Concept 22.1   Fungi Live by Absorptive Heterotrophy

• Fungi are opisthokonts with absorptive heterotrophy and with chitin in their cell walls. Fungi have various nutritional modes: some are saprobes, others are parasites, and some are mutualists. Yeasts are unicellular, free-living fungi. Review Figure 22.1
• The body of a multicellular fungus is a mycelium—a meshwork of filaments called hyphae. Hyphae may be septate (having septa) or coenocytic (multinucleate). Review Figure 22.3
• Fungi are often tolerant of hypertonic environments, and many are tolerant of extreme temperatures.

Concept 22.2   Fungi Can Be Saprobic, Parasitic, Predatory, or Mutualistic

• Saprobic fungi, as decomposers, make crucial contributions to the recycling of elements, especially carbon.
• Many fungi are parasites of plants, harvesting nutrients from plant cells by means of haustoria. Review Figure 22.5
• Certain fungi have relationships with other organisms that are both symbiotic and mutualistic.
• Some fungi associate with unicellular green algae, cyanobacteria, or both to form lichens, which live on exposed surfaces of rocks, trees, and soil. Review Figure 22.9
• Mycorrhizae are mutualistic associations of fungi with plant roots. They improve a plant’s ability to take up nutrients and water. Review Figure 22.10
• Endophytic fungi live within plants and may protect their hosts from herbivores and pathogens.

Concept 22.3   Major Groups of Fungi Differ in Their Life Cycles

• The microsporidia, chytrids, and zygospore fungi diversified early in fungal evolution. The mycorrhizal fungi, sac fungi, and club fungi form a monophyletic group, and the latter two groups form the clade Dikarya. Review Figure 22.11, Table 22.1, and ACTIVITY 22.1
• Many species of fungi reproduce both sexually and asexually, although most fungi spend most of their lives in a haploid, asexual state. When sexual reproduction does occur, it usually occurs between individuals of different mating types. Review Figure 22.12
• The microsporidia are highly reduced unicellular fungi. They are obligate intracellular parasites that infect several animal groups.
• The three distinct lineages of chytrids all have flagellated gametes. Review Figure 22.14A
• In the sexual reproduction of terrestrial fungi, hyphae fuse, allowing “gamete” nuclei to be transferred. Plasmogamy (fusion of cytoplasm) precedes karyogamy (fusion of nuclei).
• Zygospore fungi have a resting stage with many diploid nuclei, known as a zygospore. Their fruiting structures are simple stalked sporangiophores. Review Figure 22.14B and ANIMATED TUTORIAL 22.1
• Arbuscular mycorrhizal fungi form symbiotic associations with plant roots. They are only known to reproduce asexually. Their hyphae are coenocytic.
• In sac fungi and club fungi, a mycelium containing two genetically different haploid nuclei, called a dikaryon, is formed. The dikaryotic (n + n) condition is unique to the fungi. Review Figure 22.16 and ACTIVITY 22.2
• Sac fungi have septate hyphae; their sexual reproductive structures are asci. Many sac fungi are partners in lichens. Filamentous sac fungi produce fleshy fruiting structures called ascomata. The dikaryon stage in the sac fungus life cycle is relatively brief. Review Figure 22.16A
• Club fungi have septate hyphae. Many club fungi are plant pathogens, although mushroom-forming species are more familiar to most people. Their fruiting structures are called basidiomata, and their sexual reproductive structures are basidia. The dikaryon stage may last for years. Review Figure 22.16B

Concept 22.4   Fungi Can Be Sensitive Indicators of Environmental Change

• The diversity and abundance of lichen growth on trees provide sensitive indications of air quality. Review Figure 22.20
• Museum collections of fungi provide a historical record of atmospheric pollutants that were present when the fungi were growing. Fungi are often useful in environmental remediation.
• Reforestation projects require restoration of the mycorrhizal fungal community.