Useful Reading
Campbell, Biology 6th Ed - Chapter 31, pgs 616-632
Campbell, Biology 7th Ed - Chapter 31, pgs 608-625
Kingdom Fungi is made up of multicellular eukaryotes. Fungi are actually more closely related to Kingdom Animalia than to Kingdom Plantae. There are four phylums that make up the fungi kingdom:
Chytridiomycota (fossil fungus)
Zygomycota
Ascomycota (the sac fungi)
Basidiomycota (the club fungi-mushrooms)
Ecology: Fungi can be found marine, freshwater and terrestrial habitats and are heterotrophic, which means the organism cannot create its own food and is dependent on complex organic substances for nutrition. Common forms include yeasts, mildews and mushrooms. Many of these organisms are important decomposers and survive by digesting dead organisms (saprophytes), while others are parasites. In addition, some fungi form ecologically-important symbioses with plants.
Structure: In most fungi, cells are haploid. The body of a fungus is called the mycelium and it grows underground. The mycelium is made up of many individual filamentous hyphae, threadlike strings of cells, in a tangled mass. This structure makes it possible for every cell to be relatively unspecialized and in close contact with the environment. Hyphae are often surrounded by a cell wall made of tough chitin. Hyphae are often divided by walls (septa) which contain holes, allowing materials to pass through the body of the fungus. It is thought that some fungi mycelia may be the biggest living organisms on the planet, covering a span of several states in the soil.
Hyphae and mycelium
The fungal phyla differ in life
cycles and modes of reproduction. Asexual reproduction occurs when hyphae
differentiate into sporangia, which produce asexual spores. Asexual
reproduction can also occur through fragmentation of the mycelium. Sexual
reproduction occurs when haploid mycelia of different mating types (+ or -)
meet, produce gametangia and gametes. Reproduction is illustrated for
each phylum in more detail below.
Spores and sporangia
Cool fungi ecology and structure websites:
Introduction
to the Fungi
The
Fascinating World of Fungi
Phylum Chytridiomycota
Chytridiomycetes are the most primitive fungi. They are aquatic with chitinous cell walls. They act as decomposers and as parasites.
During asexual reproduction, sporangia produce motile, flagellated zoospores. These zoospores develop into new mycelia. During sexual reproduction, two zoospores meet and fuse their bodies (plasmogamy), followed by their haploid nuclei (karyogamy). The diploid zygote goes through one round of meiosis, followed by mitosis, growth and production of new zoospores.
Chytridium confervae hyphae and sporangium (left); release of zoospores (right)
For more on Chytridiomycota, click here.
Phylum Zygomycota
There are ~1000 species of Zygomycetes, most of which are terrestrial decomposers. They are coenocytic, which means that many nuclei are found within a large cytoplasm. One group of zygomycetes forms mycorrhizae, which are mutualistic associations with plant roots. They reproduce asexually, through asexual spores contained in sporangia. When reproducing sexually, hyphae of two different mating strains meet, create gametangia, fuse, and subsequently create a diploid zygosporangium and zygospore. The zygospore goes through meiosis and produces haploid sporangia.
Rhizopus stolonifer (bread mold) is a zygomycete.
Asexual reproduction in Rhizopus: Sporangia – note the root like hyphae (rhizoids) and horizontally-growing hyphae (stolons) at the base.
Sexual reproduction in Rhizopus: hyphae meeting (far left) and making a zygospore (far right).
More Rhizopus zygospores.
For more on Zygomycota, click here.
Phylum Ascomycota
The Ascomycetes (sac fungi), made up of decomposers, mutualists, and parasites, are the largest group of fungi with ~60,000 species. The diverse group includes yeasts, powdery mildew molds, penicillin molds, and cup fungi. Many parasitic forms produce haustoria, which are modified hyphae that project into host tissue:
Ascomycetes reproduce asexually, through projecting conidiophores which produces conidia spores. The sexual fruiting body, known as the ascocarp, produces many characteristic sacs, known as asci (singular = ascus), which contain many ascospores (sexual spores).
Some ascocarps are very large and are used as food items by humans.
One of the characteristics of the sac fungi, Tuber melanosporum, is they emit a strong odor and cooks would use pigs (because of their keen smell) to search the undergrowth and locate this fungi for use in the delicacy called "Truffles". The morel mushroom, highly sought after in Iowa, is also a member of this phylum.
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Cup Fungi |
Powdery mildew |
Morchella (morel) |
Yeast |
Penicillium |
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Cup Fungi asci |
Mildew Asci |
Morel ascospores |
Yeast ascus |
Conidiophores |
For more on Ascomycetes, click here.
Phylum Basidiomycota
This phylum of ~25,000 species is made up of decomposers, parasites, and mutualists. They (generally) only reproduce sexually by forming elaborate fruiting bodies, called basidiocarps (commonly known as mushrooms or toadstools). Inside the basidiocarp are millions of tiny, club-shaped cells called basidia, each producing four sexual spores.
Basidomycetes come in all shapes, sizes and colors.
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Coral Fungus |
Conch Fungus |
Birds Nest Fungi |
Bracket Fungi |
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Agaricus coprinus |
Gills |
Basidia |
Basidia with spores |
Wheat rust (Puccinia) is also a member of Phylum Basidiomycota.
For more on Basidiomycota, click here.
Ecological Associations
Lichens
Lichens are mutualistic associations between fungi and algae or cyanobacteria. The fungus provides a safe environment and minerals to the algae and the algae provide sugars to the fungus. Together, they are often considered as separate organisms or even called species, but realize lichens represent two species living together in a mutualistic relationship.
Often lichens are the first to "colonize" exposed rocks initiating the generation of soil from rock. They "reproduce" by producing soredia (a specialized asexual reproductive unit) composed of fungal hyphae surrounding algae. These small structures may be air- or water-borne to new locations thus spreading the lichens. It is a distribution mechanism and not a reproduction mechanism. The fungal members of a lichen symbiosis may reproduce sexually by producing ascocarps or basidiocarps.
Lichens can survive in extreme environments such as Antarctica or a desert living on rocks, trees, and in soil. They are good monitors of environmental pollution because of their ability to absorb nutrients from substrate, air, and rain. Their rate of growth is at a rate of 0.1-10mm/yr (VERY SLOW).
Mycorrhizae
Mycorrhizae are mutualistic relationships between fungi and plant roots. Hyphae of the fungi contact the plant roots and spread out into the soil, gathering soil mineral and water resources and providing them to the plant. The plant provides the fungi with sugars produced by photosynthesis. In endomycorrhizal relationships, the fungal hyphae penetrate the plant cells. In ectomycorrhizal relationships, the hyphae surround the plant cells.
For more on fungal phyla, click here.
A great website on fungi: Fungal Biology
Fungus-like Protists – Slime molds
Slime molds belong to the protist Candidate Kingdom Mycetozoa, but are sometimes classified in Kingdom Fungi. There are two types of slime molds – plasmodial slime molds and cellular slime molds. The plasmodium of the plasmodial slime mold is a multinucleate mass of protoplasm that creeps along surfaces and feeds on bacteria. To reproduce, they produce spores through fruiting bodies.
Slime mold Physarum.
Review Questions
-For each fungal phylum, which stages of the life cycle of haploid, and which are diploid?
-What is an example of fungi in a mutualistic relationship with a plant? What is an example of fungi in a parasitic relationship with a plant?
-What characteristics are common and which differ between slime molds and true fungi?
-How does a fungus benefit from having symbiotic cyanobacteria?
-Why is decomposition an important ecological process?
-Is sexual reproduction initiated when a + and – hyphae meet or when two + hyphae meet?
