Fungi: Biology Chapter 32

• Lecture 18 is about Fungi

Fungi

• Chapter 32, pages 712-731 in "Biology: How Life Works, Morris et al." discusses fungi

Learning Objectives

• The definition of fungi
• Fungal characteristics
• How scientists classify fungal diversity

Fungi Defined

• Fungi diverged approximately 500 million years ago from a branch shared with animals
• Fungi are a diverse, widespread group
• Fungi are essential for terrestrial ecosystems because break down organic matter and recycle vital nutrients

Fungal Characteristics

• Fungi are multicellular heterotrophic eukaryotes
• Fungi lack an internal digestive system, absorbing nutrients directly through their cell walls
• This mode of digestion presents 2 problems:
  • Fungi cannot directly access large organic molecules that cannot cross cell walls or membranes
  • Solution: Fungi secrete multiple enzymes that break down large molecules for external digestion
• Fungi cannot move to find food, unlike animals, bacteria, protists
• Solution: Fungi use the growth of their fungal body to find nourishment
• Hyphae are branched filaments that forms the structure of fungi and have chitin cell walls
• Hyphae have numerous long, thin threads for large surface area to absorb nutrients
• Hyphae grow at their tips for fungal movement toward new food sources
• The mycelium is an extensive network formed by hyphae
• Raw materials for growth are transported into cells across the cell membrane
• Cytoplasmic continuity is crucial along the hyphae
• Earliest fungi lacked cross walls to separate cells and were coenocytic
• More recently, fungal groups have evolved cross walls (septa) that divide hyphae into filaments

Nutrition

• Fungi have diverse lifestyles to access food
• Fungi play a crucial role in carbon cycling by converting dead organic matter back into CO2 and H2O
  • Decomposers are a fungal lifestyle feeding on dead matter (saprotrophs)
  • Parasites are a fungal lifestyle infecting living tissues of protists, plants, and animals
  • Mutualists are a fungal lifestyle forming symbiotic interactions with other organisms:
    • With plants, e.g., mycorrhizae forming fungal-root partners
    • With animals, e.g., leaf-cutter ants, termites, beetles
    • With cyanobacteria or algae, e.g., lichens
• Fungi are major decomposers especially on land and along with heterotrophic bacteria
• Fungi are efficient decomposers, that break down cellulose and lignin
• Fungi perform an essential recycling of chemical elements

Fungi as Parasites

• About 30% of known fungal species are parasites or pathogens, mostly plant-based
• Some fungi that attack food crops are toxic to humans

Fungal Reproduction

• Similar to plants, fungi have two main challenges in completing their life cycle:
  • Finding mates for genetic diversity
  • Dispersal to new habitats
• There are a number of solutions:
  • Fungi reproduce by spores
  • Spore production is very prolific
  • Spores can be produced asexually or sexually
  • Sexual reproduction involves three steps:
  • Plasmogamy is the fusion of the cytoplasm between two haploid cells
  • Karyogamy is the nuclear fusion, which forms a zygote
  • Meiosis then occurs to produce haploid spores to form hyphae
• Sexual reproduction is the fusion of haploid hyphae for genetic diversity
• This results in plasmogamy, which involves heterokaryotic/dikaryotic stages

Classifying Fungal Diversity

• Studied through molecular analyses that clarified the evolutionary relationships
• Evolution in water, then onto land through traits:
  • Chitin cell walls
  • Hyphae
  • Septa
  • Complex multicellular fruiting bodies developed for sexual reproduction

Fungal Lineages

• Radiated into major lineages and leading to diversity

1. Chytrids
2. Zygomycetes
3. Glomeromycetes
4. Dikarya

• Ascomycetes
• Basidiomycetes

Chytrids

• Only fungi to possess motile flagellated spores called zoospores
• Undergo asexual reproduction only
• Primarily aquatic, though some terrestrial species exist
• Function as decomposers, parasites, or mutualists

Zygomycetes

• Loss of flagellated spores
• Possess aseptate or coenocytic hyphae
• Asexual reproduction:
  • Haploid hyphae tips develop into haploid sporangia through asexual reproduction
  • Sporangia then produce haploid spores, which develop into haploid hyphae (mycelium)
• Some haploid hyphae become involved in sexual reproduction:
• Sexual reproduction involves + and - mating types
• Haploid hyphae from two different fungal types come into contact
• Hyphal tips fuse, which forms a gametangium
• Haploid nuclei fuse, it will form a diploid zygosporangium that undergoes meiosis
• It then produces haploid zygospores that are released from stalked sporangium for asexual reproduction

Glomeromycetes

• Form mutualistic mycorrhizae specifically with plants
• About half of the known fungi are glomeromycetes
• Undergo asexual reproduction
• Form symbiotic associations with plant roots
• Significant ecologically.

Dikarya

• Separate monophyletic group of fungi
• Distinguishes them by presence of septa between cells
• This controls the number of nuclei per cell
• Complex fruiting bodies are developed that function for sexual reproduction

Ascomycetes

• Also known as sac fungi
• Ascomycetes are the most diverse group of fungi
• Asexual reproduction uses haploid asexual spores, via conidia in asexual sporangia called conidiophores
• Sexual reproduction produces haploid sexual spores (ascospores) within a sexual sporangium (ascus) inside a specialized sexual fruiting body (ascocarp)

Basidiomycetes

• Known as club fungi
• No asexual reproduction is observed
• Only sexual reproduction exists
• Haploid sexual spores (basidiospores) are produced on club-shaped sexual sporangia
• These are called basidia, and that develop in a specialized sexual fruiting body (basidiocarp)
• Mushrooms are an example and fall into the category of; puffballs, shelf fungi, mutualists, and plant parasites
• Many mushrooms are important decomposers of woody plant debris