Biological Classification · Lesson 6 of 10
Kingdom Fungi: Hyphae, Nutrition and Reproduction
“Understand fungal cell structure, methods of absorbing nutrients, symbiotic relationships and the sequence of sexual reproduction.”
• Explain how hyphae and mycelium form fungal bodies. • Distinguish septate from coenocytic hyphae. • Compare saprotrophic, parasitic and mutualistic fungal nutrition. • Identify vegetative, asexual and sexual reproductive strategies. • Explain plasmogamy, the dikaryotic stage, karyogamy and meiosis in their correct order.
Why fungi are neither plants nor animals
A mushroom may seem similar to a plant because it does not run or swim. Yet it lacks chlorophyll and obtains its organic nutrients by absorption. Fungal cells are eukaryotic and commonly have walls containing chitin and other polysaccharides. Their biology is adapted to releasing enzymes outside the body, breaking large molecules down and absorbing soluble products. These features separate fungi from the primarily photosynthetic plants and food-ingesting animals.
Fungi occur in soil, air, freshwater and marine environments, on decaying material and in close associations with living organisms. Warm, moist surroundings often encourage rapid growth of bread moulds because water and available organic matter favour metabolism. Cooling food generally slows microbial growth, helping reduce spoilage, although refrigeration does not sterilise food.
Hyphae and mycelium: the architecture of a fungus
A slender tubular filament of fungal cells or continuous fungal cytoplasm that extends through a substrate or into surrounding space.
A network or mass of hyphae forming much of the vegetative body of a filamentous fungus.
Most familiar moulds form extensive mycelia. Growing hyphal tips explore new areas and penetrate food or soil, increasing the surface area through which nutrients can be absorbed. A mushroom seen above ground is only a reproductive structure of some fungi; a much larger mycelial network can remain hidden within soil or wood. Yeasts are an important exception because many exist primarily as individual cells.
Septate hyphae
Septate hyphae contain cross walls, called septa, that divide the filament into compartments. The septa of many fungi have pores, allowing cytoplasm and selected cell components to move between adjacent compartments. A segmented appearance therefore does not necessarily mean each compartment is completely isolated.
Coenocytic hyphae
Coenocytic hyphae have no regularly spaced cross walls through much of the growing filament. Many nuclei share a continuous cytoplasm. This arrangement supports growth and transport across long regions of a hypha. It must not be confused with a multicellular animal tissue, whose cells typically have separate cell membranes.
How fungi obtain food
Saprotrophic fungi release digestive enzymes onto dead organic material, such as fallen leaves or rotting wood, and absorb the soluble molecules produced. They are major decomposers in ecosystems and contribute to nutrient recycling. Parasitic fungi obtain nutrients from living hosts and may cause plant or animal diseases. Puccinia, for instance, causes rust diseases in plants.
Other fungi form mutualisms. In mycorrhizae, fungal hyphae associate with plant roots and often help a plant absorb mineral nutrients and water; in return, the plant supplies organic carbon. In lichens, a fungus associates with a photosynthetic partner such as a green alga or cyanobacterium. These associations illustrate how organisms with different nutritional abilities can cooperate, though their ecological outcomes can vary with conditions.
| Nutritional relationship | Fungal role | Illustrative situation |
|---|---|---|
| Saprotrophic | Decomposes dead organic material and absorbs nutrients | Mould on decaying bread or plant litter |
| Parasitic | Obtains resources from a living host, sometimes causing harm | Rust fungus on crops |
| Mycorrhizal mutualism | Associates with roots; often exchanges soil resources for carbon | Fungi on plant roots |
| Lichen association | Fungus partners with an alga or cyanobacterium | Lichen growing on rock or bark |
Three routes to reproduction
Vegetative reproduction
In fragmentation, a piece of living fungal mycelium separates and grows into a new individual when conditions are suitable. Many yeasts reproduce by budding: a small outgrowth develops on the parent cell, receives genetic material and may detach. Some yeasts divide by fission instead. These processes do not require mating.
Asexual spore production
Fungi can disperse by asexual spores such as conidia, sporangiospores or motile zoospores in relevant groups. Conidia develop externally on spore-producing hyphae called conidiophores. Sporangiospores form inside a sac-like sporangium. A zoospore can move by flagella in those fungal lineages that produce motile spores. Once conditions are favourable, suitable spores germinate and produce new growth.
Sexual reproduction
A central sexual sequence is plasmogamy, karyogamy and meiosis. Plasmogamy joins the cytoplasm of compatible cells or hyphae. Karyogamy subsequently fuses their nuclei. Meiosis then reduces the chromosome number and contributes to genetic variation. The interval between plasmogamy and karyogamy varies by fungal group; these steps should not be mistaken for occurring simultaneously in every species.
Fusion of the cytoplasm of two compatible fungal cells or hyphae without necessarily fusing their nuclei immediately.
Fusion of two haploid nuclei to produce a diploid nucleus.
A fungal condition in which a cellular compartment has two genetically distinct haploid nuclei, conventionally written n + n.
In many ascomycetes and basidiomycetes, plasmogamy is followed by an n + n stage, in which two haploid nuclei share a cell but remain separate. Only later does karyogamy produce a diploid nucleus, briefly expressed as 2n. Meiosis subsequently forms haploid nuclei that can enter reproductive spores. Writing n + n is therefore not the same as writing 2n: two separate haploid nuclei have not yet become one fused diploid nucleus.
Plasmogamy joins cytoplasm; karyogamy joins nuclei. Where an extended dikaryotic phase exists, a cell containing two separate haploid nuclei is not yet diploid.
Problem
After plasmogamy, a fungal compartment contains two haploid nuclei. A student writes 2n. What is incorrect?
- 1.The nuclei remain separate, so the correct state is n + n.
- 2.Only when the nuclei fuse during karyogamy is there a single diploid nucleus, 2n.
- 3.Meiosis can then reduce diploid nuclei to haploid products.
Quick Quiz
Quiz
What is a mycelium?
What distinguishes coenocytic hyphae?
Which process describes fungi digesting dead material outside their bodies and absorbing products?
Which is a vegetative reproduction method in yeast?
What is the correct order of key sexual stages?
What does n + n indicate?
Practice Problems
- Explain how hyphae and a mycelium help a fungus obtain nutrients.
- Compare septate and coenocytic hyphae.
- Differentiate saprotrophic nutrition, parasitism and mycorrhizal association.
- Explain fragmentation, budding and asexual spore production, giving one useful distinction.
- Trace plasmogamy, a possible dikaryotic stage, karyogamy and meiosis, including n, n + n and 2n.
Hyphal tips extend through a substrate and increase contact with nutrients. Secreted enzymes release soluble molecules that can be absorbed across the hyphal surface.
Key Takeaways
Fungi are eukaryotic heterotrophs that typically absorb nutrients and have chitin-containing walls. A mycelium is a network of hyphae; hyphae may be septate or coenocytic. Fungi can be decomposers, parasites or partners in mutualistic associations. Vegetative processes and asexual spores differ from sexual reproduction. Sexual reproduction involves plasmogamy, karyogamy and meiosis; an n + n stage may separate the first two events.