Fungal Diversity: Phyla Chytridiomycota and Zygomycota

Fungal Diversity

• Phylum Chytridiomycota:
  • Found in freshwater and terrestrial habitats
  • Can be decomposers, parasites, or mutualists
  • Early divergent group in fungal evolution
  • Unique feature: flagellated spores (zoospores)
• Phylum Zygomycota:
  • Diverse life histories (fast-growing molds, parasites, commensal symbionts)
  • Life cycle of black bread mold (Rhizopus stolonifer) is typical of the phylum
  • Coenocytic hyphae with haploid spores produced in asexual sporangia
  • Sexual reproduction involves zygosporangia, which are resistant to freezing and drying
• Phylum Glomeromycota:
  • Previously classified as Zygomycota, now in a separate clade
  • Form arbuscular mycorrhizae
• Phylum Ascomycota:
  • Found in marine, freshwater, and terrestrial habitats
  • Produce sexual spores in saclike asci contained in fruiting bodies (ascocarps)
  • Commonly called sac fungi
  • Vary in size and complexity (unicellular yeasts to elaborate cup fungi and morels)
  • Include plant pathogens, decomposers, and symbionts
  • Reproduce asexually by enormous numbers of conidia

General Characteristics of Fungi

• Diversity:
  • Essential for terrestrial ecosystems (break down organic material and recycle nutrients)
  • 100,000 described species, estimated 1.5 million total species
• Nutrition:
  • Heterotrophs, absorb nutrients from outside their body
  • Use enzymes to break down complex molecules into smaller organic compounds
  • Diverse lifestyles (decomposers, parasites, mutualists)
• Morphology:
  • Multicellular fungi with mycelia (networks of branched hyphae) for absorption
  • Fungal cell walls contain chitin
  • Most fungi have septate hyphae with pores for cell-to-cell movement of organelles
  • Coenocytic fungi lack septa and have a continuous cytoplasmic mass with multiple nuclei

Specialized Hyphae

• Some fungi have hyphae specialized for capturing prey
• Some fungi have haustoria that penetrate host tissues
• Mycorrhizal fungi deliver phosphate ions and minerals to plants
• Types of mycorrhizae: ectomycorrhizal fungi (do not penetrate root cells) and arbuscular mycorrhizal fungi (penetrate root cells)

Characteristics of Fungi

• Fungi are eukaryotes with cells that have a nucleus and membranous organelles.
• There are approximately 100,000 named species of fungi, but scientists believe there may be around 1.5 million different species on Earth.
• The largest known organism is a fungus, covering over 2,000 acres of land and estimated to be 3,000 years old.

Nutrition and Digestion

• Fungi are heterotrophic, obtaining nutrients by breaking down and absorbing organic matter.
• They absorb nutrients through their plasma membrane, unlike animals, which ingest their food.
• Digestion occurs both outside and inside fungal cells.

Lifestyles of Fungi

• Decomposers break down organic matter.
• Parasitic fungi benefit at the expense of another organism.
• Mutualistic fungi provide benefits to their host and receive benefits in return.

Body Structure

• A fungus's body is called a mycelium, composed of thread-like filaments called hyphae.
• Hyphae are too small to be seen with the naked eye and are often hidden underground or inside dead trees.
• Visible parts of fungi, such as mushrooms, are only the reproductive structures.

Cell Walls and Chitin

• Fungal cell walls are composed of chitin, a unique characteristic among organisms.
• Chitin is also found in the exoskeletons of arthropods, such as crabs and insects.

Types of Fungi

• Septate fungi have partial divisions between their nuclei, with dividers called septa.
• Coenocytic fungi lack these dividers and have a single, large cell containing many nuclei.
• Specialized forms of fungi have modified hyphae for trapping prey or forming associations with plants.

Mycorrhizae

• Mycorrhizae is a symbiotic relationship between fungi and plant roots.
• There are two types: ectomycorrhizae, which only penetrate the spaces between cells, and endomycorrhizae, which penetrate the cell walls.
• Both types form a mutualistic relationship, with the fungus collecting water and providing it to the plant, and the plant providing sugars and amino acids to the fungus.

Fungi Overview

• The golden mushroom in Oregon is one of the oldest living organisms on earth and the most massive.

Fungal Reproduction

• Fungi can engage in both sexual and asexual reproduction.
• In sexual reproduction, fungi use pheromones to attract a mate in dark environments.
• When two fungi mate, their cytoplasm joins, and their nuclei form a dikaryon, which can last from hours to centuries.
• Eventually, the nuclei fuse, forming a zygote, the only diploid stage in the fungal life cycle.

Zygote Stage

• The zygote is the only diploid stage in the fungal life cycle.
• The zygote undergoes meiosis, halving the number of chromosomes, to form haploid spores.

Asexual Reproduction

• In asexual reproduction, mycelium produces spore-modifying structures or spore-producing structures using haploid nuclei.
• These structures produce haploid spores through mitosis.
• Examples of asexual reproduction include:
  • Penicillin mold growing on orange surfaces
  • Molds producing large amounts of spores
  • Yeast cells reproducing through budding
• Asexual reproduction allows fungi to quickly reproduce when resources are abundant.

Yeast Cells

• Yeast cells are single-cell organisms and an exception to the rule that fungi are multicellular.
• During budding, a mature yeast cell produces an outgrowth that eventually becomes a new cell, making yeast cells briefly multicellular.

Characteristics of Basidiomycota

• Known as club fungi due to the club-like structure of the basidia
• Most edible mushrooms belong to this phylum
• Includes bioluminescing mushrooms

Basidia and Spores

• Basidia: structures where spores are found, can be singular or plural
• Gills: reproductive structures, not for respiration; found in mushrooms
• Hundreds of gills in each mushroom, containing basidia and spores
• Basidiospores created through meiosis in the basidia

Life Cycle Stages

• Hyphae: underground, haploid, and of opposite mating types
• Fusion of hyphae creates a diploid structure
• Basidio carp: reproductive body growing above ground
• Basidia on the gills produce spores through meiosis
• Four haploid, single-celled basidiospores created in each basidia
• Spores released into the environment; can grow into new hyphae under suitable conditions

Important Terms

• Basidio: refers to the reproductive body
• Carp: entire structure growing above ground
• Basidia: club-like structure where spores are found (singular or plural)
• Basidiospor: spores created in the basidia
• Meiosis: process of basidiospore creation in the basidia

Characteristics of Ascomycota

• Referred to as the sac fungi
• Spores are held in a structure called an ascus

Examples of Ascomycota

• Truffles
• Morel mushrooms
• Cup fungi

Sexual Reproduction in Ascomycota

• Hyphae (haploid) grow and touch, forming an ascocarp (diploid)
• Ascocarp grows into a visible fungus
• Meiosis occurs in an ascus within the ascocarp, producing eight ascospores in each ascus

Key Structures in Ascomycota

• Ascocarp: the reproductive structure in Ascomycota
• Ascus: the spore-making structure within the ascocarp where meiosis occurs

Life Cycle of Ascomycota

• Hyphae of opposite mating types come together and form an ascocarp
• Ascocarp grows into a visible fungus
• Meiosis occurs in the ascus, producing ascospores
• Ascospores are released into the environment
• Ascospores grow into hyphae in suitable conditions, restarting the cycle