• Smuts and rusts are club fungi that parasitize cereal crops like corn, wheat, oats, and rye

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ergot

© R. Calentine/Visuals Unlimited

• Lichens are an association between fungus, usually sac fungus, and a cyanobacterium or green algae

• They were assumed to be mutualistic, but may involve a controlled form of parasitism of the algal cells by the fungus with no benefit to algae

• Mycorrhizae are mutualistic relationships between soil fungi and roots of most plants

• Plants grow more successfully in poor soils especially those deficient in phosphates with mycorrhizae