Mastering Concepts
20.1
1. In what ways are fungi important in ecosystems?
Fungi are important as decomposers that break down dead organic matter, releasing
inorganic nutrients. Some are parasites of plants and animals, whereas others help plants
absorb nutrients or fight disease.
2. What characteristics define the fungi?
Fungi are eukaryotic and heterotrophic; they digest their food externally. They have cell
walls made of chitin, and they store carbohydrates as glycogen. Most fungi are
multicellular, although the yeasts are unicellular. In most species, the only diploid cell is
the zygote.
3. What evidence suggests that fungi are more closely related to animals than to plants?
Fungi and animals produce chitin, are heterotrophs, and use glycogen as a storage
carbohydrate. Molecular data support the close relationship between fungi and animals.
4. Describe the major structures of a fungus.
Hyphae are filamentous structures that make up a fungal body. A mycelium is a visible
mass of hyphae. Spores are the reproductive structures of fungi, and in many species a
fruiting body produces the spores.
20.2
1. How do flagellated cells adapt chytrids to moist environments?
A chytrid’s gametes and zoospores are motile cells that swim in a film of water, allowing
chytrids to reproduce and spread in moist or wet environments.
2. What do chytrids eat?
Chytrids are decomposers that eat cellulose, chitin, and keratin.
20.3
1. Where do zygomycetes occur?
Zygomycetes occur in decaying organic matter in soil (and they often decay spoiled food
as well).
2. How do zygomycetes reproduce asexually and sexually?
They reproduce asexually when haploid hyphae create spore sacs that release haploid
spores. They reproduce sexually when two hyphae fuse, merging their nuclei to form a
diploid zygospore.
3. How does the zygospore fit into the zygomycete life cycle?
A zygospore is formed by the union of two haploid hyphae. Inside the zygospore, two
nuclei fuse into a zygote. The zygote then undergoes meiosis and develops into a spore
sac that releases haploid spores.
20.4
1. What are the distinctive features of glomeromycetes?
Glomeromycetes live only in association with plant roots, forming arbuscular
mycorrhizae. They also have unusually large asexual spores, and they lack sexual life
cycles.
2. Describe arbuscular mycorrhizae.
In arbuscular mycorrhizae, hyphae colonize a plant’s roots, exchanging water and
minerals from the soil for sugars that the plant produces in photosynthesis. The fungus
produces arbuscules inside root cells; the arbuscules are the sites where materials are
exchanged between the fungus and the plant.
20.5
1. List examples of ascomycetes that are important to humans.
Ascomycetes that are important to humans include the fungi that cause Dutch elm
disease, chestnut blight, athlete’s foot, and other diseases. Some produce antibiotics and
immune-suppressing drugs. Others add flavor to some cheeses. Morels and truffles are
ascomycetes. Yeasts are used in baking, brewing, winemaking, and the production of soy
sauce.
2. Describe asexual and sexual reproduction in an ascomycete.
In asexual reproduction, haploid hyphae produce haploid conidia by mitosis. In sexual
reproduction, two haploid hyphae of compatible mating types fuse, producing a
dikaryotic hypha. The nuclei fuse, forming the diploid zygote, which immediately
undergoes meiosis (and usually mitosis) to produce eight haploid ascospores.
3. How do dikaryotic cells fit into the ascomycete life cycle?
Dikaryotic cells form when two hyphae fuse but the nuclei remain separate. Inside the
fruiting body, the nuclei fuse to form a diploid zygote. Meiosis follows, and the haploid
spores form within asci.
4. How does an ascus come to contain eight ascospores?
An ascus comes to contain eight ascospores after a diploid nucleus within an ascus
undergoes meiosis. The result is four haploid nuclei, which each immediately undergo
mitosis to produce eight nuclei (that mature into eight ascospores).
20.6
1. What are some familiar basidiomycetes?
Familiar basidiomycetes include mushrooms, toadstools, puffballs, stinkhorns, bracket
fungi, and bird’s nest fungi.
2. How are basidiomycetes different from ascomycetes?
Basidiomycetes typically produce four sexual spores per club-shaped basidium, whereas
ascomycetes typically produce eight sexual spores in each saclike ascus.
3. Describe the sexual life cycle of basidiomycetes.
Haploid basidiospores germinate, producing haploid hyphae. Two compatible hyphae
fuse and form a dikaryotic mycelium, which grows within the food source. When
conditions are right, a mushroom is produced. Lining the gills are the basidia, where the
two nuclei fuse to form the diploid zygote. The zygote undergoes meiosis, producing four
haploid basidiospores.
4. How does a “fairy ring” form?
A “fairy ring” forms as hyphae grow outward from a food source, producing mushrooms
near the outer edge of the ring.
20.7
1. What are endophytes?
Endophytes are fungi living within the tissues of plants without causing disease.
2. Which types of fungi form arbuscular mycorrhizae and ectomycorrhizae?
Glomeromycetes are arbuscular mycorrhizal fungi, while some basidiomycetes and
ascomycetes are ectomycorrhizal fungi.
3. How do ants, plants, fungi, and bacteria interact in leaf-cutter ant colonies?
In leaf-cutter ant colonies, the ants harvest plant materials and transport them into their
underground nests. There, they process the leaves into a paste and inoculate the paste
with fungal spores. The fungi develop into structures that the ants eat. Bacteria on the
ants’ cuticles secrete a potent antibiotic that kills an ascomycete that attacks the
cultivated fungus.
4. How do fungi and autotrophs interact in a lichen?
Fungi and autotrophs exist in a symbiotic relationship in a lichen. The fungus absorbs
water and minerals and the photosynthetic organism produces the carbohydrates.
5. How do scientists use lichens to monitor pollution?
Lichens absorb toxins from surrounding air and water. The disappearance of lichens
from a habitat is therefore an indicator of polluted air.
20.8
1. According to figure 20.22, were young leaves with endophytes significantly less
damaged than those without endophytes? Explain how you know.
Yes, young leaves with endophytes were significantly less damaged than those without
endophytes. Different letters above the bars indicate a statistically significant difference.
2. How would you design an experiment to determine whether one species of endophyte
or some combination of multiple species protects leaves against pathogens?
The test above could be repeated, but the treatment levels could be modified to include
leaves sprayed with different numbers of fungal species.
Write It Out
1. Give examples of fungi that are important economically, ecologically, and as food for
humans.
Economically important fungi – Yeast are used in the baking industry. Penicillium and
other fungi are used in making antibiotics. Ecologically important fungi – Many species
of fungi are decomposers in nature; many others form mycorrhizae in association with
plant roots. Several species of fungi cause athlete’s foot, ringworm, and other infections.
Rusts, molds, blights, and smuts are common plant pathogens. Food for humans – Some
mushrooms, morels, and truffles are valuable sources of food for humans. Fermenting
fungi also participate in the production of many foods and beverages.
2. Fungi and animals are both heterotrophs. What does this mean? How do fungi and
animals differ in how they obtain food?
Heterotrophs are organisms that must consume organic molecules as a source of carbon
and energy. Fungi obtain food by secreting digestive enzymes into their food source and
then absorbing the products of digestion. In contrast, animals ingest food, digest it
internally, absorb the products of digestion, and eliminate the indigestible wastes.
3. Suppose you are digging in forest soil and find a whitish structure. If you had access to
a microscope, what features would you look for to determine whether the object is a plant
root or a fungus?
The presence of starch granules or a cellulose cell wall would suggest it was a plant. To
determine if it were a fungus you could check for spores, chitin in the cell walls, the
absence of starch, and the presence of glycogen.
4. Create a Venn diagram including the characteristics of fungi, animals, and plants.
The diagram should include the following unique features and areas of overlap. All fungi,
animals, and plants are eukaryotic. Only fungi and animals are heterotrophic, whereas
most plants are autotrophic. Fungi have cell walls are made of chitin, whereas plants have
cell walls made of cellulose. Animals don’t have cell walls. Both fungi and animals store
carbohydrates as glycogen; plants store carbohydrates as cellulose. Plants and fungi have
spores. All plants and animals are multicellular, whereas some fungi (the yeasts) are
unicellular.
5. What characteristics distinguish each phylum of fungi?
Chytrids are fungi that produce swimming cells (gametes and zoospores). Zygomycetes
feature two haploid hyphae merging to form a diploid zygospore. Ascomycetes are
known as the sac fungi because they produce sexual spores (ascospores) in saclike asci.
Basidiomycetes are called the club fungi because they produce basidia, which are clubshaped cells that produce sexual spores called basidiospores.
6. Many fungi produce chemicals that inhibit bacterial growth. Why might the genes
encoding these chemicals be adaptive to fungi?
Fungi compete for resources with bacteria. Inhibiting bacterial growth increases the
reproductive success of fungi.
7. Review figure 19.5, which shows the alternation of generations in plants. Compare and
contrast the life cycles of zygomycetes, ascomycetes, and basidiomycetes with the basic
plant life cycle.
In plants, a multicellular haploid generation alternates with a multicellular diploid
generation. Zygomycetes, ascomycetes, and basidiomycetes all have multicellular
haploid generations, but none has a multicellular diploid generation. In zygomycetes, the
majority of the life cycle consists of haploid hyphae that fuse to form the diploid
zygospore. Meiosis in the zygospore regenerates haploid nuclei. In ascomycetes, the
majority of the life cycle consists of haploid hyphae. The hyphae of two fungi fuse, but
the individual nuclei from the two parents do not immediately merge. The resulting cell is
dikaryotic (containing two nuclei from separate parents). The dikaryotic cell divides and
gives rise to the fruiting body. In some of the dikaryotic cells, the nuclei fuse, forming a
diploid zygote. The zygote immediately undergoes meiosis, producing haploid
ascospores that germinate to yield a new haploid individual. In basidiomycetes, the basic
life cycle is similar to that of ascomycetes, except that the haploid hyphae fuse earlier in
the life cycle.
8. Sketch each of the following structures: mycelium; ascus with ascospores; zoospore;
zygospore; basidium with basidiospores.
[See illustrations in chapter]
9. Create a table with three columns. Write the following terms in the first column:
mycelium, zygospore, basidium, conidium, and hyphae. Define the terms in the second
column, and classify the terms as diploid, dikaryotic, or haploid in the third column.
A mycelium is a mass of aggregated hyphae; the mycelium may be haploid or dikaryotic.
A zygospore is the sexually produced spore of a zygomycete; it is diploid. A basidium is
a club-shaped cell of a basidiomycete; it is diploid. A conidium is an asexual spore; most
conidia are haploid. Hyphae are thin filaments making up the fungal body; hyphae may
be haploid or dikaryotic.
10. How are hyphae similar to plant roots? How are these structures different?
Both roots and hyphae grow through a medium (soil or an organic substrate) and absorb
nutrients. Hyphae only absorb nutrients at the tip, whereas roots absorb nutrients through
root hairs, which form just behind the root tip.
11. Describe the difference between sexual and asexual reproduction in zygomycetes.
Why might asexual reproduction be more common than sexual reproduction?
Haploid spores germinate into haploid hyphae, which may produce additional haploid
spores asexually. Sometimes a hypha fuses with a neighboring hypha, producing a zygote
inside a zygospore. The zygote undergoes meiosis and produces haploid spores by
meiosis. One possible explanation for the prevalence of asexual reproduction is that any
hypha can produce asexual spores, whereas sexual reproduction requires that two
compatible fungi encounter one another.
12. Other than shared DNA sequences, what characteristics place ascomycetes and
basidiomycetes together as sister groups?
Both groups of fungi produce dikaryotic cells, and both may produce large, fleshy
fruiting bodies.
13. Each ascus within an ascomycete fruiting body contains eight cells. Are these cells
haploid, diploid, or dikaryotic? Is every cell within an ascus genetically unique? What is
the fate of each cell?
Meiosis produces four nonidentical haploid cells, and then each cell divides mitotically.
So each ascus contains four pairs of identical haploid spores. Wind, water, or animals
then disperse the spores. Each cell might germinate to form haploid hyphae.
14. An ascomycete causes chestnut blight, a disease that has killed most of the chestnut
trees in the United States. If you were a plant breeder trying to bring back the chestnut,
how would you use artificial selection to create disease-resistant varieties of the tree?
Start by collecting seeds from trees that have survived the blight. Let the seeds germinate
in a controlled environment. As the seedlings grow, expose them to the chestnut blight
fungus. Cross-pollinate the trees that are the most blight-resistant (i.e., have the least
amount of damage). Collect the new seeds and repeat the process until seedlings are all
resistant to the blight.
15. Compare and contrast endophytes, mycorrhizae, and lichens.
Endophytes, mycorrhizae, and lichens are all fungal interactions with other organisms.
Endophytes are fungi that live within plants but do not produce disease symptoms.
Mycorrhizae are mutualistic fungal associations with plant roots. Lichens are symbiotic
interactions between fungi and green algae or cyanobacteria.
16. Use the Internet to find examples of chytrids, zygomycetes, ascomycetes, and
basidiomycetes that cause diseases in plants or animals. How does each fungus infect a
host and spread to new hosts? What can humans do to fight each disease? What are the
costs and benefits of doing so?
[Answers will vary]
17. Some endophytes produce compounds that fight bacteria, making them potential new
sources of antibiotics. Describe an experimental method that would allow you to screen
endophytes for antibacterial activity.
One possible strategy would be to spread bacteria uniformly over plates of agar. Before
the bacteria have a chance to grow, place bits of endophyte-infected leaves (or endophyte
fungi alone) in different areas of each plate. After incubating the plates, look for clear
areas around the endophyte fungi. These clear areas present areas where bacteria are
unable to grow.
18. Describe how experiments might show that:
a. chytrids are killing amphibians.
b. fungi benefit more from a lichen relationship than do algae.
c. bacteria help leaf-cutting ants cultivate one fungus while killing another.
d. overharvesting of mycorrhizal basidiomycetes harms forest health.
e. mold on a building’s walls can cause illness in the building’s occupants.
Many answers are possible; a few examples follow. (a) Laboratory experiments can
verify that chytrid zoospores can infect frogs and cause a lethal skin disease. Isolation of
the chytrids from frogs in field experiments can reveal how commonly the disease occurs
in nature. (b) Laboratory experiments can determine whether fungi and algae grown
separately grow as well as the same organisms together as a lichen. (c) Scientists can
apply antibiotics to kill the bacteria living on leaf-cutting ants and determine whether the
fungus gardens grow as well as they do when bacteria are present. (d) Scientists can
divide a forest into large plots and restrict harvesting of mushrooms from some of the
plots. They can then compare the plots for long-term measures of forest health (e.g.
photosynthetic rate, tree biomass). (e) Scientists can keep experimental animals in two
rooms, one with mold and one without. They can then observe the animals in the two
rooms for signs of illness.
19. Genome sequencing projects are complete or in progress for many fungi other than
Neurospora crassa. Search the Internet for a list of these fungi, choose one, and describe
some discoveries that have come from research on the species you chose.
[Answers will vary].
20. Why might it be challenging to produce drugs that treat fungal infections without
harming human cells?
Fungi and animals are closely related, so they share many cellular structures and
metabolic pathways. Producing drugs that target only fungal cells can therefore be
challenging. In contrast, antibiotics that kill bacteria often leave human cells unharmed
because bacteria and animal cells have many physical and metabolic differences.
21. White nose syndrome is an illness that weakens and kills bats roosting in caves.
Affected animals have a white fungus around their muzzle and on their wings, but no one
knows if the fungus causes the illness or simply infects bats that are weakened by some
other disease. White nose syndrome was discovered in 2006 and has since spread to
several states in the eastern and Midwestern United States. Search the Internet for the
latest information about white nose syndrome, then propose a testable hypothesis to
explain how the fungus might have entered bat populations. What information would you
need to determine whether the fungus actually causes the disease? How can people help
prevent the spread of the fungus from cave to cave? If the disease continues to spread,
how might the widespread death of bats affect ecosystems?
[Answers will vary]
22. Create a graph showing the number of species in each group of fungi. Which group is
least diverse? Based on the habitats and reproductive structures of these fungi, propose an
explanation for their relatively low diversity.
Table 20.1 and figure 20.24 show the number of known species in each group of fungi.
The glomeromycetes are the least diverse group. Glomeromycete fungi always live in
association with plant roots, which might limit the number of niches the fungus can
occupy and the ability of the fungus to disperse to new niches, if they do arise. They also
are known only to reproduce asexually, providing limited opportunities for genetic
variation. Finally, many undiscovered species are likely to remain, meaning that we
currently underestimate their biodiversity.
Pull It Together
1. Add at least one characteristic of each group of fungi to the concept map
Many answers are possible. Chytrids are mostly aquatic. Zygomycetes are fast-growing
fungi that mostly reproduce asexually. Glomeromycotes always form mycorrhizal
partnerships with plant roots. Ascomycetes spend most of their lives as haploid hyphae.
In basidiomycetes, most of the life cycle is dikaryotic.
2. Why are fungi important to other species?
Fungi are important to other species mainly because they decompose organic matter,
releasing inorganic nutrients that become available for plants to absorb. Some are also an
important source of nutrition for heterotrophs, some live symbiotically with plants
(mycorrhizal fungi), and others live symbiotically with cyanobacteria or green algae
(lichens).
3. Add hyphae and mycelium to the concept map.
“Spores” connects with the phrase “germinate to form” to “hyphae,” which in turn
connects with the phrase “aggregate into a” to “mycelium.”