Bio II ch. 22

Chapter 22 – Fungi 1

I. Overview

II. Form & Function

A. Nutrition

  • Heterotrophs: Fungi obtain nutrition via absorption.

    • Secrete enzymes to break down complex compounds outside the body.

    • Absorb simple organic compounds post-digestion.

    • Enzymes also penetrate living tissues.

  • Important roles:

    • Decomposers: Break down organic materials, recycling nutrients.

    • Mutualists: Engage in beneficial partnerships with other organisms.

    • Pathogens: Can cause diseases in plants and animals.

B. Fungal Construction

  • Yeasts: Single-celled fungi.

  • Most species are multicellular:

    • Hyphae: Filaments (tubes of cytoplasm) that form a dense mat called mycelium.

      • Mycelium can penetrate the surrounding area and is usually concealed.

      • Filamentous nature provides a large surface area for absorption.

    • Fruiting bodies: Above-ground reproductive structures formed from hyphae.

    • Cell wall: Composed of chitin.

    • Haustoria: Specialized hyphae used for penetrating host tissues (common in parasites and some mycorrhizae).

    • Fungal growth: Mycelium grows rapidly; hyphae elongate without increasing girth, allowing better access to nutrients.

III. Fungal Sex & General Life Cycle

A. Spores & Hyphae

  • Majority are haploid, with temporary diploid stages during sexual reproduction.

  • Produce numerous spores, which are dispersed by wind or water.

B. Sexual Reproduction

  1. Initiated by contact between genetically distinct mycelia (pheromone signaling).

  2. Plasmogamy: Fusion of hyphae from different individuals.

  3. New hyphae have two unique nuclei.

  4. Karyogamy: Fusion of nuclei occurs much later, before meiosis that produces haploid spores.

C. Asexual Reproduction

  1. Some species exclusively reproduce asexually.

  2. Molds: Produce haploid spores through mitosis.

  3. Yeasts: Reproduce via asexual cell division.

  4. Deuteromycetes (imperfect fungi): Lacking a known sexual stage; previously unclassified, now identifiable through molecular techniques.

IV. Evolution & Diversity of Fungi

A. Closest Relatives

  • Fungi and animals are more closely related than fungi to plants or most other eukaryotes.

B. Origin of Fungi

  1. Evolved from a unicellular, flagellated protist.

  2. Chytrids: The most basal fungal group, characterized by flagella.

  3. Opisthokonts: Group including fungi, animals, and protist relatives; suggests independent evolution of multicellularity in fungi and animals.

C. Fungi Transition to Land

  1. Possible adaptive radiation during the colonization of land by plants and animals.

  2. Fossils indicate early vascular plants had mycorrhizae, suggesting mutualism started with the initial colonization.

V. Fungal Lineages

A. Major Groups

  • Blastocladiomycota, Zygomycota, Neocallimastigomycota, Chytridmycota, Glomeromycota, Ascomycota, Basidiomycota.

  • Microsporidia: Uncertain phylogenetic placement; possibly a sister group to Fungi, small eukaryotic pathogens.

B. Fungal Groups with Zoospores

  • Zoospores: Asexual spores with flagella, indicating ancestry with choanoflagellates.

    • Blastocladiomycota: Mostly decomposers, some pathogens, both terrestrial and aquatic.

    • Neocallimastigomycota: Anaerobic, lack mitochondria, found in herbivore guts; help digest cellulose and lignin acquired from bacteria.

    • Chytridmycota: Mainly aquatic, decomposers, some are parasites affecting amphibians.

C. Zygomycota

  • Not a monophyletic group; recognized by the formation of zygospores during stress.

  • Examples: Rhizopus stolonifer (bread mold) and Pilobolus (shoots spores towards light).

D. Glomeromycota**

  • Also called Arbuscular mycorrhizae; symbiotic with 90% of plant species.

E. Ascomycota

  • Known as sac fungi: form ascocarps housing asci that contain sexually produced spores.

  • Reproduce asexually via conidia; include important pathogens, saprobes, and form lichens.

F. Basidiomycota

  • Referred to as club fungi; characterized by a long-lived dikaryotic mycelium producing fruiting bodies (basidiocarps).

  • Basidium: Diploid spore-producing part.

  • Examples include wood decomposers, mycorrhizae, and plant pathogens like rusts and smuts.

VI. Ecological Impacts

A. Decomposers

  1. Critical in breaking down cellulose and lignin; process facilitates nutrient recycling.

B. Symbionts

  1. Form vital associations with plants, algae, and animals:

  • Mycorrhizae: Mutualism with plant roots, aiding mineral uptake essential for plant health.

  • Endomycorrhizae and Ectomycorrhizae: Two types based on interaction with plant roots.

  1. Lichens: Associations of algae or cyanobacteria with fungi; initially viewed as mutualistic, now understood as parasitic in nature.

C. Pathogens

  1. Approximately 30% of fungi are parasitic, predominantly affecting plants.

  • Examples: Dutch elm disease, chestnut blight, ergot.

  1. Humans are affected by fungal infections but are less prone than plants; common infections include athlete's foot and yeast infections.

D. Commercial Importance

  1. Culinary uses: Basidiocarps and truffles from ascomycetes.

  2. Yeasts: Vital in bread, beer, and wine production.

  3. Antibiotics: E.g., Penicillium.

  4. Research: Saccharomyces cerevisiae is a model organism for biological studies.