PEBO 102: Fungal Diversity and Biology

Fungal Diversity

• Fungi were originally classified with photosynthetic plants, but later separated and grouped with algae and protozoa (protista) and then placed in the Kingdom Mycota.
• Fungi lack chlorophyll and their cell walls contain chitin, mannan, and β-glucans, which are characteristics of some animal cells.
• Cellulose is generally absent from the cell walls of true fungi.

Structural Forms

• Fungi can exist as unicellular or multicellular forms with several different cell types.
• Multicellular fungi are primarily filamentous in their growth form, with long, slender filaments called hyphae that form an entangled mass called mycelium.

Morphological Features of Hyphae

• Hyphae are of two types: aseptate (non-septate or coenocytic) and septate.
• Aseptate hyphae are continuous or branching filaments filled with cytoplasm and multiple nuclei.
• Septate hyphae are made up of long chains of cells joined end-to-end and divided by cross-walls called septa, which have pores.

Hyphae Classification Based on Function

• Vegetative hyphae: grow as submerged or surface filaments, digesting, absorbing, and distributing nutrients obtained from the substrate to other parts for growth and development.
• Aerial hyphae: project above the surface in culture medium.
• Reproductive/fertile hyphae: aerial hyphae that produce asexual spores on the surface of the substrate.

Nutrition

• Fungi acquire nutrients by three means: parasitism, saprophytism, and symbiotic associations.
• Parasitism: obtaining nutrients from another living organism (e.g., Phytophthora sp. being parasitic on cocoa plants).
• Saprophytism: absorbing nutrients from dead organic matter (e.g., Termitomyces sp. feeding on litter).
• Symbiotic associations: exchanging nutrients with other organisms, especially roots of higher plants (e.g., Boletus spp. and Glomus spp. associating with roots to form mycorrhizae).

Habitat

• Fungi occur both on land (terrestrial) and in water (aquatic - freshwater and marine).

Reproduction

• Fungi reproduce in three ways: sexual, asexual, and vegetative (budding, as seen in yeast).
• Both sexual and asexual reproduction occur in fungi.

Life Cycle

• During the asexual life cycle, spores settle on a substrate, develop an outgrowth known as a germ tube, which then elongates into a hypha.
• During the sexual life cycle, the haploid nuclei of compatible hyphae fuse, forming a diploid nucleus (zygote), which subsequently participates in sexual spore development through meiosis.

Fungal Classification

• The four major groups are: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota.
• Chytridiomycota: characterized by motile spores (zoospores) and commonly called chytrids.
• Zygomycota: characterized by the production of non-motile spores, with a coenocytic mycelial organization.
• Ascomycota: characterized by the production of sexually produced spores (ascospores) contained within a sac called an ascus.
• Basidiomycota: characterized by the production of basidiospores in a structure called a basidium.

Economic Importance of Fungi

• Recycling of nutrients/maintenance of soil fertility: fungi decompose dead organic material, releasing nutrients back into the ecosystem.
• Formation of symbiotic associations with plant roots: fungi provide the host with water, nutrients, and pathogen protection, in exchange for photosynthetic products.
• Food: fungi are consumed directly or used indirectly in the food and beverage industry.
• Medicines: fungi produce antibiotics that are widely used medicinally to control diseases.
• Biocontrol of soil-borne pathogens: fungi are used to control pests, such as nematodes and insects.
• Crop diseases: fungi cause the majority of known plant diseases, resulting in significant economic losses.
• Animal diseases: fungi can parasitize domestic animals, causing diseases, but this is not usually a major economic problem.
• Spoilage of food and goods: fungi cause spoilage of food and other consumable goods, such as fabric, leather, and paper.