CR

5, Ch 31 Fungi for students

Fungi

Overview

  • Mycologists estimate there may be as many as 1.5 million fungal species.

  • Fungi can be single-celled or multicellular.

  • They reproduce sexually or asexually.

  • Exhibit unique form of mitosis unlike many other organisms.

  • Specialized to extract and absorb nutrients from their surroundings.

  • Shared a common ancestor with animals approximately 460 million years ago.

Phylogenetic Diversity

Major Groups of Fungi

  • Six major monophyletic phyla:

    • Blastocladiomycota

    • Neocallismastigomycota

    • Chytridiomycota

    • Glomeromycota

    • Basidiomycota

    • Ascomycota

  • One paraphyletic phylum:

    • Zygomycota

  • Microsporidia sometimes included based on classifications.

General Features of Fungi

Structure

  • Multicellular fungi consist of long, slender filaments called hyphae.

  • Hyphae can be continuous or divided by septa.

  • Cytoplasm can flow throughout hyphae, allowing rapid growth in favorable conditions.

Hyphal Structure

  • Hyphae consist of:

    • Dikaryotic cells – containing two nuclei.

    • Septa with pores for cytoplasmic streaming.

Mycelium and Cell Walls

  • Mycelium: mass of connected hyphae that grow through and digest their substrates.

  • Cell walls composed of chitin, a material also found in arthropod exoskeletons.

Genetics of Fungi

Nucleus Variability

  • Hyphae may contain more than one nucleus:

    • Monokaryotic: 1 nucleus.

    • Dikaryotic: 2 nuclei (both genomes transcribed).

    • Heterokaryotic: mixture of genetically distinct nuclei.

    • Homokaryotic: genetically similar nuclei.

Unique Mitotic Processes

  • Fungi exhibit unusual mitosis; nuclei are the key reproduction unit.

  • Nuclear envelope remains intact, spindle apparatus forms within it.

  • Lack of centrioles in most fungi, spindle plaques regulate microtubule formation.

Reproductive Strategies

Reproduction Methods

  • Capable of both sexual and asexual reproduction:

    • Sexual reproduction involves the fusion of haploid hyphae from compatible mating types.

    • Some form diploid cells immediately, others enter a dikaryotic stage before forming diploid nuclei.

    • May produce structures like mushrooms or puffballs.

Spores

  • Spores serve as the primary means of reproduction, formed through either sexual or asexual processes.

  • Most spores are dispersed by wind.

Nutrition and Ecology

Nutritional Acquisition

  • Fungi exhibit external digestion, secreting enzymes into their environment.

  • They can absorb organic molecules produced by this digestion, leading to a high surface area-to-volume ratio:

    • Capable of breaking down cellulose and lignin, contributing to wood decomposition.

Ecological Contributions

  • Fungi, along with bacteria, are principal decomposers in the biosphere.

  • Many fungi are difficult to culture in labs, complicating species diversity assessments. Environmental DNA studies can identify them without culturing.

  • Help break down cellulose and lignin, releasing vital nutrients such as carbon, nitrogen, and phosphorus into the ecosystem.

Symbiotic Relationships

Interactions with Other Species

  • Fungi engage in various symbioses:

    • Obligate symbiosis: essential for fungal survival.

    • Facultative symbiosis: nonessential for survival.

    • Pathogens: lead to disease in hosts.

    • Parasites: harm hosts without causing disease.

    • Commensal relationships: one partner benefits without harming the other.

    • Mutualistic relationships: both partners benefit.

Endophytic Fungi

  • Live within plant intercellular spaces; can be parasitic, commensal, or mutualistic.

  • Some provide protection against herbivores by producing toxins.

  • Example: Perennial rye grass becomes more resistant to aphid feeding when endophytes are present.

Lichens

  • Symbiotic associations between fungi and photosynthetic partners (e.g., cyanobacteria, green algae).

  • Most interactions are mutualistic, although some fungi act as parasites on the photosynthetic partner.

  • Ascomycetes are predominant in over 15,000 lichen species.

Mycorrhizal Relationships

Types of Mycorrhizae

  • Mutualistic associations between fungi and plant roots, found on roots of about 90% of vascular plant species:

    • Arbuscular mycorrhizae (common and formed with glomeromycetes). Increase crop yields by enhancing nutrient absorption.

    • Ectomycorrhizae (most hosts are forest trees like pines and oaks) surround but do not penetrate root cells; play roles in nutrient exchange.

Fungal Interactions with Animals

Mutual Symbioses

  • Ruminant animals host neocallimastigomycete fungi aiding digestion of plant material.

  • Leaf-cutter ants cultivate basidiomycete fungi in their underground gardens, where ants provide plant material for the fungi, which in turn serves as a food source.

Pathogenic Fungi

Health Implications

  • Fungi can cause various health issues in humans:

    • Allergens can trigger immune responses.

    • Different types of infections, known as mycoses, can occur in skin, nails, and more.

    • Toxins produced can lead to further health complications.

Fungal Toxins

  • Mycotoxins secreted can render food harmful or distasteful:

    • Specific examples include aflatoxin produced by Aspergillus flavus and corn smut (Ustilago maydis).

Fungal Diseases in Animals

  • Fungi also cause diseases such as athlete’s foot, ringworm, and nail fungus; treatments are challenging due to the phylogenetic closeness of fungi to animals.

Amphibian Population Decline

Chytridiomycosis

  • Caused by Batrachochytrium dendrobatidis; associated with a worldwide decline in amphibian populations.

Fungal Plant Diseases

  • Numerous fungal species affect plants, making them significant pests and causing spoilage in harvest/products.