Mastering Concepts
19.1
1. In what ways are fungi important in ecosystems?
Fungi are important in ecosystems as decomposers that break down dead organic matter,
releasing inorganic nutrients. They are also parasites of plants and animals.
2. What characteristics define the fungi?
Fungi are eukaryotic and heterotrophic. They have cell walls made of chitin, and they
store carbohydrates as glycogen. 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.
5. Which features distinguish the five phyla of fungi?
The five phyla are distinguished by the presence of and type of sexual structures. The
basidiomycetes produce sexual spores on a club-shaped structure; the ascomycetes
produce sexual spores in sacs; the glomeromycetes have large asexual spores and lack a
sexual cycle; the zygomycetes produce thick-walled sexual zygospores; and the
chytridiomycetes produce sexual and asexual spores with flagella.
19.2
1. What are zoospores, and how do they adapt chytrids to moist environments?
Zoospores are motile spores that swim in a film of water, allowing chytrids to spread in
moist or wet environments.
2. What do chytrids eat?
Chytrids are decomposers that eat cellulose, chitin, and keratin.
19.3
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.
19.4
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 a mycorrhiza.
In mycorrhizae the hyphae colonize the roots, exchanging water and minerals from the
soil for sugars that the plant produces in photosynthesis.
19.5
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. Penicillium notatum produces
antibiotics and other Penicillium species flavor cheeses. Morels and truffles are
ascomycetes. Yeasts are used in baking, brewing, winemaking, and production of soy
sauce. The drug cyclosporine comes from ascomycetes.
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).
19.6
1. What are some familiar basidiomycetes?
Familiar basidiomycetes include mushrooms, toadstools, puffballs, stinkhorns, shelf
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.
19.7
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 produce arbuscules while the basidiomycetes and ascomycetes form
ectomycorrhizae.
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?
Pollution kills lichens, which absorb toxins from surrounding air and water. The
disappearance of lichens from an environment is an indicator of polluted air.
19.8
1. How did the researchers test their hypothesis that endophytes help prevent plant
disease?
Researchers first removed native fungi by soaking plant seeds in bleach. These fungusfree seeds were then placed in a protected greenhouse where different leaves could be
sprayed with endophytes in a controlled manner. Disease-causing protists were then
introduced to the sprayed and not-sprayed leaves and the dead leaves were counted 15
days later.
2. This study used a mix of endophyte species. How would you design an experiment to
determine whether one species of endophyte or some combination of multiple species
protects the leaves against pathogens?
The test above could be repeated but rather than simply having one fungus sprayed on
some leaves and no fungus sprayed on other leaves, the experimental group could be
modified to include leaves sprayed with different numbers of fungi.
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. Aspergillus is used
in making soy sauce and other commercial products. Fungi such as Penicillium and
others are used in making antibiotics. Ecologically important fungi – Several species of
fungi are decomposers in nature. Aspergillus can cause lung infections, and several
species of fungi cause athlete’s foot and ringworm. Rusts, molds, blights, and smuts are
common plant pathogens. Food for humans – Morels, some mushrooms, and truffles are
valuable sources of food for humans.
2. 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 walls, the absence
of starch, and the presence of glycogen.
3. What evidence suggests that fungi are more closely related to animals than to plants?
Like animals, fungi are heterotrophs. Fungal cell walls are composed primarily of chitin,
a tough, flexible molecule that also forms the exoskeletons of some animals. The storage
carbohydrate of fungi is glycogen, the same as for animals. In addition, molecular
evidence clearly places the fungi closer to animals than to plants.
4. Which characteristics do all fungi share? Which characteristics distinguish each
phylum of fungi?
All fungi are eukaryotic and heterotrophic. The cell walls are made of chitin.
Chytrids are fungi that produce swimming spores (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 club-shaped cells that
produce sexual spores called basidiospores.
5. Penicillin, an antibiotic derived from a fungus, kills bacteria by destroying their cell
walls. Why doesn’t penicillin harm fungal cell walls?
Fungal cell walls are composed of chitin, not peptidoglycan as in bacteria.
6. Review figure 18.5, which shows the alternation of generations in plants. Compare and
contrast the life cycles of zygomycetes, ascomycetes, and basidiomycetes to 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 have 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 hyphae. 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.
7. Sketch each of the following structures: mycelium; ascus with ascospores; zoospore;
zygospore; basidium with basidiospores.
[see sketches in chapter]
8. Distinguish between the following pairs of terms: yeast and mycelium; ascomycete and
basidiomycete; mycorrhiza and lichen; zoospore and zygospore; diploid and dikaryotic.
Yeast and mycelium – Yeasts are unicellular fungi. A mycelium is a mass of aggregated
hyphae that may form visible strands.
Ascomycete and basidiomycete - Ascomycetes produce ascospores in asci;
basidiomycetes produce basidiospores on basidia.
Mycorrhiza and lichen – Mycorrhizae are associations between fungal hyphae and plant
roots. A lichen is a dual organism that forms when a fungus harbors green algae or
cyanobacteria.
Zoospore and zygospore – Zoospores are motile spores produced by chytrids; a
zygospore is a distinctive diploid spiny spore produced by zygomycetes.
Diploid and dikaryotic – A diploid cell has one nucleus that contains two sets of
chromosomes, with one set coming from each parent. A dikaryotic cell contains two
separate haploid nuclei, one from each parent.
9. Hyphae and arbuscules are highly branched structures. How does their extensive
surface area contribute to their functions?
The surface area of hyphae increases their ability to secrete digestive enzymes and absorb
nutrients. The surface area of arbuscules maximizes their ability to exchange materials
with the host plant.
10. 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.
11. An ascomycete called Cryphonectria parasitica 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 diseaseresistant varieties of the tree?
Start by collecting seeds from trees that have survived the blight. Grow the seeds in an
environment that controls for the blight and does not allow natural pollination. As the
seedlings grow expose them to the blight and cross-pollinate the trees that are the most
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.
12. 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]
13. Describe the relationships that connect:
a. leaves and endophytes.
b. roots and mycorrhizal fungi.
c. leaf-cutter ants, their fungal food, bacteria, and pathogenic fungi.
d. the fungal and photosynthetic partners in a lichen.
(a) Endophytes are fungi that live symbiotically inside plant tissue without causing
disease. When inside leaves, like in the cacao plant, the endophytes can help with
pathogen resistance. (b) Mycorrhizae are associations between fungal hyphae and plant
roots. The relationship benefits both partners: the plant obtains water and minerals that
the hyphae absorb from the soil, and the fungus gains carbohydrates that the plant
produces in photosynthesis. (c) Leaf-cutter ants cultivate the basidiomycete Lepiota on
leaves in special underground chambers. Streptomyces bacteria coat parts of the ants’
cuticles and secrete a potent antibiotic that kills an ascomycete, Escovopsis, that attacks
the cultivated fungus. (d) A lichen is a dual organism that forms when a fungus, either an
ascomycete or a basidiomycete, harbors green algae or cyanobacteria. The photosynthetic
partner contributes food to the lichen, and the fungus partner absorbs essential minerals.
14. 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.
Plate out different bacteria, placing bits of the endophyte in different areas of each plate.
Let the bacteria reproduce and then look for clear areas around the endophytes.
15. Figure 19.22 shows that endophytes were associated with a small but statistically
significant reduction in cacao leaf disease. In what ways might this seemingly small
difference be important to the plant?
A decrease in leaf disease means more surface area available for photosynthesis, which in
turn would increase the number of sugars available to the plant for growth and
reproduction.
16. 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 zoospores of the chytrid Batrachochytrium dendrobatidis 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 leafcutting 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.
17. 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].
18. Use the Internet to learn how Alexander Fleming discovered penicillin. Many years
passed before other scientists developed penicillin into a drug. How does the story of
penicillin, from discovery to drug development, illustrate the process of science?
Fleming and his associates noticed that bacteria did not grow near colonies of Penicillium
fungi. They isolated penicillin and conducted experiments, which showed that penicillin
kills bacteria. Because they published their work in a peer-reviewed journal, other
scientists were able to build on their work by repeating these experiments and testing new
hypotheses. Different research teams worked on different aspects of the problem, such as
purifying the compound, producing large quantities, stabilizing the compound, and
testing it on lab animals and humans.
19. 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 northeastern 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]
Pull It Together
1. Which spore types are produced by each group of fungi?
Nearly all fungi produce asexual spores, but the groups differ in their sexual spores.
Basidiomycetes make basidiospores on club-shaped basidia. Ascomycota make asci
containing ascospores. Glomeromycota have no sexual spores. Zygomycota make
zygospores. Chytridiomycota make motile zoospores.
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. How do dikaryotic cells fit into the sexual life cycle of fungi?
A dikaryotic cell contains two nuclei. It results from the fusion of two haploid hyphae of
two different fungi (ascomycetes or basidiomycetes). From this cell, a large dikaryotic
fruiting body grows via mitotic cell division. Inside the fruiting body, the two nuclei fuse
to produce a diploid zygote. The zygote immediately undergoes meiosis, yielding haploid
spores.