Introduction

• Fungi are eukaryotes

• Nearly all multicellular (yeasts are unicellular)

• Distinguished from other kingdoms by:

  • modes of nutrition

  • structural organization

  • growth

  • reproduction

The Characteristics of Fungi

• 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

Nutrition

• 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

Heterotrophic Absorbers

• Fungi get carbon from organic sources

• Hyphal tips release enzymes

• Enzymatic breakdown of substrate

• Products diffuse back into hyphae

• Fungus is often hidden from view. It grows through its food source (substratum)

Septate vs Aseptate

• 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

Hyphal Growth

• Hyphae grow from their tips

• Mycelium: extensive, feeding web of hyphae

• Mycelia are the ecologically active bodies of fungi

Mycorrhizae

• 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

• 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.

Lichen Internal Structure

• 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

Fungal Spores

• 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

Sexual Reproduction

• 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

The Move to Land

• 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