Fungi

Fungi are eukaryotes
Nearly all multicellular (yeasts are unicellular)
Distinguished from other kingdoms by:
- modes of nutrition
- structural organization
- growth
- reproduction

Body form
- Unicellular
- Filamentous (tube-like strands called hypha (singular) or hyphae (plural)
- Mycelium: aggregate of hyphae
- Sclerotium: hardened mass of mycelium that generally serves as an overwintering stage.
- multicellular, such as mycelial cords, rhizomorphs, and fruit bodies (mushrooms)
Cell wall present, composed of cellulose and/or chitin.
Eukaryotes - true nucleus and other organelles present.
No obligate anaerobes.
Fungi grow in almost every habitat imaginable, as long as there is some type of organic matter present.
Heterotrophy - 'other food’
- Saprophytes or saprobes: feed on dead tissues or organic waste (decomposers)
- Symbionts: mutually beneficial relationship between a fungus and another organism
- Parasites: feeding on living tissue of a host.
- Pathogens: parasites that cause disease

Absorptive nutrition: enables fungi to live as decomposers and symbionts
Fungi are heterotrophs that acquire nutrients by absorption
Secrete hydrolytic enzymes and acids to decompose complex molecules into simpler ones that can be absorbed
Specialized into three main types:
- Saprobes: absorb nutrients from dead organic material
- Parasitic fungi: absorb nutrients from cells of living hosts; some are pathogenic
- Mutualistic fungi: absorb nutrients from a host, but reciprocate to benefit the host

Fungal hyphae may be septate or aseptate
Hyphae of septate fungi: divided into cells by crosswalls called septa
Hyphae of aseptate fungi: lack cross walls (coenocytic)
Parasitic fungi have modified hyphae called haustoria, which penetrate the host tissue but remain outside cell membrane

Mutualism between:
- Fungus: nutrient & water uptake for plant)
- Plant: carbohydrate for fungus)
Extremely important ecological role of fungi!
Mycorrhizae: mutually beneficial relationships between fungi and plant roots
Ectomycorrhizal fungi: form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex
Arbuscular mycorrhizal fungi: extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane

Lichens: mutually beneficial relationship between fungus and alga/cyanobacterium
Mutualism between
- Fungus: structure
- Alga or cyanobacterium: provides food
Three main types of lichens:
- Crustose lichens: form flat crusty plates.
- Foliose lichens: leafy in appearance, although lobed or branched structures are not true leaves.
- Fruticose lichens: even more finely branched and may hang down like beards from branches or grow up from the ground like tiny shrubs.

Lichens are nature’s biological monitors of pollution and air quality
- Thalli act like sponges
- Some species more sensitive to pollution
- Which species are present can indicate air quality
- Most resistant species can also be analyzed for pollutants, including bioaccumulation of heavy metals and radioactive isotopes

Spores: asexual (product of mitosis) or sexual (product of meiosis) in origin.
Purpose of Spores
- Allows the fungus to move to new food source
- Resistant stage: allows fungus to survive periods of adversity
- Means of introducing new genetic combinations into a population
Fungi reproduce by releasing spores
Usually unicellular, haploid and of various shapes and sizes
Spores are produced either sexually (by meiosis) or asexually (by mitosis)
- In favorable conditions, fungi produce enormous numbers of spores asexually
- For many fungi, sexual reproduction only occurs as a contingency - results in greater genetic diversity
Spores are the agent of dispersal responsible for geographic distribution of fungi:
- Carried by wind or water
- Germinate in moist places with appropriate substrata
Spores are formed:
- Directly on hyphae
- Inside sporangia
- Fruiting bodies

Fungal nuclei are normally haploid, with the exception of transient diploid stages formed during the sexual life cycles
Sexual reproduction requires the fusion of hyphae from different mating types
Fungi use sexual signaling molecules called pheromones to communicate their mating type
Plasmogamy: is the union of cytoplasm from two parent mycelia
In most fungi, the haploid nuclei from each parent do not fuse right away; they coexist in the mycelium, called a heterokaryon
In some fungi, the haploid nuclei pair off two to a cell; such a mycelium is said to be dikaryotic

Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants
Molecular analyses have helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain