General Facts

• saprobic fungus - absorptive heterotroph that feeds on dead, organic matter. Such a fungus is said to be a "saprobe."
• parasitic fungus - absorptive heterotroph that feeds on living matter.

Some interesting notes

• mold - rapidly growing, asexually reproducing hyphal fungus.
• yeast - a unicellular fungus, usually found growing in liquid or moist environments.

Tiptoe through the Taxa Several major taxa of fungi are generally recognized...

• Chytridiomycota (the "chytrids")
• Zygomycota (the black bread molds)
• Ascomycota (the sac fungi)
• Basidiomycota (the club fungi)

• Deuteromycota (the asexual fungi)
  These are probably all ascomycota which have secondarily lost the ability to reproduce sexually. Included are such economically important forms as Penicillium (the original source of penicillin and its derivatives) and Aspergillis (the original source of ascorbic acid, an agent widely used commercially to give tangy flavor to a wide variety of foods.

Let's meet the Fungi!

Chytridiomycota
These are the most primitive of fungi, and may provide clues as to the origins of fungi.

• Most are aquatic, suggesting an aquatic origin of the Fungi.
• Some are free-living saprobes, others are parasites of protists, plants and aquatic invertebrates.
• Like all fungi, they have chitin in the cell walls
• one aberrant group or Chytrids has cellulose in the cell walls--UNIQUE among fungi
• They have flagellated gametes (zoospores)--a PRIMITIVE character not shared by any other true fungi
• Molecular data support the link between chytrids and other fungal taxa.

Zygomycota - reproduce sexually via zygospores

• reproduction is via fusion (plasmogamy) of + and - hyphae
• at the junction of the hyphae, a zygosporangium is formed
• fusion of the + and - haploid nuclei (karyogamy) inside the zygosporangium results in formation of diploid zygotes inside the zygosporangium
• still inside the zygosporangium, the zygotes undergo meiosis to produce genetically new, haploid spores, called zygospores.
• these are released, and will germinate to become genetically new + and - hyphae.

Ascomycota - reproduce sexually via ascospores

• About 75% of all described fungal species are ascomycetes.
• The + and - hyphae fuse to produce a dikaryotic region of cytoplasm with two genetically distinct types of nuclei (a.k.a. heterokaryotic)
• This dikaryotic cytoplasm proliferates, with each haploid nucleus dividing and multiplying independently until...
• The dikaryotic hyphae develop into a fruiting body known as an ascocarp.
• The ascocarp bears specialized cells (called asci, because they resemble little sacs) in which karyogamy occurs.
• The diploid asci then divide meiotically to produce genetically unique ascospores.

Basidiomycota - reproduce sexually via basidiospores

• The + and - hyphae fuse to produce a dikaryotic region, which proliferates and grows, with each haploid nucleus dividing and multiplying independently until...

• The dikaryotic hyphae grow into a fruiting body known as an basidiocarp.
• The basidiocarp bears specialized cells (called basidia, because they resemble little clubs) in which karyogamy occurs.
• The basidia then divide meiotically to produce genetically unique basidiospores.
  
• Each spore germinates to develop into a genetically unique + or - hypha.

Glomeromycota - Vital to Ecosystems Mycorrhizae - "fungus roots"

This is a symbiotic relationship between a fungus and a plant root. (What does each partner get out of the relationship?)

• Vesicular Arbuscular Mycorrhizae (V.A.M.) - association between a glomeromycete and a plant

• Ectomycorrhizae - association between asco- or basidiomycete and a conifer or flowering plant (usually large trees).
  

Some of the most valuable edible organisms in the world are truffles, various species of mycorrhizal fungi.

An old name you might hear: Deuteromycota ("Fungi Imperfecti") - This artificial taxon is not monophyletic. Most members appear to be ascomycetes that have secondarily lost the ability to sexually reproduce.

Like typical Ascomycetes, they reproduce asexually via spores called conidia borne on hyphal structures called conidiophores. Common examples are Penicillium and Aspergillis, which are now generally classified within the Ascomycota.

Symbiotic Fungi

Before we proceed, let's have a quick overview of symbiosis.

• Arthrobotrys (a Deuteromycete) nabs nematodes and other small prey

• We eat many species--but be sure you know what you're doing!

• rainforest ascomycetes that take over the nervous system of arthropod hosts
• athlete's foot
• fungi that inhabit warm, moist places (ew)
• histoplasmosis (infects the lungs)
• Dutch Elm disease threatens native U.S. populations of elm trees (it's carried by a bark beetle) (This is NOT the same as citrus canker or lethal yellowing of palm trees, both of which are viral.)
• some fungi which infect rotting food (such as grain crops) can be carcinogenic (e.g., some species of Aspergillis or highly toxic (ascomycete ergots - a smut disease which infects plants). Ergots are sclerotia (resting stage of the fungus) which replace the ovaries of infected grasses. These are loaded with highly toxic alkaloids which can cause central nervous system damage, smooth muscle contraction, necrosis, hallucinations and temporary insanity!). Fungal toxins are collectively known as mycotoxins, and are extremely economically important.
• A chytrid fungus, Batrachochytrium dendrobatidis, has been implicated in the recent large-scale extinctions of amphbians, worldwide. (A possible "canary in the coal mine" regarding global climate change?)
• If you are not of delicate sensibility, click HERE to show what fungus can do to an immunosuppressed person.

• Lichens - a symbiotic association between a fungus and a photoautotroph.
  • Lichens are found almost everywhere, but most notably are successful in harsh climates (dry, cold) where other organisms find it difficult or impossible to survive.
  • Relationship: fungus provides a habitat for the algae (or cyanobacteria) (in an area where they could not otherwise survive) and the algae provide photosynthates for the fungus.
  • Lichens are very effective at absorbing nutrients directly from the atmosphere, and for this reason are very sensitive to smog.
  • Lichens are important primary producers in harsh environments such as tundra.
  • Lichens are the original source of the compound used to make litmus paper (an old-fashioned way to tell if a solution is acidic or basic).

And don't forget the invaluable service fungi provide, along with bacteria, as the earth's major decomposers.

And now, a moment of silence to fully appreciate the fungi.