Introduction to Fungi: Structure, Classification, and Reproduction

Domain: Eukarya – Kingdom Fungi (Myceteae)

Overview of Fungi

Fungi are a diverse group of eukaryotic organisms classified under the Kingdom Fungi (Myceteae). They play essential ecological roles and exhibit unique structural and reproductive features distinct from plants and animals.

• Fungus: Latin for mushroom.

• Species Diversity: Approximately 148,000 described species, with estimates exceeding 5 million.

• Cellularity: Mostly multicellular, though some (e.g., yeasts) are unicellular.

• Nutrition: Heterotrophic, obtaining nutrients as saprophytes (decomposers) or parasites; not photosynthetic.

• Structure: Filamentous (except chytrids and yeasts), with cell walls containing chitin and often melanin.

• Storage: Store energy as glycogen.

• Ecological Role: Decomposition and soil fertility.

Fungi in Nature

Largest Organism Example

• Armillaria ostoyae: A pathogenic fungus affecting conifers in Oregon's Malheur National Forest.

• Recognized as the world's largest organism by mass, area, and volume (covers 3.7 square miles and weighs 35,000 tons).

Fungi Evolution

Phylogenetic Relationships

• DNA and protein sequence analyses show fungi are more closely related to animals than to plants.

• Fossil and molecular data indicate fungi and animals last shared a common ancestor about 460 million years ago.

Fungus Anatomy

Vegetative Structures

• Unicellular Fungi: Called yeasts.

• Multicellular Fungi: Produce hyphae (filamentous structures).

• Stolon: Horizontal stem-like hyphae.

• Mycelium: A mass of hyphae that can grow on surfaces, in liquids, or into living/dead tissue or soil.

• Septa: Cross-walls dividing hyphae into cells; some fungi are coenocytic (no septa, many nuclei per hypha).

Specialized Hyphae

• Rhizoids: Found in saprophytic species; break down decaying organic matter, including cellulose and lignin.

• Haustoria: Found in parasitic species; penetrate host tissues (e.g., cause diseases like smut, Dutch Elm Disease, athlete's foot).

Fungus Reproduction

General Modes

• Fungi reproduce both sexually and asexually, with mechanisms varying by phylum.

Asexual Reproduction

• Fragmentation: Hyphae break off and form new colonies.

• Budding: In unicellular fungi, a bulge forms, detaches, and grows into a new cell.

• Spores: Primary method; produced within sporangia (sporangiospores) or directly from hyphae (conidia/conidiospores).

Sexual Reproduction

• Often triggered by adverse environmental conditions.

• Involves two mating types (not male/female):

  • Homothallic: Self-fertile.

  • Heterothallic: Requires two compatible types.

• Three main stages (in club and sac fungi):

  1. Plasmogamy: Fusion of two haploid cells, resulting in a dikaryotic stage (two haploid nuclei per cell).

  2. Karyogamy: Fusion of nuclei to form a diploid zygote.

  3. Meiosis: Produces haploid spores of different mating types.

Classification of Fungi

Major Phyla

Fungi are classified into five main phyla, each with distinct reproductive and structural features.

Phylum (Common Name)	Reproduction	Key Features	Examples
Basidiomycota (club fungi)	Basidiospores, sexual	Basidiocarp (fruiting body), septate hyphae	Mushrooms, puffballs, shelf fungi
Ascomycota (sac fungi)	Ascospores, asexual conidia	Ascocarp (fruiting body), septate hyphae	Yeasts, morels, truffles, Penicillium
Glomeromycota (AM fungi)	Asexual, large multinucleate spores	Arbuscular mycorrhizae, symbiotic with plants	Glomeromycetes
Chytridiomycota (chytrids)	Zoospores with flagella, sexual and asexual	Single-celled, aquatic, flagellated spores	Chytridium, Batrachochytrium
Zygomycota (zygospore fungi)	Zygospores, asexual sporangiospores	Coenocytic hyphae, zygosporangium	Rhizopus (bread mold)

Phylum Chytridiomycota

Chytrids

• Most closely related to ancestral fungi; evolved ~500 million years ago.

• Simple, mostly single-celled; both saprophytes and parasites.

• Primarily aquatic; only fungi with flagellated spores (zoospores).

• Sexual reproduction by gamete fusion; zygote undergoes meiosis and often forms a resting spore.

• Batrachochytrium dendrobatidis: Causes chytridiomycosis in amphibians, contributing to global frog population declines.

Phylum Zygomycota – Coenocytic True Fungi

• Thallus of coenocytic hyphae (many nuclei, no septa except at reproductive structures).

• Primarily saprophytes; some are parasites (infect insects).

• Grow on decaying organic material (e.g., bread, fruit).

• Rhizopus (bread mold):

  • Three types of hyphae: stolons, rhizoids, sporangiophores.

  • Reproduces both sexually and asexually.

Rhizopus Reproduction

• Sexual: Hyphae of different mating types form gametangia, merge to form a diploid zygote, which develops into a zygosporangium. Meiosis produces spores.

• Asexual: Sporangiophores grow from mycelium; haploid spores are released from sporangia.

Phylum Glomeromycota

• Arbuscular mycorrhizal (AM) fungi; form mutualistic relationships with plant roots.

• Hyphae penetrate root cells, forming arbuscules for nutrient exchange.

• Provide plants with nutrients (especially phosphorus and nitrogen); plants provide carbohydrates.

• Reproduce asexually only.

Phylum Ascomycota – Sac Fungi

• Comprise 75% of known fungi; include yeasts, molds, morels, truffles.

• Unicellular and multicellular forms; primarily saprophytes, some plant pathogens.

• Hyphae with perforated septa; extensive dikaryotic stage (two nuclei per cell).

• Reproduce both sexually and asexually.

Ascomycota Asexual Reproduction

• Budding: Unicellular species (e.g., yeasts).

• Conidia: Multicellular species produce asexual spores (conidia) at hyphal tips.

Ascomycota Sexual Reproduction

• Antheridium (n) and ascogonium (n) from two hyphae undergo plasmogamy.

• Ascogenous hyphae (n+n) grow, forming the ascocarp/ascoma (n+n).

• Ascocarps contain sacs called asci (singular: ascus), each with eight ascospores (n).

• Karyogamy forms zygotes (2n), which undergo meiosis and mitosis to produce ascospores.

Ascocarp/Ascoma Types

• Apothecium: Open, disk-shaped (e.g., Peziza).

• Perithecium: Flask-shaped with a pore (ostiole) at the top (e.g., Nectria).

• Cleistothecium: Completely enclosed (e.g., Microsphaera).

Taxonomy and Notable Classes

• Sordariomycetes: Includes Ophiostoma ulmi (Dutch Elm Disease) and Cryphonectria parasitica (American Chestnut Blight).

• These fungi are significant plant pathogens, causing widespread tree mortality.