The Wonderful World of Fungi
Before we wade in, let's see where we are, phylogenetically.
Several groups of organisms known collectively as
"slime molds" were once included in Kingdom Fungi, but since
have been determined to be only distantly related to them. Some of these
might end up in their own Kingdoms some day.
What is a Fungus?
Along with bacteria, these are the most important DECOMPOSERS in the
biosphere. They convert dead, organic matter into its inorganic
mycelium - the entire body of the fungus
hyphae - (singular, hypha) threadlike structures of which the mycelium is composed
thallus - another term for the mycelium; "body"
spore - haploid propagule produced via meiosis
sporangium - structure within which spores are produced (either
sexually or asexually, depending on life cycle stage)
Fungi live as haploid hyphal masses for most of the life cycle
Classification is based upon the mode of sexual reproduction
Hyphae come in two complementary "mating types" generally called "+" and "-" (since
they are not easily definable as male and female, to which "+" and "-" can be considered analogous.)
all species--whether free-living or parasitic--are absorptive
heterotrophs: they secrete digestive enzymes onto their nutrient source
(whether alive or dead) and absorb the digestive products of those
enzymes' interactions with the nutrient source.
The main structural support in the cell walls is chitin. There is no
cellulose present in Fungi except for one small, aberrant group (of
Fungi lack true tissues, and are composed of threadlike hyphae.
Main storage carbohydrate is glycogen (as in animals), not plant starch
Some phyla are characteristically coenocytic (a mass of cytoplasm
with multiple nuclei) without any physical separation of of the cytoplasm.
Other phyla are characteristically septate: the cytoplasm is divided
into hyphal compartments by septa (walls). The cytoplasm within each compartment may still be multinucleate (coenocytic).
- 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
Some interesting notes
Pathogenic/parasitc fungi have specialized hyphae
called haustoria, which are used to invade the host's cells and
create a nutrient pathway between fungus and host.
Fungi exist in different forms
- mold - rapidly growing, asexually reproducing hyphal fungus.
- yeast - a unicellular fungus, usually found growing in liquid or moist environments.
Major Fungal Taxa
Fungal taxa have undergone a great deal of revision in recent years, and a currently accepted phylogeny can be seen here. We'll focus on only a few groups for now.
These fungi retain primitive characters that may provide clues about fungal origins.
A chytrid parasite, Batrachochytrium dendrobatidis, has been implicated in the recent large-scale extinctions of amphbians, worldwide. More on this later.
- Most are aquatic, suggesting an aquatic origin of the Fungi.
- Some are free-living saprobes, others are parasites of protists, plants and aquatic
- 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
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
Each ascospore germinates and develops into a new, genetically
unique + or - hypha.
- 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
An old name you might hear: Deuteromycota (a.k.a., "Fungi Imperfecti")
This form taxon was created to include fungi that have secondarily lost the ability to
sexually reproduce. Molecular data have placed most of the former fungi imperfecti in Ascomycota.
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 classified within the Ascomycota.
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 -
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
Be sure to review the different types of symbiosis.
There are predatory fungi
- Arthrobotrys (a Deuteromycete) nabs nematodes and other small prey
There are prey fungi
- We eat many species--but be sure you know what you're doing!
There are parasitic fungi
- rainforest ascomycetes that take over the nervous system of arthropod
- 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.
There are mutualistic fungi
- 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
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.