Fungi

Learning Objectives

  1. Place and identify the clade Fungi on a phylogenetic tree within the domain Eukarya

  2. Define traits and adaptations common to fungi

  3. Describe the general fungal morphology and life cycle of multicellular fungi, and differentiate between single and multicellular fungal morphology

  4. Describe the metabolic traits of fungi with respect to their ecosystem services and human relevance

  5. Describe the mutualistic and symbiotic relationship of fungi with land plants, and explain why plant transition to land was facilitated by fungi

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Fungi on the Phylogenetic Tree of Life

  • Fungi are a monophyletic group of eukaryotic heterotrophs, meaning they evolved from a common ancestor and require organic carbon.

  • Fungi are more closely related to animals than to plants.

  • They diverged from animals 800–900 million years ago and from plants around 1.5 billion years ago.

Shared Traits of Fungi

  • Eukaryotic cells with a nucleus, mitochondria, and membrane-bound organelles.

  • Heterotrophic: absorb nutrients after external digestion.

  • Cell walls made of chitin (also found in arthropod exoskeletons).

  • Can be unicellular (e.g., yeast) or multicellular (e.g., mushrooms).

  • Most fungi grow as hyphae, forming a network called a mycelium.

  • Reproduce via spores—either sexually or asexually.

Fungal Reproduction

  • Asexual: by budding, fragmentation, or mitotic spores.

  • Sexual: involves three key stages:

    1. Plasmogamy – fusion of cytoplasm from two mycelia.

    2. Heterokaryotic stage – cells contain two separate nuclei.

    3. Karyogamy – nuclei fuse to form a diploid zygote, followed by meiosis to form spores.

Ecological Roles and Metabolism

  • Decomposers (saprophytes): break down dead organic matter, recycling nutrients like nitrogen and phosphorus.

  • Mutualists: form mycorrhizae with plant roots, exchanging nutrients for sugars.

  • Parasites: cause diseases in plants (e.g., Dutch elm disease) and humans (e.g., athlete’s foot).

  • Predators: trap small organisms like nematodes in nutrient-poor environments.

Importance to Humans

  • Food and fermentation: yeasts in bread, beer, and cheese.

  • Medicine: source of antibiotics (e.g., penicillin), immunosuppressants, and other drugs.

  • Bioremediation: some fungi can break down pollutants and heavy metals.

Fungi and Plant Evolution

  • Mycorrhizal fungi helped early plants colonize land ~450 million years ago.

  • They created soil and aided water and nutrient uptake for primitive plants.

  • About 80–90% of modern plant species depend on fungal symbiosis.

Key Terms

  • Hyphae: thread-like fungal filaments.

  • Mycelium: mass of hyphae forming the fungal body.

  • Spores: reproductive cells dispersed by wind or animals.

  • Mycorrhizae: symbiotic relationship between fungal hyphae and plant roots.