General Characteristics of Fungi

General Characteristics of Fungi

• Mostly multicellular and terrestrial
• Absorptive heterotrophs (obtain nutrients from organic or mineral sources); can be saprobes, parasites, or mutualists
• Cell walls composed of chitin
• Reproduce either sexually or asexually by spores

Reproductive Structure

• Hyphae: thread-like filaments that form a mycelium (mass of hyphae)
• Spore-producing structures: specialized structures for producing spores for reproduction

Hyphae Structure

• Septate hyphae: have cross-walls (septa) dividing the hypha into cells with nuclei
• Coenocytic hyphae: lack septa; nuclei are dispersed throughout the hypha

The Mycelium

• Extensive hyphal growth forms a mycelium
• Mycelia can grow on soil surfaces, other objects, or beneath the soil
• A single mycelium can cover hundreds of acres, with thousands of miles of hyphae!
• Hyphal growth maximizes surface area for absorption of nutrients

General Fungal Life Cycle

• Alternating between haploid (n) and diploid (2n) stages
• Common in changing environments
• Plasmogamy: Fusion of cytoplasm of two parents
• Karyogamy: Fusion of nuclei

Origins of Fungi

• DNA evidence suggests fungi are most closely related to unicellular nucleariids
• Animals are most closely related to unicellular choanoflagellates
• Multicellularity arose separately in animals and fungi
• Oldest undisputed fungal fossils are about 460 million years old

Early-Diverging Fungal Groups

• Genomic studies identified chytrids (genus Rozella) as early diverging fungal lineage
• Rozella and other unicellular cryptomycota lack chitin walls
• Fungi were among the earliest land colonizers

Phylum Classifications:

• Chytrids: 1,000 species
• Zygomycetes: 1,000 species
• Glomeromycetes: 160 species
• Ascomycetes: 65,000 species
• Basidiomycetes: 30,000 species

Zygomycota - Zygote Fungi

• Mostly terrestrial, found in soil or detritus
• Some are mycorrhizae (mutualistic symbiosis with plant roots)
• Coenocytic hyphae, except where reproductive cells are formed
• Zygosporangia (resistant reproductive structures) are extremely resistant to environmental conditions

Rhizopus and Reproduction

• Rhizopus is a common zygomycete
• Mycelia of different mating types form gametangia
• Plasmogamy & karyogamy occur, leading to zygosporangium formation

Arbuscular Mycorrhizae

• Symbiotic fungi that penetrates root cells of many plants
• Important for nutrient uptake for plant
• Extensive hyphal network for increased surface area for nutrient exchange

Ascomycota – Sac Fungi

• Marine, freshwater, and terrestrial environments
• Includes yeast, cup fungi, truffles, and morels
• Includes saprobes, plant pathogens and mutualists (lichens or mycorrhizae)
• Dikaryotic hyphae form ascocarps
• Sexual spores borne in asci (sac-like structures) on ascocarps
• Asexual spores borne in naked groups on hyphae

Morels, Truffles, and Related Fungi.

• Examples of ascomycete diversity

Basidiomycota - Club Fungi

• Mostly terrestrial, including saprobes and plant parasites
• Includes mushrooms, shelf fungi, puffballs, and rusts
• Septate hyphae (separated cells)
• Basidiocarps: prominent fruiting bodies in life cycle
• Spores borne in basidia on basidiocarp, often on gills beneath the cap.

Unique Lifestyles in Fungi

• Molds: rapid-growing, asexually reproducing saprobes or parasites, and lacking known sexual stages
• Yeasts: unicellular, economically important (breadmaking, alcohol production). Some can cause infections (e.g., Candida).
• Lichens: symbiotic relationship of fungus and algae/cyanobacteria, important pioneer species. Different growth forms (fruticose, foliose, crustose).
• Mycorrhizae: Mutualistic fungi that enhances root absorption of water and nutrients in many plants.

Importance of Fungi

• Decomposition of detritus
• Pathogens
• Food for animals and humans

Summary of Fungal Phyla

• Each phylum has unique distinguishing features based on reproduction, and ecological roles.