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23.1 Introducing the Fungi

1. Fungi are widespread and profoundly affect ecosystems because they are decomposers and parasites. Essential Study Partner
Characteristics of Fungi
2. The four fungal phyla are Chytridiomycota (chytrids), Zygomycota (zygomycetes), Ascomycota (ascomycetes), and Basidiomycota (basidiomycetes). Essential Study Partner
3. Fungal characteristics include chitin cell walls, glycogen, and a saprotrophic or parasitic mode of acquiring nutrients. Fungi have both asexual and sexual reproduction, and some have unique dikaryotic cells with two nuclei. Fungi do not photosynthesize.  
4. A fungal body includes a mycelium built of threads called hyphae, which may form a fruiting body. Some fungi occur both as hyphae and as unicellular yeasts. Hyphae may be continuous (nonseptate) or divided into cells (septate). Fungi reproduce using spores. The nonreproductive part is a thallus.  
5. Molecular evidence indicates that ascomycetes and basidiomycetes are sister groups, and that fungi diverged from animals, their closest relatives, about 1 billion years ago.  

23.2 Chytridiomycetes Are Flagellated Fungi

6. Chytrids are microscopic fungi that produce flagellated, motile zoospores. They may resemble primitive fungi.  
7. Chytrids decompose major biological carbohydrate coverings.  

23.3 Zygomycetes Are Prolific, Nonseptate Fungi

8. Zygomycetes include bread molds. They grow and reproduce rapidly.  
9. Asexual reproduction occurs via haploid, thin-walled spores (conidia).  
10. Sexual reproduction occurs when gametangia of different mating types fuse their nuclei into a zygospore. The zygospore undergoes meiosis, then generates a spore sac. Spores germinate to yield hyphae, continuing the cycle. Art Quiz
Rhizopus Life Cycle

23.4 Ascomycetes Are Sac Fungi

11. Ascomycetes may be multicellular, unicellular, or both. They include important pathogens of plants and animals and have many uses in industry.  
12. Hyphae are septate and haploid for most of the life cycle, although there is a brief dikaryotic stage.  
13. Asexual reproduction occurs as asexual spores which are released from hyphal tips, or by budding.  
14. In sexual reproduction, ascomycetes produce haploid ascospores in saclike asci.  

23.5 Basidiomycetes Are Club Fungi

15. Basidiomycetes include mushrooms and other familiar fungi. Many important plant pathogens are basidiomycetes.  
16. Basidiomycetes are dikaryotic for most of the life cycle.  
17. Asexual reproduction occurs by budding, fragmenting of a thallus, or with asexual spores.  
18. Sexual reproduction occurs when haploid nuclei in basidia along the gills of mushrooms fuse, then undergo meiosis. The resulting haploid basidiospores germinate, and hyphae of compatible mating types fuse to regenerate the dikaryotic state.  

23.6 Fungi Interact with Other Organisms

19. Symbiotic associations between fungi and roots are called mycorrhizae.  
20. A lichen is a compound organism, with unique characteristics, that consists of a fungus in intimate association with a cyanobacterium or a green alga.  
21. Leaf-cutter ants cultivate basidiomycetes that they eat, and bacteria on the ants kill a parasitic ascomycete.  

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