Characteristics of Fungi
• The fungi live by absorptive nutrition, secreting
digestive enzymes that break down large food molecules and absorbing the breakdown products.
• Some are saprobes (feeding on dead matter); others are
parasites.
• A few have mutually beneficial (symbiotic) relationships with other organisms. Mycorrhizae are mutualistic
associations of fungi and plant roots. The fungus obtains organic compounds, while the plant is provided with water and soil minerals
.
Characteristics of Fungi
• The production of chitin is a shared derived trait for fungi, choanoflagellates, and animals. Chitin is a
complex polysaccharide found in the exoskeletons of arthropods (insects) and some Protists. The
presence of chitin in fungi is evidence that all fungi are more closely related to animals than to plants.
• The kingdom Fungi consists of four phyla:
Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota.
Characteristics of Fungi
• Unicellular forms are
found in all of the
fungal phyla.Those of
the Zygomycota,
Ascomycota, and
Basidiomycota are
called
yeasts
. Yeasts
may reproduce by
• The vegetative body of a multicellular fungus is called the mycelium (plural mycelia). This compose the majority of the fungi and can cover large areas!
• The mycelium is composed of threadlike filaments of cells called hyphae (singular hypha).
• Within the hyphae of two clades, cell-like compartments are formed by incomplete cross-walls called septa
(singular septum). Fungi which have the cross-walls are called Septate fungi.
• Pores in septa allow free movement of organelles, sometimes even nuclei, and other materials.
• Some hyphae with out the septa and are like a continous tube of chitin. They are described as
coenocytic—they have no internal separations (septa) , therefore no compartments. Fungi which are coenocytic are called Aseptate fungi.
Characteristics of Fungi
• The hyphae may be widely dispersed to forage fornutrients or they may be clumped together in a cottony mass to exploit a rich nutrient source.
• Sometimes the mycelium becomes reorganized into a fruiting (reproductive) structure, such as a
mushroom.
• Rhizoids are modified hyphae, which anchor
Chytridiomycota to a substrate. These rhizoids are not homologous to the rhizoids of plants because they are
not specialized to absorb water and nutrients. Throughout the mycelium (except in fruiting structures), all the hyphae are very close to their environmental food source.
• Fungi are tolerant to highly hypertonic environments. (Can grow on materials which have a high sugar
concentration, when bacteria can not.)
• Many can tolerate temperatures as low as 5–6C below
freezing. Some can tolerate temperatures as high as 50
Characteristics of Fungi
• The majority of fungi are saprobes, living on dead organisms.
• Saprobic fungi (along with bacteria) are Earth’s primary decomposers.
• Fungi are the principal decomposers of cellulose and lignin.
• Facultative parasites can attack living organisms but they can also be grown on defined media.
• Obligate parasites grow only on their specific host.
• Most predatory fungi secrete sticky substances from the hyphae. Trapped prey are penetrated by hyphae and
eventually killed.
• Some predatory species form a ring with modified
Impact of Fungal Parasites
• Fungal pathogens are a major cause of death among people with compromised immune systems.
• Most patients with AIDS die of fungal diseases such as Pneumocystis carinii.
• Candida albicans and other yeasts also cause severe diseases in those with AIDS.
• Other less severe and common diseases include ringworm and athlete’s foot.
• Plant diseases include black stem rust, smuts, blights, mildews, and others. Many are responsible for extensive crop and forest damage. The American Chestnut blight is due to a fungal parasite that was introduced
Fungal Asexual Reproduction
• Asexual reproduction among the fungi
includes:
– The production of haploid spores within
sporangia
.
– The production of naked spores at the tips of
hyphae (not within sporangia) called
conidia
.
– Cell division by unicellular fungi—either equal
division (
fission
) or production of a daughter
cell (
budding
).
Fungal Sexual Reproduction
• Such fungi have some unusual sexual
reproduction features:
– There are no gamete cells, just gamete
nuclei.
– The hyphae are neither diploid (2
n
) or haploid
(
n
), but dikaryotic (
n
+
n
). Each hyphae
Fungal Sexual Reproduction
• Sexual reproduction involves fusion between different mating types.
• Some fungi have more than two mating types.
• Mating types cannot be distinguished morphologically. • Mating can occur only between different mating types,
which prevents self-fertilization.
• Fungi reproduce sexually when hyphae (or motile cells in chytrids) of different mating types meet and fuse.
• In many fungi, the zygote nuclei are the only diploid nuclei of the life cycle.
• These nuclei undergo meiosis, producing haploid nuclei. • Haploid spores divide mitotically to form haploid hyphae. • This type of life cycle is called a haplontic life cycle and
Phylum: Chytridiomycota (Water molds)
• The chytrids (phylum
Chytridiomycota) are the earliest diverging fungal lineage.
• They are aquatic
microorganisms, formerly classed with protists but now classed with fungi because of the chitin in their cell walls. • They are the only fungi that
have flagella at any stage of the life cycle.
• Chytrids are parasitic or
saprobic, but some are found in the rumen of ruminants. Most live in fresh water or moist soil; some are marine.
• Some are unicellular; others have coenocytic hyphae.
Phylum: Zygomycota (Terrestrial Molds)
• Zygomycetes (phylum
Zygomycota) have
coenocytic hyphae; they have only one diploid cell, the
zygote.
• Most form occasional stalked reproductive structures called
sporangiophores.
• Sporangiophores may have one or many sporangia.
• One group are the fungal species in the most common mycorrhizal associations.
Phylum: Ascomycetes (Sac fungi)
• The ascomycetes (phylum Ascomycota) are a large and diverse
group with septate hyphae, and distinguished by the production of asci
(singular ascus).
• The ascus contains the products of meiosis. • There are two groups of ascomycetes:
– Those with an ascocarp are called euascomycetes (“true ascomycetes”). Euascomycetes reproduce asexually by means of conidia that form at the tips of specialized hyphae. Sexual reproduction of euascomycetes includes formation of a dikaryon.
- Those without are called hemiascomycetes (“half ascomycetes”). Most hemiascomycetes are microscopic and some are unicellular.
Baker’s or brewer’s yeast (Saccharomyces cerevisiae) is an
ascomycete. Hemiascomycete yeasts reproduce asexually by budding or fission.
Penicillium is a genus of green molds. Some species produce the antibiotic penicillin.
Phylum: Basidiomycota (Club Fungi
)
• About 25,000 species of basidiomycetes (phylum Basidiomycota) have been described.
• They produce a wide variety of fruiting structures (basidiocarps): puffballs, mushrooms, and giant bracket fungi.
• There are more than 3,250 species of mushrooms.
• Agaricus bisporus is the common edible one; some Amanita mushrooms are deadly poisonous.
• Bracket fungi are tree parasites. • Smut fungi parasitize cereal grains.
• Basidiomycetes have septate hyphae. Basidia, swollen cells at the tips of hyphae, are the sexual reproductive structures.
• Meiosis gives rise to four haploid nuclei, incorporated into
basidiospores on the outside of the basidium.
• Basidiospores give rise to haploid hyphae. Hyphae of different mating types meet and fuse, forming dikaryotic hyphae.This stage may persist for years, decades, or even centuries.
• Eventually, the dikaryotic mycelium produces a fruiting body called a
basidiocarp. These are what you typically recognize as or call a mushroom.
Lichens
• Lichens are a meshwork of two different organisms. One is a fungus, and the other is a photosynthetic organism.
• Lichens can survive harsh environments.
• In spite of this hardiness, lichens are sensitive to air pollution because they cannot excrete toxic substances. Hence they are good
biological indicators of air pollution.
• The fungi of most lichens are ascomycetes. • The photosynthetic component may be either
a cyanobacterium or a unicellular green alga. The fungal components cannot grow
independently of their photosynthetic partners.
• Lichen growth forms include crustose,
foliose, and fruticose.
• Lichens reproduce simply by fragmentation of the vegetative body called the thallus.
• They can also reproduce by means of specialized structures called soredia
