Chapter 31 – Fungi
- Eukaryotes that grow as single cells or as large, branching networks of multicellular filaments
- Along with land plants and animals, fungi are one of 3 major lineages of large, multicellular euks
that occupy terrestrial environment.
- Fungi absorb nutrition from other dead or alive animals
- Fungi that absorb nutrients from dead organisms are the world’s most important decomposers.
Fungi and a handful of bacterial species are the only organisms capable of completely digesting
both the lignin and cellulose that make up wood.
- Act as parasites
- Vast majority of fungi are mutualists (benefit host)
- Because they recycle key elements such as carbon, nitrogen and phosphorous, fungi have a
profound influence on productivity and biodiversity.
31.1 Why Do Biologists Study Fungi?
Fungi Provide Nutrients for Land Plants
- Fungi that live in close associated with plant roots are said to be mycorrhizal.
- Fungi are critical to the productivity of forests, cropland and rangelands.
Fungi Speed the Carbon Cycle on Land
- Saprophytes: Fungi that digest dead plant material
- Coal exists today because conditions were too acidic for fungi to do their job (decompose)
- Amassive die-off in trees and shrubs produced gigantic quantities of rotting wood and led to the
explosion of fungal abundance.
- When trees die, fungi break down wood into sugars and other small organic compounds that they
and other organisms can use as food.
- Two basic components of carbon cycle on land: 1) the fixation of carbon by land plants –
meaning that carbon in the atmospheric CO2 is converted to cellulose, lignin, and other complex
organic compounds in the bodies of plants. 2) the release of CO2 from plants, animals, and fungi
as the result of cellular respiration – meaning the oxidation of glucose and production of theATP
that sustains life.
- On land, fungi make the carbon cycle turn much faster. The nutrients they release feed a host of
Fungi Have Important Economic Impacts - Soil-dwelling fungi have been the source of many of the most important antibiotics currently
- Some fungi, including saprophytes, damage crops.
- Mushrooms, yeast.
- Enzymes derived from fungi are used to improve the characteristics of foods ranging from fruit
juice and candy to meat.
FungiAre Key Model Organisms in Eukaryotic Genetics
- In good condition, yeast cells grow and divide almost as rapidly as bacteria
- Simple morphology provides an example of a pure eukaryotic cell - one that is suitable for
experiments on how cell division occurs and how particular gens are turned on and off.
- Biologists study fungi because they provide a window for understanding euk cells and because
they affect a wide range of species in nature, including humans
31.2 How Do Biologists Study Fungi?
Analyzing Morphological Traits
- Only two growth forms:
1. Single-celled forms called yeasts
2. Multi-cellular, filamentous structures called mycelia
The Nature of Fungal Mycelium
- Some mycelia can grow to be extremely large (530 hectares in Oregon). Although most are much
smaller and shorter lived, all mycelia are dynamic. Grow constantly in direction of food sources
and die back in areas where food is running out. Body shape can change almost simultaneously
throughout its life.
- Hyphae: Individual filaments that make up a mycelium. Most are haploid, some are dikaryotic,
meaning that each cell contains 2 haploid nuclei – one from eac parent.
- Hyphae are long narrow filaments hat branch frequently.
- Usually, each filament is broken into cell-like compartments by cross-walls called septa. Septa do
not close off segments of hyphae completely. Instead, gaps called pores enable a wide variety of
materials, even organelles and nuclei to flow from one compartment to the next.
- Septa may have single large opening or series of small gaps that give them sieve-like appearance. - Because nutrients, mitochondria and genes can flow through mycelium, the fungal mycelium is
intermediate between a multicellular land plant or animal and an enormous single-celled
- Some fungal species are coenocytic – meaning they lack septa. Have many nuclei scattered
throughout the mycelium.
- Fungal mycelia can penetrate tiny fissures in soil and absorb nutrients that are inaccessible to
- Because mycelia are composed of complex, branching networks of extremely thin hyphae, the
body of a fungus has the highest surface-area-t-volume ration possible in a multicellular
organism. Consider that hyphae found in any fist-sized ball of rich soil typically have a surface
area equivalent to half page of this book. Surface area is important because it makes absorption
efficient, and because fungi make their living on absorption.
- The extraordinarily high surface area has a downside: Fungi are prone to drying out, because the
amount of water that evaporates from an organism is a function of its surface area. Due to high
surface area of mycelia, fungi are most abundant in moist habitats. Often endure conditions in the
form of tough, watertight spores.
- The mass of filaments on the inside of mushrooms is protected from drying by the densely
packed hyphae forming the surface.
- Most fungi fall into 4 major groups:
1. Chytridiomycota (chytrids) – water or wet soils. Only fungi with motile cells. The spores that
chytrids produce during sexual reproduction have flagella, as do the gametesproduced by
2. Zygomycota – Hyphae are haploid and come in several mating types. Hyphae of different
mating types look identical but will not combine unless the individuals have different alleles
of one or more genes involved in mating. Individuals of different mating types may become
yoked together. Cell from yoked hyphae fuse to form spore-producing structure called
zygosporangium. Yoked hyphae that form a zygosporangium are the reproductive structure
unique to this group.
3. Basidomycota (club fungi) - Mushrooms, bracket fungi, puffballs are among reproductive
structures. Inside these structures, specialized cells called basidia form at the ends of hyphae
and produce spores. Multiple mating types occur.
4. Ascomycota (sac fungi) – produce complex reproductive structures – the largest are often
cup-shaped. The tips of hyphae inside these structures produce distinctive sac-like cells called
asci.An ascus is a spore-producing structure found only inAscomycota.
Evaluating Molecular Phylogenies - Fungi are much more closely related to animals than they are to land plants.
- Most animals and fungi synthesize chitin. Chitin is a prominent component of the cell walls of
- The flagella that develop in chytrid spores and gametes re very similar to those of animals: single,
located at the back of the body, move in a whiplash manner
- Both store food by synthesizing starch as their storage product.
- To understand the relationship between the four types of fungi, scientists have sequences series of
genes and used the data to estimate the phylogeny. Several conclusions:
Chytrids include the most basal groups of fungi. Fungi evolved from aquatic ancestors.
Also suggests that fungi made the transition to land early in their evolution.
Chytridiomycota and Zygomycota are paraphyletic. Neither represents a single common
ancestor and all of its descendents. Means, either swimming gametes or yoked hyphae
evolved more than once, or both were present in a common ancestor but then were lost in
Microsporidians are within the monophyletic group of fungi. Based on the molecular
phylogeny, researches are testing the hypothesis that fungicides – meaning molecules that
are lethal to fungi – may prove to be effective in combating microspirdian infections.
Glomeromycota is monophyletic. Sexual reproduction has yet to be observed
Basidiomycota andAscomycota are monophyletic and are the most highly derived groups
Experimental Studies of Mutualism
- Symbiotic- living together relationship. Land plants and fungi.
- Biologists hypothesize that the evolution of symbiotic associations has played a large role in the
diversification of fungi.
- Mutualistic – benefit both species; parasitic – benefits one harms host; commensal – one benefits,
other is unaffected.
- Fungi that are typically symbiotic are usually unable to grow and reproduce if their regular