31
Fungi: Recyclers, Pathogens,
Parasites, and Plant Partners
About 300 million Africans in 25 countries are suffering because of
the invasion of crops by witchweed (Striga), a parasitic flowering
plant. This parasite has attacked more than two-thirds of the
sorghum, maize, and millet crops in sub-Saharan Africa, doing damage estimated at U.S. $7 billion each year.
In 1991 a team of Canadian scientists began a search for a solution to the Striga
problem. By 1995 they had begun fieldwork in Mali. What was their strategy? They
had isolated a strain of a fungus, the mold Fusarium oxysporum, that has two outstanding properties. First, it grows on Striga, wiping out a high percentage of the parasites. Second, it is not toxic to humans, nor does it attack the crop plants on which
Striga is growing. Now farmers apply the fungus to their crops and are rewarded by
greatly increased crop yields as Striga is held in check.
It may be possible to repeat this story—using a fungus to wipe out a particular
type of flowering plant—in a very different context.
A different strain of F. oxysporum preferentially attacks coca plants (the source of cocaine). There is a
controversial proposal to use F. oxysporum to wipe
out the coca plantations of Andean South America
and some countries in other parts of the world.
Some other fungi attack people, not plants.
Every breath we take contains large numbers of
fungal spores. Some of those spores can be dangerous, and fungal diseases of humans, some of
which are as yet incurable, have become a major
global threat. However, other fungi are of immense
commercial importance to us. Fungi are essential
to plants as well. They interact with roots, greatly
enhancing the roots’ ability to take up water and
mineral nutrients. Fungi and plants probably invaded the land together in the Paleozoic era (see
Table 22.1).
Earth would be a messy place without the fungi.
They are constantly at work in forests, fields, and
garbage dumps, breaking down the remains of
dead organisms (and even manufactured substances, such as some plastics). For almost a billion
years, the ability of fungi to decompose organic
substances has been essential for life on Earth,
chiefly because by breaking down carbon com-
Fungus Trumps Plant The fungus
Fusarium oxysporum is a potent pathogen
of witchweed (Striga), a parasitic plant that
attacks crops. The fungus spores are shown
in blue; the fungal filaments are in tan. Both
colors were added to this electron micrograph.
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CHAPTER THIRT Y-ONE
pounds, they return carbon and other elements to the environment, where they can be used again by other organisms.
In this chapter we will examine the general biology of the
kingdom Fungi, which differs in interesting ways from the
other kingdoms. We will also explore the diversity of body
forms, reproductive structures, and life cycles among the four
phyla of fungi, as well as the mutually beneficial associations
of certain fungi with other organisms. As we begin our study,
recall that the fungi and the animals are descended from a
common ancestor—molds and mushrooms are more closely
related to us than they are to the flowers we admired in the
last chapter.
General Biology of the Fungi
The kingdom Fungi encompasses heterotrophic organisms with
absorptive nutrition and with chitin in their cell walls. The fungi
live by absorptive nutrition: They secrete digestive enzymes
that break down large food molecules in the environment,
and then absorb the breakdown products. Many fungi are
saprobes that absorb nutrients from dead matter, others are
parasites that absorb nutrients from living hosts (Figure 31.1),
and still others are mutualists that live in intimate association
with other organisms.
The production of chitin, a polysaccharide, is a synapomorphy (shared derived trait) for fungi, choanoflagellates,
and animals. That is, its presence in fungi is the evidence that
all fungi are more closely related to animals than any fungi
are to plants. Chitin is used in the cell walls of fungi, but it is
used in other ways in animals. The use of chitin in cell walls
is a synapomorphy for fungi, and it allows us to distinguish
between the fungi and the basal eukaryotes (protists) that resemble them. Some protists that were formerly confused with
fungi include the slime molds (see Figures 28.31 and 28.32)
and water molds (oomycetes; see Figure 28.23).
The alternation between gametophyte (n) and sporophyte
(2n) generations that evolved in plants (see Chapter 29) is
found in only the most basal group of fungi, the chytrids. The
derived condition, which is found in the other three fungal
clades, involves a unique state in which two haploid nuclei
are present in a single cell, discussed later in this chapter. As
one might expect, the chytrids, which are aquatic, possess
flagellated gametes (or spores). Flagella have been lost in the
terrestrial fungi.
The kingdom Fungi consists of four phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. We distinguish the phyla on the basis of their methods and structures for sexual reproduction and, to a lesser extent, by
criteria such as the presence or absence of cross-walls separating their cell-like compartments. This morphologically
based phylogeny has proved largely consistent with phylogenies based on DNA sequencing. The term “fungal systematics” has an interesting anagram, “fantastic ugly mess,” but
we’ll see that the situation isn’t all that bad.
In the sections that follow, we’ll consider some aspects of
the general biology of the fungi, including their body structure and its intimate relationship with their environment,
their nutrition, and some special aspects of their unusual sexual reproductive cycles.
Some fungi are unicellular
Unicellular forms are found in all of the fungal phyla. Unicellular members of the Zygomycota, Ascomycota, and Basidiomycota are called yeasts. Yeasts may reproduce by budding, by fission, or by sexual means (Figure 31.2). Their
means of reproduction help us to place them in their appropriate phyla, as we will see below.
The body of a multicellular fungus
is composed of hyphae
(a)
Fungus
(b)
Fungal fruiting body
Most fungi are multicellular. The body of a
multicellular fungus is called a mycelium
(plural, mycelia). It is composed of rapidly
growing individual tubular filaments called
31.1 Parasitic Fungi Attack Other Living
Organisms (a) The gray masses on this ear of corn
are the parasitic fungus Ustilago maydis, commonly
called corn smut. (b) The tropical fungus whose fruiting body is growing out of the carcass of this ant
has developed from a spore ingested by the ant. The
spores of this fungus must be ingested by insects
before they will germinate and develop. The growing fungus absorbs organic and inorganic nutrients
from the ant’s body, eventually killing it, after which
the fruiting body produces a new crop of spores.
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
605
Nuclei
Cell wall
Pore
Septum
Septa are not complete:
Pores allow movement
of organelles and other
materials between celllike compartments.
Saccharomyces sp.
31.2 Yeasts Are Unicellular Fungi Unicellular members of the fungal phyla Zygomycota, Ascomycota, and Basidiomycota are known as
yeasts. Many yeasts reproduce by budding—mitosis followed by
asymmetrical cell division—as those shown here are doing.
hyphae (singular, hypha). Within hyphae of two clades, incomplete cross-walls called septa (singular, septum) divide the
hypha into separate cells. Pores in the septa allow organelles—sometimes even nuclei—to move in a controlled
way between cells (Figure 31.3). Other hyphae are coenocytic
and have no septa.
Certain modified hyphae, called rhizoids, anchor chytrids
and some other fungi to their substratum (the dead organism or other matter upon which they feed). These rhizoids
are not homologous to the rhizoids of plants because they are
not specialized to absorb nutrients and water. Parasitic fungi
may possess modified hyphae that take up nutrients from
their host.
The total hyphal growth of a mycelium (not the growth
of an individual hypha) may exceed 1 km per day. The hy-
Grass cells
(a) Coenocytic hypha
(b) Septate hypha
31.3 Most Hyphae Are Incompletely Divided into Separate Cells
(a) Coenocytic hyphae have no septa between their nuclei. (b) Even in
septate hyphae, the septa do not block the movement of organelles
within the hypha.
phae may be widely dispersed to forage for nutrients over a
large area, or they may clump together in a cottony mass to
exploit a rich nutrient source. Sometimes, when sexual spores
are produced, the mycelium becomes reorganized into a fruiting (reproductive) structure such as a mushroom.
The way in which a parasitic fungus attacks a plant illustrates the absorptive role of fungal hyphae (Figure 31.4). The
hyphae of a fungus invade a leaf through the stomata, through
wounds, or in some cases, by direct penetration of epidermal
cells. Once inside the leaf, the hyphae form a mycelium. Some
hyphae produce haustoria, branching projections that push
into the living plant cells, absorbing the nutrients within the
cells. The haustoria do not break
through the plant cell plasma membranes; they simply press into the
cells, with the membrane fitting them
like a glove. Fruiting structures may
form, either within the plant body or
on its surface.
Fungal
hyphae
Spore
Fungal spores
germinate on the
surface of the leaf.
Stoma
Hypha
Elongating hyphae pass
through stomata into
the interior of the leaf.
Some hyphae
penetrate cells
within the leaf.
31.4 A Fungus Attacks a Leaf The white structures in the micrograph are hyphae of the fungus Blumeria graminis, which is growing
on the dark surface of the leaf of a grass.
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CHAPTER THIRT Y-ONE
Fungi are in intimate contact with their environment
The filamentous hyphae of a fungus give it a unique relationship with its physical environment. The fungal mycelium
has an enormous surface area-to-volume ratio compared
with that of most large multicellular organisms. This large ratio is a marvelous adaptation for absorptive nutrition.
Throughout the mycelium (except in fruiting structures), all
the hyphae are very close to their environmental food source.
Another characteristic of some fungi is their tolerance for
highly hypertonic environments (those with a solute concentration higher than their own; see Chapter 5). Many fungi
are more resistant than bacteria to damage in hypertonic surroundings. Jelly in the refrigerator, for example, will not become a growth medium for bacteria because it is too hypertonic to the bacteria, but it may eventually harbor mold
colonies. Their presence in the refrigerator illustrates another
trait of many fungi: tolerance of temperature extremes. Many
fungi tolerate temperatures as low as 5–6°C below freezing,
and some tolerate temperatures as high as 50°C or more.
Fungi are absorptive heterotrophs
All fungi are heterotrophs that obtain food by direct absorption
from their immediate environment. The majority are saprobes,
obtaining their energy, carbon, and nitrogen directly from dead
organic matter through the action of enzymes they secrete.
However, as we’ve learned already, some are parasites, and still
others form mutualistic associations with other organisms.
Saprobic fungi, along with bacteria, are the major decomposers of the biosphere, contributing to decay and thus to the
recycling of the elements used by living things. In the forest,
for example, the mycelia of fungi absorb nutrients from fallen
trees, thus decomposing their wood. Fungi are the principal
decomposers of cellulose and lignin, the main components
of plant cell walls (most bacteria cannot break down these
materials). Other fungi produce enzymes that decompose
keratin and thus break down animal structures such as hair
and nails.
Because many saprobic fungi are able to grow on artificial
media, we can perform experiments to determine their exact
nutritional requirements. Sugars are their favored source of
carbon. Most fungi obtain nitrogen from proteins or the products of protein breakdown. Many fungi can use nitrate
(NO3–) or ammonium (NH4+) ions as their sole source of nitrogen. No known fungus can get its nitrogen directly from
nitrogen gas, as can some bacteria and plant–bacteria associations (see Chapter 37). Nutritional studies also reveal that
most fungi are unable to synthesize their own thiamin (vitamin B1) or biotin (another B vitamin) and must absorb these
vitamins from their environment. On the other hand, fungi
can synthesize some vitamins that animals cannot. Like all
organisms, fungi also require some mineral elements.
Nutrition in the parasitic fungi is particularly interesting
to biologists. Facultative parasites can attack living organisms
but can also be grown by themselves on artificial media. Obligate parasites cannot be grown on any available medium;
they can grow only on their specific living hosts, usually
plants. Because their growth is limited to living hosts, they
must have specialized nutritional requirements.
Some fungi have adaptations that enable them to function
as active predators, trapping nearby microscopic protists or
animals. The most common strategy is to secrete sticky substances from the hyphae so that passing organisms stick
tightly to them. The hyphae then quickly invade the prey,
growing and branching within it, spreading through its body,
absorbing nutrients, and eventually killing it.
A more dramatic adaptation for predation is the constricting ring formed by some species of Arthrobotrys, Dactylaria, and Dactylella (Figure 31.5). All of these fungi grow in
soil. When nematodes (tiny roundworms) are present in the
soil, these fungi form three-celled rings with a diameter that
just fits a nematode. A nematode crawling through one of
these rings stimulates the fungus, causing the cells of the ring
to swell and trap the worm. Fungal hyphae quickly invade
and digest the unlucky victim.
Two other kinds of relationships between fungi and other
organisms have nutritional consequences for the fungal partner. These relationships are highly specific, symbiotic (the partners live in close, permanent contact with one another), and
mutualistic (the relationships benefit both partners). Lichens
are associations of a fungus with a cyanobacterium, a unicellular photosynthetic protist, or both. Mycorrhizae (singular,
mycorrhiza) are associations between fungi and the roots of
plants. In these associations, the fungus obtains organic com-
Roundworm
Fungal loop
31.5 Some Fungi Are Predators A nematode (roundworm) is
trapped in sticky loops of the soil-dwelling fungus Arthrobotrys
anchonia.
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
pounds from its photosynthetic partner, but provides it with
minerals and water in return, so that the partner’s nutrition
is also promoted. In fact, many plants could not grow at all
without their fungal partners. We will discuss lichens and mycorrhizae more thoroughly later in this chapter.
Most fungi reproduce both asexually and sexually
Both asexual and sexual reproduction are common among
the fungi. Asexual reproduction takes several forms:
The production of (usually) haploid spores within structures called sporangia.
The production of naked spores (not enclosed in sporangia) at the tips of hyphae; such spores are called conidia
(from the Greek konis, “dust”).
Cell division by unicellular fungi—either a relatively equal
division (called fission) or an asymmetrical division in
which a small daughter cell is produced (called budding).
Simple breakage of the mycelium.
Asexual reproduction in fungi can be spectacular in terms
of quantity. A 2.5-centimeter colony of Penicillium can produce as many as 400 million conidia. The air we breathe contains as many as 10,000 fungal spores per cubic meter.
Sexual reproduction in many fungi features an interesting
twist. There is often no morphological distinction between
female and male structures, or between female and male individuals. Rather, there is a genetically determined distinction between two or more mating types. Individuals of the
same mating type cannot mate with one another, but they
can mate with individuals of another mating type within the
same species. This distinction prevents self-fertilization. Individuals of different mating types differ genetically from
one another, but are often visually and behaviorally indistinguishable. Many protists also have mating type systems.
Fungi reproduce sexually when hyphae (or, in the
chytrids, motile cells) of different mating types meet and
fuse. In many fungi, the zygote nuclei formed by sexual reproduction are the only diploid nuclei in the life cycle. These
nuclei undergo meiosis, producing haploid nuclei that become incorporated into spores. Haploid fungal spores,
whether produced sexually in this manner or asexually, germinate, and their nuclei divide mitotically to produce hyphae. This type of life cycle, called a haplontic life cycle, is also
characteristic of many protists (see Figure 28.27).
The presence of a dikaryon is a synapomorphy
of three phyla
Certain hyphae of some Zygomycota, Ascomycota, and Basidiomycota have a nuclear configuration other than the familiar haploid or diploid states. In these fungi, sexual repro-
607
duction begins in an unusual way: The cytoplasms of two individuals of different mating types fuse (plasmogamy) long
before their nuclei fuse (karyogamy), so that two genetically different haploid nuclei coexist and divide within the same hypha.
Such a hypha is called a dikaryon (“two nuclei”). Because
the two nuclei differ genetically, such a hypha is also called
a heterokaryon (“different nuclei”).
Eventually, specialized fruiting structures form, within
which the pairs of genetically dissimilar nuclei—one from
each parent—fuse, giving rise to zygotes long after the original “mating.” The diploid zygote nucleus undergoes meiosis, producing four haploid nuclei. The mitotic descendants
of those nuclei become spores, which give rise to the next
generation of hyphae.
The reproduction of such fungi displays several unusual
features. First, there are no gamete cells, only gamete nuclei.
Second, there is never any true diploid tissue, although for a
long period the genes of both parents are present in the
dikaryon and can be expressed. In effect, the hypha is neither
diploid (2n) nor haploid (n); rather, it is dikaryotic (n + n). A
harmful recessive mutation in one nucleus may be compensated for by a normal allele on the same chromosome in the
other nucleus. Dikaryosis is perhaps the most significant of
the genetic peculiarities of the fungi.
Finally, although zygomycetes, ascomycetes, and basidiomycetes grow in moist places, their gamete nuclei are not
motile and are not released into the environment. Therefore,
liquid water is not required for fertilization.
Some fungi are pathogens
Although most human diseases are caused by bacteria or
viruses, fungal pathogens are a major cause of death among
people with compromised immune systems. Most people
with AIDS die of fungal diseases, such as the pneumonia
caused by Pneumocystis carinii or the incurable diarrhea
caused by some other fungi. Candida albicans and certain
other yeasts also cause severe diseases in individuals with
AIDS and in individuals taking immunosuppressive drugs.
Such fungal diseases are a growing international health problem. Our limited understanding of the basic biology of these
fungi still hampers our ability to treat the diseases they cause.
Various fungi cause other, less threatening human diseases,
such as ringworm and athlete’s foot.
In plants, the situation is reversed. Fungi are by far the
most important plant pathogens, causing crop losses amounting to billions of dollars. Major fungal diseases of crop plants
include black stem rust of wheat and other diseases of wheat,
corn, and oats. Bacteria and viruses are less important as plant
pathogens.
The fungus that causes root and butt rot in pine trees is an
important forest pathogen with an interesting, recently dis-
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CHAPTER THIRT Y-ONE
31.1
Classification of Fungi
PHYLUM
COMMON NAME
FEATURES
EXAMPLES
Chytridiomycota
Chytrids
Aquatic; gametes have flagella
Allomyces
Zygomycota
Zygote fungi
Zygosporangium; no regularly occurring septa;
usually no fleshy fruiting body
Rhizopus
Ascomycota
Sac fungi
Ascus; perforated septa
Neurospora, baker’s yeast
Basidiomycota
Club fungi
Basidium; perforated septa
Armillariella, mushrooms
covered property. The virulence (relative ability to cause disease) of some strains of the fungus is controlled by genes in
its mitochondria—even though its dikaryotic cells have two
different nuclei.
Diversity in the Kingdom Fungi
In this section on fungal diversity, we’ll consider four phyla—
Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota (Figure 31.6; Table 31.1). The first two groups are
probably not clades, but the Ascomycota and Basidiomycota
are clades.
Chytrids probably resemble the ancestral fungi
The earliest-diverging fungal group is
Chytridiomycota
the chytrids (phylum ChytridiomyZygomycota
cota). These aquatic microorganisms
Ascomycota
were formerly classified with the protists. However, morphological (cell walls
Basidiomycota
that consist primarily of chitin) and molecular evidence
support their inclusion in the kingdom Fungi as its basal
members. In this book, we use the term “chytrid” to refer
to the entire phylum, but some mycologists reserve the term
to apply to one of the major clades in the phylum.
Like their sister taxon, the animals, the chytrids possess
flagellated gametes. The retention of this character reflects the
aquatic environment in which fungi first evolved. Chytrids
are the only fungi that have flagella at any life cycle stage.
Chytrids are either parasitic (on organisms such as algae,
mosquito larvae, and nematodes) or saprobic, obtaining nutrients by breaking down dead organic matter. Chytrids in
the compound stomachs of foregut-fermenting animals such
as cows may be an exception, living in a mutualistic association with their hosts. Most chytrids live in freshwater habitats or in moist soil, but some are marine. Some chytrids are
unicellular; others have mycelia made up of branching,
coenocytic hyphae. Chytrids reproduce both sexually and
asexually, but they do not have a dikaryon stage.
Allomyces, a well-studied genus of chytrids, displays alternation of generations. A haploid zoospore (a spore with flagella)
comes to rest on dead plant or animal material in water and
germinates to form a small, multicellular haploid mycelium.
That mycelium produces female and male gametangia (gamete
cases) (Figure 31.7). Mitosis in the gametangia results in the formation of haploid gametes, each with a single nucleus.
Chytridiomycota
Common
ancestor
The female gametangium
contains female gametes.
Zygomycota
The male gametangium
contains male gametes.
Ascomycota
Basidiomycota
Allomyces sp.
31.6 Phylogeny of the Fungi
among the fungi.
Four phyla are recognized
31.7 Reproductive Structures of
a Chytrid The haploid gametes
produced in these gametangia
will fuse with other gametes to
form diploid mycelia. The male
gametangia are smaller than the
female gametangia and possess a
light orange pigment.
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
Both female and male gametes have flagella. The motile
female gamete produces a pheromone, a chemical that attracts
the swimming male gamete. The two gametes fuse, and then
their nuclei fuse to form a diploid zygote. Mitosis and cytokinesis in the zygote gives rise to a small, multicellular
diploid organism, which produces numerous diploid flagellate zoospores. These diploid zoospores disperse and germinate to form more diploid organisms. Eventually, the diploid
organism produces thick-walled resting sporangia that can
survive unfavorable conditions such as dry weather or freezing. Nuclei in the resting sporangia eventually undergo meiosis, giving rise to haploid zoospores that are released into the
water and begin the cycle anew.
The presence of flagellated gametes is a distinguishing feature of the chytrids. The loss of flagella is a synapomorphy
that unites the remaining three fungal lineages.
Zygomycetes reproduce sexually by fusion
of two gametangia
Most zygomycetes (“zygote
Chytridiomycota
fungi,” phylum Zygomycota) have
Zygomycota
coenocytic hyphae. They produce no
Ascomycota
motile cells, and only one diploid cell—
Basidiomycota
the zygote—appears in the entire life cycle.
The mycelium of a zygomycete spreads over its substratum,
growing forward by means of vegetative hyphae. Most zygomycetes do not form a fleshy fruiting structure; rather, the
hyphae spread in an apparently random fashion, with occasional stalked sporangiophores reaching up into the air (Figure 31.8). These reproductive structures may bear one or
many sporangia.
609
Almost 900 species of zygomycetes have been described.
A very important group of zygomycetes serve as the fungal
partners in the most common type of mycorrhizal association with plant roots. A zygomycete that you may be more
familiar with is Rhizopus stolonifer, the black bread mold. Rhizopus reproduces asexually by producing many stalked sporangiophores, each bearing a single sporangium containing
hundreds of minute spores (Figure 31.9a). As in other filamentous fungi, the spore-forming structure is separated from
the rest of the hypha by a wall.
Zygomycetes reproduce sexually when adjacent hyphae
of two different mating types release pheromones, which
cause them to grow toward each other. These hyphae produce gametangia, which fuse to form a zygosporangium.
Sometime later, the gamete nuclei now contained within the
zygosporangium fuse to form a single multinucleate zygospore (Figure 31.9b). The zygosporangium develops a
thick, multilayered wall that protects the zygospore. The
highly resistant zygospore may remain dormant for months
before its nuclei undergo meiosis and a sporangiophore
sprouts. The sporangium contains the products of meiosis:
haploid nuclei that are incorporated into spores. These
spores disperse and germinate to form a new generation of
haploid hyphae.
The next two fungal lineages that we’ll discuss are related
groups with many similarities, including a dikaryon stage
and hyphae with septa. A key feature distinguishing between them is whether the sexual spores are borne inside a
sac (in the ascomycetes) or on a pedestal (in the basidiomycetes).
The sexual reproductive structure of ascomycetes
is an ascus
Sporangia
Sporangiophores
Pilobolus sp.
31.8 A Zygomycete This small forest of filamentous structures
is made up of sporangiophores. The stalks end in tiny, rounded
sporangia.
The ascomycetes (“sac fungi,” phylum
Chytridiomycota
Ascomycota) are a large and diverse
Zygomycota
group of fungi distinguished by the
Ascomycota
production of sacs called asci (singular,
Basidiomycota
ascus), which contain sexually produced
ascospores (Figure 31.10). The ascus is the characteristic sexual reproductive structure of the ascomycetes. Ascomycete
hyphae are segmented by more or less regularly spaced
septa. A pore in each septum permits extensive movement of
cytoplasm and organelles (including the nuclei) from one
segment to the next.
The approximately 30,000 known species of ascomycetes
can be divided into two broad groups, depending on
whether the asci are contained within a specialized fruiting
structure. Species that have this fruiting structure, the ascocarp, are collectively called euascomycetes (“true ascomycetes”); those without ascocarps are called hemiascomycetes (“half ascomycetes”).
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CHAPTER THIRT Y-ONE
(a)
(b)
Spores
1 Hyphae of differing
mating types produce
branches that grow
toward each other.
Hypha of –
mating type
Hypha of +
mating type
2 The tips develop into
gametangia.
Spores
Gametangia (n)
Sporangium
Rhizopus stolonifer
Sporangiophore
HAPLOID (n)
6 The zygospores undergo
meiosis, forming haploid
spores that are released
from the sporangium.
31.9 Sexual Reproduction
in a Zygomycete (a) The
micrograph shows the fruiting
body of a black bread mold.
(b) Sexual reproduction in
zygomycetes begins when
pheromones released by
hyphae of two different mating types cause them to fuse
and form zygosporangia.
3 The
gametangia
fuse. . .
DIPLOID DIKARYOTIC
(2n)
(n + n)
Meiosis
Zygospores (2n)
within
zygosporangium
5 The resulting zygote
develops into a
zygosporangium that
contains zygospores.
Zygosporangium
(n + n)
Fertilization
Most hemiascomycetes are microscopic, and many species are unicellular. Perhaps the best
known are the ascomycete yeasts, especially baker’s or
brewer’s yeast (Saccharomyces cerevisiae; see Figure 31.2).
These yeasts are among the most important domesticated
fungi. S. cerevisiae metabolizes glucose obtained from its
environment to ethanol and carbon dioxide by fermentation. It forms carbon dioxide bubbles in bread dough and
gives baked bread its light texture. Although they are
baked away in bread making, the ethanol and carbon dioxide are both retained when yeast ferments grain into beer.
Other yeasts live on fruits such as figs and grapes and play
an important role in the making of wine.
Hemiascomycete yeasts reproduce asexually either by fission (splitting in half after mitosis) or by budding (an asymmetrical cell division in which a small daughter cell is produced; see Figure 31.2). Sexual reproduction takes place when
two adjacent haploid cells of opposite mating types fuse. (We
discussed the genetics of yeast mating types in Chapter 14.)
In some species, the resulting zygote buds to form a diploid
cell population; in others, the zygote nucleus undergoes
HEMIASCOMYCETES.
Ascospores
Ascus
31.10 Asci and Ascospores The ascomycetes are characterized by
the production of ascospores within sacs called asci. Ascospores are
the products of meiosis followed by a single mitotic division.
Ascospores and asci do not mature all at once, and they may abort, so
not every ascus in this micrograph contains eight mature ascospores.
4 . . . as do the
gametes within
them, forming a
zygosporangium.
meiosis immediately. When this diploid
nucleus undergoes meiosis, the entire
cell becomes an ascus. Depending on
whether the products of meiosis then
undergo mitosis, a yeast ascus usually
contains either eight or four ascospores
(see Figure 31.10). The ascospores germinate to become haploid cells. Hemiascomycetes have no dikaryon stage.
Yeasts, especially Saccharomyces cerevisiae, are frequently used in molecular
biological research. Just as E. coli is the
best-studied prokaryote, S. cerevisiae is
the most completely studied eukaryote.
The euascomycetes include many of the
filamentous fungi known as molds. Among them are several common pink molds, one of which (Neurospora) Beadle
and Tatum used in their pioneering genetic studies (see
Figure 12.1). Many euascomycetes are parasites on flowering plants. Chestnut blight and Dutch elm disease are both
caused by euascomycetes. The powdery mildews are euascomycetes that infect cereal grains, lilacs, and roses, among
many other plants. They can be a serious problem to grape
growers, and a great deal of research has focused on ways
to control these agricultural pests.
The euascomycetes also include the cup fungi (Figure
31.11a,b). In most of these organisms the ascocarps are cupshaped and can be as large as several centimeters across. The
inner surfaces of the cups are covered with a mixture of vegetative hyphae and asci, and they produce huge numbers of
spores. Although these fleshy structures appear to be composed of distinct tissue layers, microscopic examination
shows that their basic organization is still filamentous—a
tightly woven mycelium.
Two particularly delicious euascomycetes ascocarps are
morels (Figure 31.11a) and truffles. Truffles grow underground in a mutualistic association with the roots of some
species of oaks. Europeans traditionally used pigs to find
truffles because some truffles secrete a substance that has an
odor similar to a pig’s sex pheromone. Unfortunately, pigs
also eat truffles, so dogs are now the usual truffle hunters.
Penicillium is a genus of green molds, of which some species
produce the antibiotic penicillin, presumably for defense
against competing bacteria. Two species, P. camembertii and P.
roquefortii, are the organisms responsible for the characteristic
flavors of Camembert and Roquefort cheeses, respectively.
Brown molds of the genus Aspergillus are important in
some human diets. A. tamarii acts on soybeans in the production of soy sauce, and A. oryzae is used in brewing the Japanese alcoholic beverage sake. Some species of Aspergillus that
grow on nuts such as peanuts and pecans produce extremely
EUASCOMYCETES.
(b) Sarcoscypha coccinea
(a) Morchella esculenta
31.11 Two Cup Fungi (a) Morels, which have a spongelike ascocarp and a subtle flavor, are considered a delicacy by humans. (b)
These brilliant red cups are the ascocarps of another cup fungus.
carcinogenic (cancer-inducing) compounds called aflatoxins.
In the United States, moldy grain infected with Aspergillus is
thrown out. In Africa, where food is scarcer, the grain gets
eaten, moldy or not, and causes severe health problems.
The euascomycetes reproduce asexually by means of conidia that form at the tips of specialized hyphae (Figure 31.12).
Small chains of conidia are produced by the millions and can
survive for weeks in nature. The conidia are what give molds
their characteristic colors.
The sexual reproductive cycle of euascomycetes includes
the formation of a dikaryon. Most euascomycetes form mat-
Conidia
Leaf
Hyphae
Erysiphe sp.
31.12 Conidia Chains of conidia are developing at the tips of specialized hyphae arising from this powdery mildew growing on a leaf.
612
Spores (n)
CHAPTER THIRT Y-ONE
Asexual
reproduction
31.13 The Life Cycle of a Euascomycete
This cup fungus is so named because of its
cup-shaped ascocarp.
Spores (n)
Mating type a ( )
Mating type A ( )
Mating
structures
Germinating
ascospore (n)
Germinating
ascospore (n)
Ascospores (n)
Ascospores (n)
HAPLOID (n)
DIKARYOTIC (n + n)
Dikaryotic
hyphae (n + n)
Mitosis
DIPLOID
(2n)
Dikaryotic
ascus (n + n)
Haploid
hyphae (n)
Meiosis
Fertilization
Ascocarp
ing structures, some “female” and some “male” (Figure
31.13). Nuclei from a male structure on one hypha enter a female mating structure on a hypha of a compatible mating
type. Dikaryotic ascogenous (ascus-forming) hyphae develop
from the now dikaryotic female mating structure. The introduced nuclei divide simultaneously with the host nuclei.
Eventually asci form at the tips of the ascogenous hyphae.
Only with the formation of asci do the nuclei finally fuse. Both
nuclear fusion and the subsequent meiosis of the resulting
diploid nucleus take place within individual asci. The meiotic
products are incorporated into ascospores that are ultimately
shed by the ascus to begin the new haploid generation.
The sexual reproductive structure of basidiomycetes
is a basidium
About 25,000 species of basidioChytridiomycota
mycetes (“club fungi,” phylum BaZygomycota
sidiomycota) have been described.
Ascomycota
Basidiomycetes produce some of the
Basidiomycota
most spectacular fruiting structures found
anywhere among the fungi. These fruiting structures, called
basidiocarps, include puffballs (which may be more than
half a meter in diameter), mushrooms of all kinds, and the
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
613
(a) Lycoperdon perlatum
(b) Amanita muscaria
31.14 Basidiomycete Fruiting Structures The basidiocarps of the basidiomycetes are probably the most familiar structures produced by fungi.
(a) When raindrops hit them, these puffballs will release clouds of spores for
dispersal. (b) These mushrooms were produced by a member of a highly poisonous genus, Amanita, that forms mycorrhizal relationships with trees.
(c) This edible bracket fungus is parasitizing a tree.
(c) Laetiporus sulphureus
giant bracket fungi often encountered on trees and fallen
logs in a damp forest (Figure 31.14). There are more than
3,250 species of mushrooms, including the familiar Agaricus
bisporus you may enjoy on your pizza, as well as poisonous
species, such as members of the genus Amanita. Bracket
fungi do great damage to cut lumber and stands of timber.
Some of the most damaging plant pathogens are basidiomycetes, including the rust fungi and the smut fungi (see
Figure 31.1a) that parasitize cereal grains. In contrast, other
basidiomycetes contribute to the survival of plants as fungal partners in mycorrhizae.
Some of the largest organisms on Earth are basidiomycetes. One such fungus, a member of the genus Armillariella growing in Michigan, covers an area of 37 acres. Its effect on plants is evident from the air, but from ground level,
it is difficult to realize how large the fungus is. At the surface,
only seemingly isolated clumps of mushrooms are visible.
The vast body of the fungus, which weighs approximately
the same as a blue whale, grows underground and consists
almost entirely of microscopic hyphal filaments. Molecular
studies indicate that this giant fungus is or was a single individual that arose from a single spore. It is possible that fragmentation over time may have broken it into a few separate—but still gigantic—individuals. Another, larger fungus
of the same genus, growing in the state of Washington, occupies parts of three counties.
Basidiomycete hyphae characteristically have septa with
small, distinctive pores. The basidium (plural, basidia), a
swollen cell at the tip of a hypha, is the characteristic sexual
reproductive structure of the basidiomycetes. It is the site of
nuclear fusion and meiosis. Thus, the basidium plays the
same role in the basidiomycetes as the ascus does in the
ascomycetes and the zygosporangium does in the zygomycetes.
The life cycle of the basidiomycetes is shown in Figure
31.15. After nuclei fuse in the basidium, the resulting diploid
nucleus undergoes meiosis, and the four resulting haploid
nuclei are incorporated into haploid basidiospores, which
form on tiny stalks on the outside of the basidium. These basidiospores typically are forcibly discharged from their basidia and then germinate, giving rise to haploid hyphae. As
these hyphae grow, haploid hyphae of different mating types
meet and fuse, forming dikaryotic hyphae, each cell of which
contains two nuclei, one from each parent hypha. The dikaryotic mycelium grows and eventually, when triggered by rain
or another environmental cue, produces a basidiocarp. The
dikaryon stage may persist for years—some basidiomycetes
live for decades or even centuries. This pattern contrasts with
the life cycle of the ascomycetes, in which the dikaryon is
found only in the stages leading up to formation of the asci.
The elaborate basidiocarp of some fleshy basidiomycetes,
such as the mushroom shown in Figure 31.15, is topped by a
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CHAPTER THIRT Y-ONE
2 Basidiospores give rise
to haploid hyphae.
Basidiospores
3 Haploid hyphae of different
mating types fuse, forming
dikaryotic hyphae.
+ Mating type
Mycelial
hyphae
1 The basidium is the
characteristic sexual
reproductive structure
of the basidiomycetes.
Basidiospores form
outside the basidium.
Dikaryotic
mycelium
– Mating type
4 The dikaryotic mycelium
grows and eventually
produces a fruiting
structure, the basidiocarp.
HAPLOID
(n)
DIKARYOTIC
(n + n)
DIKARYOTIC
(n + n)
Gills lined
with basidia
Young
basidiocarp
Pileus
Basidiospores
5 The basidiocarp is topped
by a cap, or pileus, which
has gills on its underside.
DIPLOID
(2n)
Nuclei
Basidium
Fused
nucleus
Meiosis
31.15 The Basidiomycete Life Cycle
Basidiospores form on tiny stalks and
are then dispersed to germinate into
haploid hyphae.
7 Nuclear fusion and
meiosis take place
in the developing
basidium.
cap, or pileus, which has structures called gills on its underside. Enormous numbers of basidia develop on the surfaces
of the gills. The basidia discharge their basidiospores into the
air spaces between adjacent gills, and the spores sift down
into air currents for dispersal and germination as new haploid mycelia. A single basidiocarp of the common bracket
fungus Ganoderma applanatum can produce as many as 4.5
trillion basidiospores in one growing season.
Imperfect fungi lack a sexual stage
As we have just seen, mechanisms of sexual reproduction
readily distinguish members of the four phyla of fungi from
one another. But many fungi, including both saprobes and
parasites, appear to lack sexual stages entirely; presumably
Fertilization
Basidiocarp
(fruiting structure)
Developing
basidium
6 Basidia develop on the
surfaces of the gills.
these stages have been lost during the evolution of these
species or have not yet been observed. Classifying these fungi
used to be difficult, but biologists now can assign most such
fungi to one of the four phyla on the basis of their DNA sequences.
Fungi that have not yet been placed in any of the existing
phyla are pooled together in a polyphyletic group called
deuteromycetes, informally known as “imperfect fungi.”
Thus, the deuteromycete group is a holding area for species
whose status is yet to be resolved. At present, about 25,000
species are classified as imperfect fungi.
If sexual structures are found on a fungus classified as a
deuteromycete, that fungus is reassigned to the appropriate
phylum. That happened, for example, to a fungus that produces plant growth hormones called gibberellins (see Chapter 38). Originally classified as the deuteromycete Fusarium
moniliforme, this fungus was later found to produce asci,
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
whereupon it was renamed Gibberella fujikuroi and transferred
to the phylum Ascomycota.
Fungal Associations
Earlier in this chapter we mentioned mycorrhizae and
lichens, two kinds of symbiotic, mutualistic associations between fungi and other organisms. Now that we have learned
a bit about fungal diversity, let’s consider mycorrhizae and
lichens in greater detail.
Mycorrhizae are essential to many plants
Almost all tracheophytes require a symbiotic association with
fungi. Unassisted, the root hairs of such plants do not absorb
enough water or minerals to sustain growth. However, their
roots usually do become infected with fungi, forming an association called a mycorrhiza.
In ectomycorrhizae, the fungus (usually a basidiomycete)
wraps around the root, and its mass is often as great as that
of the root itself (Figure 31.16a). The fungal hyphae do not
penetrate the root cells. An extensive web of hyphae penetrates the soil in the area around the root, so that up to 25 percent of the soil volume near the root may be fungal hyphae.
The hyphae of the fungi attached to the root increase the surface area for the absorption of water and minerals, and the
mass of the mycorrhiza, like a sponge, holds water efficiently
in the neighborhood of the root. Infected roots characteristically branch extensively and become swollen and clubshaped, and they lack root hairs.
(a)
Hyphae of the fungus
Pisolithus tinctorius cover
a eucalyptus root.
(b)
615
In endomycorrhizae, the fungal (zygomycete) hyphae enter
the root and penetrate the root cells, forming tree-like structures inside the cells, which become the primary site of exchange between plant and fungus (Figure 31.16b). As with
the ectomycorrhizae, the fungus forms a vast web of hyphae
leading from the root surface into the surrounding soil.
The mycorrhizal association is important to both partners.
The fungus obtains important organic compounds, such as
sugars and amino acids, from the plant. In return, the fungus, because of its very high surface area-to-volume ratio and
ability to penetrate the fine structure of the soil, greatly increases the plant’s ability to absorb water and minerals (especially phosphorus). The fungus may also provide the plant
with certain growth hormones and may protect it against attack by microorganisms. Plants that have active endomycorrhizae typically are a deeper green and may resist drought
and temperature extremes better than plants of the same
species that have little mycorrhizal development. Attempts
to introduce some plant species to new areas have failed until a bit of soil from the native area (presumably containing
the fungus necessary to establish mycorrhizae) was provided.
Trees without ectomycorrhizae normally will not grow at all,
so the health of our forests depends on the presence of ectomycorrhizal fungi.
The partnership between plant and fungus results in a
plant that is better adapted for life on land. It has been suggested that the evolution of mycorrhizae was the single most
important step leading to the colonization of the terrestrial
environment by living things. Fossils of mycorrhizal structures more than 300 million years old have been found, and
some rocks dating back 460 million years contain structures
that appear to be fossilized fungal
spores. Some liverworts, which are
among the most ancient terrestrial
plants (see Chapter 29), form mycorrhizae.
Certain plants that live in nitrogenpoor habitats, such as cranberry
bushes and orchids, invariably have
mycorrhizae. Orchid seeds will not
germinate in nature unless they are already infected by the fungus that will
form their mycorrhizae. Plants that
lack chlorophyll always have mycorrhizae, which they often share with the
The shapes filling much of this
soybean root cell are sections through
the hyphae of the endomycorrhizal
fungus Glomus caledonium.
31.16 Mycorrhizal Associations (a)
Ectomycorrhizal fungi wrap themselves
around the plant root, increasing the area
available for absorption of water and nutrients. (b) Endomycorrhizae infect the root
internally and penetrate the root cells.
616
(a)
CHAPTER THIRT Y-ONE
Foliose
Crustose
roots of green, photosynthetic plants. In effect, these plants
without chlorophyll are feeding on nearby green plants, using the fungus as a bridge.
Lichens can grow where plants cannot
A lichen is not a single organism, but rather a meshwork of
two radically different organisms: a fungus and a photosynthetic microorganism. Together the organisms constituting a
lichen can survive some of the harshest environments on
Earth. The biota of Antarctica, for example, features more
than 100 times as many species of lichens as of plants.
In spite of this hardiness, lichens are very sensitive to air
pollution because they are unable to excrete toxic substances
that they absorb. Hence they are not common in industrialized cities. Because of their sensitivity, lichens are good biological indicators of air pollution.
The fungal components of most lichens are ascomycetes,
but a few are basidiomycetes or imperfect fungi. The photosynthetic component is most often a unicellular green alga
but may be a cyanobacterium, or may include both. Relatively little experimental work has focused on lichens, perhaps because they grow so slowly—typically less than 1 centimeter per year.
There are about 13,500 “species” of lichens. Their fungal
components may constitute as many as 20 percent of all fungal species, but none of these species are able to grow independently without a photosynthetic partner. Lichens are
found in all sorts of exposed habitats: on tree bark, open soil,
and bare rock. Reindeer “moss” (actually not a moss at all,
but the lichen Cladonia subtenuis) covers vast areas in arctic,
subarctic, and boreal regions, where it is an important part of
the diets of reindeer and other large mammals. Lichens come
in various forms and colors. Crustose (crustlike) lichens look
(b)
31.17 Lichen Body Forms Lichens fall into three principal classes
based on their body form. (a) These foliose and crustose lichens are
growing on otherwise bare rock. (b) A miniature jungle of fruticose
lichens.
like colored powder dusted over their substratum (Figure
31.17a); foliose (leafy) and fruticose (shrubby) lichens may have
complex forms (Figure 31.17b).
The most widely held interpretation of the lichen relationship is that it is a mutually beneficial symbiosis. The hyphae of the fungal mycelium are tightly pressed against the
algae or cyanobacteria and sometimes even invade them. The
bacterial or algal cells not only survive these indignities, but
continue their growth and photosynthesis. In fact, the algal
cells in a lichen “leak” photosynthetic products at a greater
rate than do similar cells growing on their own. On the other
hand, photosynthetic cells from lichens grow more rapidly
on their own than when associated with a fungus. On this basis, we could consider lichen fungi as parasitic on their photosynthetic partners.
Lichens can reproduce simply by fragmentation of the
vegetative body, which is called the thallus, or by means of
specialized structures called soredia (singular, soredium).
Soredia consist of one or a few photosynthetic cells surrounded by fungal hyphae (Figure 31.18a). The soredia become detached, are dispersed by air currents, and upon arriving at a favorable location, develop into a new lichen.
Alternatively, if the fungal partner is an ascomycete or a basidiomycete, it may go through its sexual cycle, producing
either ascospores or basidiospores. When these spores are
discharged, however, they disperse alone, unaccompanied
by the photosynthetic partner, and thus may not be capable
of reestablishing the lichen association, or even of surviving
on their own. Nevertheless, many lichens produce characteristic fruiting structures containing asci or basidia.
FUNGI: RECYCLERS, PATHOGENS, PARASITES, AND PLANT PARTNERS
31.18 Lichen Anatomy (a) Soredia of a fruticose
lichen. (b) Cross section showing the layers of a
foliose lichen.
617
Soredia detach readily from the parent lichen
and travel in air currents, founding new lichens
when they settle in a suitable environment.
(b)
(a)
Each soredium consists of one or
a few photosynthetic cells
surrounded by fungal hyphae.
Upper layer of
fungal hyphae
Hyphae
Lichens are arranged
in distinct layers.
Soredium
Photosynthetic
cell layer
Loose layer of
fungal hyphae
Lower layer of
fungal hyphae
Substratum
Visible in a cross section of a typical foliose lichen are a
tight upper region of fungal hyphae, a layer of cyanobacteria or algae, a looser hyphal layer, and finally hyphal rhizoids
that attach the whole structure to its substratum (Figure
31.18b). The meshwork of fungal hyphae takes up some nutrients needed by the photosynthetic cells and provides a
suitably moist environment for them by holding water tenaciously. The fungi derive fixed carbon from the photosynthesis of the algal or cyanobacterial cells.
Lichens are often the first colonists on new areas of bare
rock. They satisfy most of their nutritional needs from the air
and rainwater, augmented by minerals absorbed from dust.
A lichen begins to grow shortly after a rain, as it begins to
dry. As it grows, the lichen acidifies its environment slightly,
and this acid contributes to the slow breakdown of rocks, an
early step in soil formation. After further drying, the lichen’s
photosynthesis ceases. The water content of the lichen may
drop to less than 10 percent of its dry weight, at which point
it becomes highly insensitive to extremes of temperature.
Whether living on their own or in symbiotic associations,
fungi have spread successfully over much of Earth since their
origin from a protist ancestor. That ancestor also gave rise to
the choanoflagellates and the animal kingdom, as we will see
in Chapter 32.
Chapter Summary
General Biology of the Fungi
Fungi are heterotrophic eukaryotes with absorptive nutrition
and with chitin in their cell walls. They may be saprobes, parasites, or mutualists.
These four fungal phyla differ in their reproductive structures, mechanisms of spore formation, and less importantly, the
presence and form of septa in their hyphae.
The yeasts are unicellular fungi.
The bodies of multicellular fungi are composed of multinucleate hyphae, often massed to form a mycelium. The hyphae
usually have incomplete partitions (septa) that allow the movement of organelles between cells. They give fungi a large surface area-to-volume ratio, enhancing their ability to absorb
nutrients. Review Figures 31.3, 31.4
Fungi reproduce asexually by means of spores formed within
sporangia, by conidia formed at the tips of hyphae, by fission or
budding, or by fragmentation.
Fungi reproduce sexually when hyphae of different mating
types meet and fuse.
In addition to the haploid and diploid states, many fungi demonstrate a third nuclear condition: the dikaryotic, or n + n, state.
Diversity in the Kingdom Fungi
The kingdom Fungi consists of four phyla: Chytridiomycota,
Zygomycota, Ascomycota, and Basidiomycota. Review Figure
31.6, Table 31.1. See Web/CD Activity 31.1
The chytrids, with their flagellated zoospores and gametes,
probably resemble the ancestral fungi.
The zygomycetes reproduce sexually by fusion of gametangia. Review Figure 31.9
The sexual reproductive structure of ascomycetes is an ascus
containing ascospores. The ascomycetes are divided into two
groups, euascomycetes and hemiascomycetes, on the basis of
whether they have an ascocarp, or fruiting structure. Review
Figure 31.13. See Web/CD Activity 31.2
The sexual reproductive structure of basidiomycetes is a
basidium, a swollen cell bearing basidiospores. Review
Figure 31.15
Imperfect fungi (deuteromycetes) lack sexual structures, but
DNA sequencing can sometimes identify the phylum to which
they belong.
See Web/CD Tutorial 31.1
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CHAPTER THIRT Y-ONE
Fungal Associations
Mycorrhizae, which are symbiotic associations of a fungus
with plant roots, enhance the ability of the roots to absorb water
and nutrients. In return, the plant supplies the fungus with photosynthetic products.
Lichens, which are symbiotic associations of a fungus with a
green alga or a cyanobacterium, are found in some of the most
inhospitable environments on the planet. Review Figure 31.18
Self-Quiz
1. Which statement about fungi is not true?
a. A multicellular fungus has a body called a mycelium.
b. Hyphae are composed of individual mycelia.
c. Many fungi tolerate highly hypertonic environments.
d. Many fungi tolerate low temperatures.
e. Some fungi are anchored to their substrate by rhizoids.
2. The absorptive nutrition of fungi is aided by
a. dikaryon formation.
b. spore formation.
c. the fact that they are all parasites.
d. their large surface area-to-volume ratio.
e. their possession of chloroplasts.
3. Which statement about fungal nutrition is not true?
a. Some fungi are active predators.
b. Some fungi form mutualistic associations with other
organisms.
c. All fungi require mineral nutrients.
d. Fungi can make some of the compounds that are vitamins
for animals.
e. Facultative parasites can grow only on their specific hosts.
4. Which statement about dikaryosis is not true?
a. The cytoplasm of two cells fuses before their nuclei fuse.
b. The two haploid nuclei are genetically different.
c. The two nuclei are of the same mating type.
d. The dikaryon stage ends when the two nuclei fuse.
e. Not all fungi have a dikaryon stage.
5. Reproductive structures consisting of one or more photosynthetic cells surrounded by fungal hyphae are called
a. ascospores.
b. basidiospores.
c. conidia.
d. soredia.
e. gametes.
6. The zygomycetes
a. have hyphae without regularly occurring septa.
b. produce motile gametes.
c. form fleshy fruiting bodies.
d. are haploid throughout their life cycle.
e. have sexual reproductive structures similar to
those of the ascomycetes.
7. Which statement about ascomycetes is not true?
a. They include yeasts.
b. They form reproductive structures called asci.
c. Their hyphae are segmented by septa.
d. Many of their species have a dikaryotic state.
e. All have fruiting structures called ascocarps.
8. The basidiomycetes
a. often produce fleshy fruiting structures.
b. have hyphae without septa.
c. have no sexual stage.
d. produce basidia within basidiospores.
e. form diploid basidiospores.
9. The deuteromycetes
a. have distinctive sexual stages.
b. are all parasitic.
c. have “lost” some members to other fungal groups.
d. include the ascomycetes.
e. are never components of lichens.
10. Which statement about lichens is not true?
a. They can reproduce by fragmentation of the
vegetative body.
b. They are often the first colonists in a new area.
c. They render their environment more basic (alkaline).
d. They contribute to soil formation.
e. They may contain less than 10 percent water by weight.
For Discussion
1. You are shown an object that looks superficially like a pale
green mushroom. Describe at least three criteria (including
anatomical and chemical traits) that would enable you to tell
whether the object is a piece of a plant or a piece of a fungus.
2. Differentiate among the members of the following pairs of
related terms:
a. hypha/mycelium
b. euascomycete/hemiascomycete
c. ascus/basidium
d. ectomycorrhiza/endomycorrhiza
3. For each type of organism listed below, give a single characteristic that may be used to differentiate it from the other,
related organism(s) in parentheses.
a. Zygomycota (Ascomycota)
b. Basidiomycota (deuteromycetes)
c. Ascomycota (Basidiomycota)
d. baker’s yeast (Neurospora crassa)
4. Many fungi are dikaryotic during part of their life cycle. Why
are dikaryons described as n + n instead of 2n?
5. If all the fungi on Earth were suddenly to die, how would the
surviving organisms be affected? Be thorough and specific in
your answer.
6. How might the first mycorrhizae have arisen?
7. What might account for the ability of lichens to withstand the
intensely cold environment of Antarctica? Be specific in your
answer.