Fungi Notes

Fungi

Fungal Structure and Function

  • Multicellular fungi extend filaments called hyphae into their surroundings.
  • Spores enable fungi to colonize new environments.
  • Spores germinate and grow under favorable conditions.
  • Hyphae secrete enzymes to break down organic matter.
  • Nutrients are absorbed after the breakdown of organic matter.
  • Almost any organic molecule can be digested by some fungi, making them effective decomposers.

General Characteristics of Fungi

  • Fungi are diverse and essential for most ecosystems.
  • Most are multicellular, but some are single-celled.
  • Fungi are heterotrophs that feed by absorption.
  • Key shared traits include their mode of nutrition.

Nutrition and Ecology

  • Fungi are heterotrophs that absorb nutrients from outside their bodies.
  • They use hydrolytic enzymes to break down complex molecules into smaller organic compounds.
  • These enzymes digest compounds from living or dead sources.

Ecological Roles

  • Decomposers: Break down and absorb nutrients from nonliving organic material.
  • Parasitic fungi: Absorb nutrients from living hosts.
  • Mutualistic fungi: Absorb nutrients from hosts and benefit the host.

Body Structure

  • Common structures include multicellular filaments and single cells (yeasts).
  • Many grow as both, but most as filaments.
  • Yeasts inhabit moist environments with soluble nutrients like sugars or amino acids.
  • Fungal bodies form networks of tiny filaments called hyphae.
  • Hyphae have tubular cell walls strengthened with chitin, a structural polymer.
  • Chitin-rich walls prevent cells from lysing due to osmotic pressure during nutrient absorption.

Hyphae

  • Hyphae form a mycelium.

Septa

  • Hyphae are divided into cells by cross-walls, or septa.
  • Septa have pores for cell-to-cell movement of organelles.
  • Coenocytic fungi lack septa and have numerous nuclei in a continuous cytoplasmic mass.

Mycelium

  • Fungal hyphae form a mycelium that infiltrates the food source.
  • The mycelium structure maximizes the surface-to-volume ratio for efficient absorption.
  • Hyphae grow in length using cytoplasmic streaming to move materials to the tips.
  • Multicellular fungi are non-motile but colonize new areas through hyphal growth.

Specialized Hyphae

  • Some fungi have specialized hyphae for feeding on live animals.
  • Haustoria are specialized hyphae that extract nutrients from plants.

Mutualistic Fungi

  • Mutualistic fungi have specialized branching hyphae to exchange nutrients with plant hosts.
  • Arbuscules are specialized hyphae that penetrate plant cell walls but not the cell membrane.

Mycorrhizae

  • Mutually beneficial relationships between fungi and plant roots are called mycorrhizae (“fungus roots”).
  • Mycorrhizal fungi deliver phosphate ions and minerals to plants.
  • Plants supply organic nutrients to the fungi.
  • Most vascular plants depend upon mycorrhizae.
  • Mycorrhizal fungi colonize soils by dispersing haploid spores.

Types of Mycorrhizal Fungi

  • Ectomycorrhizal fungi: Form sheaths of hyphae over the root surface and extend into extracellular spaces of the root cortex.
  • Arbuscular mycorrhizal fungi: Extend arbuscules through the root cell wall and into tubes formed by invagination of the root cell plasma membrane.

Spore Production

  • Fungi propagate by producing vast numbers of spores sexually or asexually.
  • Spores are carried long distances by wind or water.
  • In moist conditions with food, spores germinate and form new mycelia.

Sexual Reproduction

  • Fungal nuclei and spores are usually haploid.
  • Some species have transient diploid nuclei during sexual life cycles.
  • Sexual reproduction requires the fusion of hyphae from different mating types.
  • Fungi use pheromones to communicate mating types.

Plasmogamy and Heterokaryon

  • Plasmogamy is the union of cytoplasm from two parent mycelia.
  • Fusion between haploid nuclei of the parents is often delayed.
  • A mycelium with coexisting, genetically different nuclei is called a heterokaryon.
  • In some fungi, haploid nuclei pair off two to a cell, forming a dikaryotic mycelium.

Karyogamy and Meiosis

  • Hours, days, or centuries may pass before nuclei fuse in karyogamy.
  • When haploid nuclei fuse, a diploid cell, like a zygote, is produced.
  • The short-lived diploid cell undergoes meiosis, producing haploid spores.
  • Karyogamy and meiosis result in genetic variation.

Asexual Reproduction

  • Many fungi use both sexual and asexual reproduction, but some use only one.
  • Molds produce haploid spores asexually by mitosis, forming visible, “furry” mycelia.

Asexual Reproduction in Yeasts

  • Single-celled yeasts reproduce asexually without spores.
  • Reproduction occurs through cell division or budding.

Deuteromycetes

  • Yeasts and filamentous fungi with no known sexual stage form a group called deuteromycetes.
  • They are reclassified if a sexual stage is discovered.
  • Genomic techniques are used to classify fungi.

Evolutionary Relationships

  • Fungi and animals are more closely related than either is to plants or most other eukaryotes.
  • The ancestor of fungi was an aquatic, single-celled, flagellated protist.

Origin of Fungi

  • The opisthokonts clade includes fungi, animals, and their protistan relatives.
  • Opisthokonts evolved from a unicellular flagellated ancestor.

Fungi and Protists

  • Fungi are most closely related to single-celled protists like nucleariids, which feed on algae and bacteria.
  • Animals are most closely related to choanoflagellates.
  • Multicellularity likely evolved independently in fungi and animals.

Colonization of Land

  • Animals and fungi diverged more than a billion years ago based on molecular clock analysis.
  • Fungi likely originated in aquatic habitats, with terrestrial fossils dating back 440 million years.
  • Fungi may have colonized land as early as 505 million years ago.

Early Colonizers

  • Fungi were among the earliest colonizers of land.
  • Fossil evidence supports mutualistic relationships between fungi and early plants.
  • Sym genes required for mycorrhizal formation were present in early plants.

Fungal Diversity

  • Molecular analyses clarify evolutionary relationships among fungal groups.
  • Metagenomic studies have led to the discovery of new groups.
  • There are 145,000 known species of fungi; estimates range from 2.2 to 3.8 million.

Interactions and Roles

  • Fungi interact with other organisms as decomposers, mutualists, and pathogens.
  • Fungi play key roles in nutrient cycling, ecological interactions, and human welfare.

Fungal Groups

  • Cryptomycetes: Parasites with flagellated spores.
  • Microsporidia: Parasitic cells that form resistant spores.
  • Chytrids: Flagellated spores.
  • Zoopagomycetes: Resistant zygosporangium as a sexual stage.
  • Mucoromycetes: Include fungi that form arbuscular mycorrhizae with plants.
  • Ascomycetes: Sexual spores (ascospores) borne internally in sacs called asci; vast numbers of asexual spores (conidia) produced.
  • Basidiomycetes: Elaborate fruiting body (basidiocarp) containing many basidia that produce sexual spores (basidiospores).