Chapter 31: Fungi

Kingdom Fungi:

-Mycology: study of fungi

1) One of Earth’s obscure lifeforms
- One spanning 86 acres, mostly underground, the Honey Mushroom (Armillaria ostoyae)
- One spanning 2000 acres in Oregon, another Honey Mushroom Honey Mushroom (Armillaria mellea)
-Resemble plants, but lack chloroplasts, so they don’t photosynthesize
-Are not animal, nor do they resemble bacteria or protozoa.
2) Microscopic in size to very large (mushrooms, mildew, molds and morels). Parasitic and saprophytic, major world decomposers.

Characteristics of Fungi:

A) General:

1) Major Decomposers along with bacteria
2) Can cause plant disease, accounting for millions of dollars in crop losses each year.
3) Can cause human diseases such as ringworm, athlete’s foot, and yeast infections (most yeasts filamentous)

B) Nutrition:

-Heterotrophic
-External Digestion: excrete enzymes which break down or “digest” food which is then absorbed by fungi.
1) Extensive network of hyphae provides an enormous surface area for absorption.
2) Animals are heterotrophic by digestion, fungi are heterotrophic by absorption.
-Most fungi are saprotrophic decomposers: break down waste products and dead remains of plants and animals.
-Some are parasitic: they live off the tissues of living plants and animals. (page 606, Figure 31.5)

C) Structure:

-Some are unicellular, such as yeast (not common).
-Most are multicellular eukaryotes.
1) Body (thallus) is a multicellular structure known as the mycelium.
2) A mycelium is a network of filaments called hyphae (page 605, Figure 31.3)
3) Hyphae (plural) Hypha (singular)- slender filaments
-Some have septa- crosswalls that divide the hyphae into cells.
-Septa only form a complete barrier when separating reproductive cells.
-Otherwise, the organism is multi-nucleated with cytoplasm flowing throughout the hyphae.
4) Types of hyphae:
-Heterokayotic: 2 genetically distinct nuclei in the same hyphae.
-Homokaryotic: hyphae with nuclei genetically the same
(Next two types used when discussing reproductive hyphae)
-Dikaryotic: 2 genetically distinct nuclei in the same cell
-Monokaryotic: one nucleus in each cell.
-Coenocytic: aseptate hyphae (no septa- no division between cells- like plasmodium, the "true" slime mold)
-Haustoria: specialized hyphae found in parasitic fungi that penetrates and absorbs nutrients directly from the cells of the host.
5) Cell walls consist of polysaccharides, including chitin, the same substance found in the outer structure of insects and crustaceans.

D) Reproduction:

-Asexual reproduction: involves spores or fragmentation of mycelium
-Sexual reproduction: can occur several ways:
1) Fusion of gametes released from gametangia (only in Ascomycota)
2) Penetration of gametes (or gamete) into the gametangium (only in Basidiomycota).
3) Fusion of gametangia (Zygomycota).

-Both sexual and asexual reproduction produce spores, ensuring dispersal of the species. All spores nonmotile.

E) Four Phyla of Fungi: Classification based on the features related to sexual reproduction.

1) Phylum Chytridiomycota:
-Chytrids (common name)
-Oldest fossil fungi, are the sister phylum to the remaining fungal phyla, found in northern Russia
-Predominantly aquatic
-Only fungus that has flagellated gametes
-The closest relative to fungi in the Kingdom Protista also have flagella.
-One species has been tied to the decline in frog populations worldwide (once classifies as a protist), parasitic.

Lab Example: Allomyces.

2) Phylum Zygomycota:
-Commonly known as zygomycetes
-Approximately 1050 species (small phylum).
-Saprotrophs, living off plant and animal remains in the soil or bakery goods.
-Hyphae are multinucleate, lack septa (except where involved with reproduction).

Lab Example: Rhizopus stolonifer- black bread mold. Distinct characteristics: zygosporangium- results from fusion of gametangia. (page 608, Figure 31.7)

3) Phylum Ascomycota:
-Commonly known as ascomycetes, sac fungi.
-Examples:
a) yeast- unicellular oddity
b) morels- are not mushrooms
c) truffles
d) powdery mildews
-Plant pathogens: Ophiostoma ulmi is the organism that causes Dutch Elm Disease (The organism that causes Chestnut Blight is in this phylum also).
-Distinct characteristics:
a) Formation of conidia- structure (spores) formed during asexual reproduction
b) Formation of ascus- sac-like structure where the zygote is formed, sexual reproduction (pages 610-611, Figure 31.9)
-Yeast: most reproduce asexually via cell fission or budding
a) Most economically useful fungus
b) Used for baking/beer (Saccharomyces cerevisiae)/ wine
c) Are eukaryotic cell of choice for genetic and biomedical research (rapid generation time).
d) Yeast infections: candida, thrush

4) Phylum Basidiomycota
-Known as basidiomycetes, club fungi
-Examples: mushrooms, toadstools, puffballs, jelly fungi, and shelf fungi.
-Plant pathogens: rusts and smuts (rusts are brightly colored and smuts look like a black smudge)
-Distinct characteristics:
a) Sexual reproductive structure: basidium, which is a club-shaped spore and is located under the cap of the mushroom or in the tubes at the base of pores in shelf fungi.
-Examples:
a) Birds nest fungi's basidiospore is dispersed by raindrops
b) Stinkhorn- Phallus impudicus, emits a disagreeable odor which attracts files who unintentionally pick up spores and distribute them.
c) Smuts and rusts are club fungi that parasitize cereal crops.
-Rusts often require two different host plants to complete their life cycle; such as apple and juniper.
-Example: Black stem rust of wheat, requires barberry bushes. Blister rust of white pine and black currant.
-Other interesting mushrooms:
a) Agaricus capestris: field mushroom; commercially grown species.
b) Amanita phalloides: destroying, death angel (10-12 hours after consumption = death)
c) Psilocybe mexicanna: used by Mexican Indians for religious ceremonies (contains psilocybin, which is structurally like LSD or mescaline)
d) Claviceps purpurea: ergot fungus, infects rye. Alkaloids from this fungus are used in medicine to cause uterine contraction and to treat circulatory disorders such as migraine headaches. Causes ergotism which is thought to be the cause of the frenzy surrounding the Salem witch trials (lysergic acid)

E) Fungal Associations:

1) Lichens:
-A symbiotic association between fungus (mostly ascomycetes) and a photosynthetic partner (cyanobacteria, green algae, or both).
3 Types:
a) Crustose: compact, found on bare rocks or tree bark
b) Foliose: leaf-like
c) Fruticose: shrub-like (stalked)
-Lichen are indicators of air pollution

Lab Examples: Assorted specimens, slides of Physcis

2) Mycorrhizae
-Greek: Mykes = fungus, Rhizion = root
-Symbiotic association between fungi and plant roots, in about 90% of plants.
-Gives greater absorptive surface for intake of minerals and water
a) replace and perform the same function as root hairs.
b) Important because it helps create larger crops
-2 Types:
a) Endomycorrhizae: (arbuscular mycorrhizae) The fungus is usually a zygomycete. Hyphae penetrate the plant's roots (roughly 200,000 associations of this kind)
b) Ectomycorrhizae: Do not penetrate plant root cells, usually a basidiomycetes.
Example: Truffle- mycorrhizal fungus living in association with oak and beech tree roots. (ascomycete)
(page 616, Figure 31.16)