Chapter 31

• Fungi


  Diverse and widespread organisms that are essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrients
• Fungi


  • Some fungal filaments absorb nutrients from the soil, transfer them to trees, and receive sugars in return
  • About 100,000 species of fungi have been described
  • It is estimated that there are as many as 1.5 million species of fungi
• Nutrition and Ecology

  Fungi are heterotrophs that absorb nutrients from outside of their bodies

  Fungi use enzymes to break down a large variety of complex molecules into smaller organic compounds

  Fungi exhibit diverse lifestyles: decomposers, parasites, and mutualists
• Decomposers


  Break down and absorb nutrients from nonliving organic material
• Parasitic fungi


  Absorb nutrients from living hosts
• Mutualistic fungi


  Absorb nutrients from hosts and reciprocate with actions that benefit the host
• Body Structure

  The most common body structures are multicellular filaments and single cells (yeasts)

  Some species grow as either; others grow as both

  The morphology of multicellular fungi enhances their ability to absorb nutrients

  The body of fungi form networks of branched hyphae adapted for absorption

  Hyphae have tubular cell walls strengthened with chitin

  Fungal hyphae form an interwoven mass called a mycelium

  The structure of a mycelium maximizes surface-to-volume ratio, making feeding very efficient
• Septa


  Divide hyphae into cells, with pores allowing cell-to-cell movement of organelles
• Coenocytic fungi


  Lack septa and have a continuous cytoplasmic mass with hundreds or thousands of nuclei
• Arbuscules


  Branching hyphae used by mutualistic fungi to exchange nutrients with plant hosts
• Fungal Spores


  Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually

  Spores can be carried long distances by wind or water; they will germinate if they land in moist conditions with available food
• Sexual Reproduction

  1. Plasmogamy: union of cytoplasm from two parent mycelia

  2. Haploid nuclei from each parent do not fuse right away; they coexist in the fused part of the mycelium

  3. Karyogamy: nuclear fusion, producing diploid cells

  4. Diploid phase is short-lived and undergoes meiosis, producing haploid spores
• Dikaryotic mycelium


  Haploid nuclei pair off two to a cell
• Asexual Reproduction


  1. Molds produce haploid spores by mitosis and form visible mycelia

  2. Yeasts reproduce asexually by simple cell division and the pinching of "bud cells" from a parent cell

  3. Some fungi can grow as yeasts and as filamentous mycelia
• Opisthokonts


  Clade that includes fungi, animals, and their protist relatives

  Evolved from a unicellular, flagellated ancestor
• The oldest undisputed fossils of fungi are about 460 million years old
• Fungi were among the earliest colonizers of land and formed mutualistic relationships with early land plants
• Chytrids


  Found in terrestrial, freshwater, and marine habitats including hydrothermal vents

  Can be decomposers, parasites, or mutualists

  Unique among fungi in having flagellated spores, called zoospores
• Mucoromycetes


  Include fast-growing molds, parasites, and commensal symbionts

  Asexual sporangia produce haploid spores

  Reproduce sexually via zygosporangia which generate zygospores

  Zygosporangia are the site of karyogamy and then meiosis

  Zygosporangia can survive unfavorable conditions
• Pilobolus


  A mucoromycete that can "aim" and shoot its sporangia toward bright light
• Glomeromycetes


  Part of the mucoromycetes

  Nearly all species form arbuscular mycorrhizae
• Ascomycetes


  Commonly called sac fungi, live in marine, freshwater, and terrestrial habitats

  Vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels

  Include plant pathogens, decomposers, and symbionts

  Produce sexual spores in saclike asci contained in fruiting bodies called ascocarps

  Reproduce asexually by enormous numbers of asexual spores called conidia produced at the tips of specialized hyphae called conidiophores
• Basidiomycetes


  Also called club fungi, include mushrooms, puffballs, and shelf fungi

  Some form mycorrhizae, are plant parasites, and many decompose wood

  The mycelium can reproduce sexually by producing fruiting bodies called basidiocarps

  Defined by a clublike structure called a basidium, a transient diploid stage in the life cycle

  The numerous basidia in a basidiocarp produce sexual spores called basidiospores

  Life cycles include a long-lived dikaryotic mycelium
• Roles of Fungi as Decomposers

  Fungi serve as decomposers, mutualists, and parasites

  Efficient decomposers of organic material including cellulose and lignin

  Perform essential recycling of chemical elements between the living and nonliving world

  An ecosystem could not function without them
• Fungus-Plant and Fungus-Animal Mutualisms

  Mycorrhizae are mutually beneficial relationships between fungi and plant roots

  Plants harbor harmless symbiotic endophytes, fungi that live inside leaves or other plant parts

  Some endophytes make toxins to help defend the host plant; others help the plant tolerate heat, drought, or heavy metals

  Some fungi digest plant matter for animals, helping break down plant material in the guts of cows and other grazing mammals

  Many species of ants use the digestive power of fungi by raising them in "farms"
• Lichens


  Symbiotic associations between a photosynthetic microorganism (green algae or cyanobacteria) and a fungus, most often an ascomycete

  The algae provide carbon compounds and often organic nitrogen and fungi provide the environment for growth

  Lichens were on land 420 million years ago and may have modified rocks and soil much as they do today, helping pave the way for plants

  Millions of photosynthetic cells are held in a mass of fungal hyphae

  Algae or cyanobacteria occupy an inner layer below the lichen surface
• Fungi as Parasites

  About 30% of known fungal species are parasites or pathogens, mostly on or in plants

  Each year, 10% to 50% of the world's fruit harvest is lost due to fungi

  Some fungi that attack food crops are toxic to humans

  Animals are much less susceptible to parasitic fungi than are plants

  The chytrid Batrachochytrium dendrobatidis has been implicated in the decline or extinction of about 200 species of amphibians worldwide

  The general term for a fungal infection in animals is mycosis
• Cordyceps fungi and zombie ants


  Parasitic fungi that infect and control the behavior of ants
• Practical Uses of Fungi

  Humans eat many fungi and use others to make cheeses, alcoholic beverages, and bread

  Some fungi are used to produce antibiotics for the treatment of bacterial infections, such as the ascomycete Penicillium
• Fungal Phyla

  • Chytridiomycota (chytrids)
  • Mucoromycota (Mucoromycetes)
  • Glomeromycetes (Mucoromycota; arbuscular mycorrhizal fungi)
  • Ascomycota (ascomycetes)
  • Basidiomycota (basidiomycetes)