Campbell Biology 12e (1) - 703-721
Campbell Biology 12e (1) - 703-721
Campbell Biology 12e (1) - 703-721
KEY CONCEPTS
Fungi
Study Tip
Figure 31.1 These little mushrooms are just the small aboveground extension
Draw a diagram: To help you
of a vast network of filaments located below the forest floor. Such underground
recognize key differences between the fungal networks, called mycelia, in some cases link mushrooms that are hundreds of
life cycles of fungi and humans, draw meters apart. In fact, the largest known mycelium spreads beneath 965 hectares of
and label simple diagrams like the forest—more than the area of 1,800 football fields.
partial example shown here depicting
the life stages at which meiosis, mitosis,
gamete formation, a multicellular
organism, and fertilization occur in
How do structure and function in fungi
humans and in fungi. relate to their role in ecosystems?
Human life cycle: As they grow, multicellular fungi
Gametes extend filaments called hyphae into
n n
their surroundings.
n
Spores enable fungi to
MEIOSIS FERTILIZATION colonize new envrironments.
The spores germinate and
grow when conditions
are favorable.
Hyphae secrete
2n Hyphae enzymes
that break down
organic matter,
Diploid releasing nutrients.
multicellular
organism
Spores
Go to Mastering Biology Nutrients
For Students (in eText and Study Area)
• Get Ready for Chapter 31
• Animation: Life Cycle of a Mushroom
For Instructors to Assign (in Item Library) Hyphae absorb
• Activity: Fungal Reproduction and Almost any organic molecule can the released
Nutrition be digested by at least some fungi, nutrients.
• Scientific Skills Exercise: Synthesizing making them highly effective
Information from Multiple Data Sets decomposers in ecosystems.
654
The hidden network of fungal filaments in Figure 31.1 is a fitting as fallen logs, animal corpses, and the wastes of organisms.
symbol of the neglected grandeur of the kingdom Fungi. Most of Parasitic fungi absorb nutrients from the cells of living hosts.
us are barely aware of these eukaryotes beyond the mushrooms Some parasitic fungi are pathogenic, including many species
we eat or the occasional brush with athlete’s foot. Yet fungi are that cause diseases in plants and others that cause diseases in
a huge and important component of the biosphere. Some fungi animals. Mutualistic fungi also absorb nutrients from a host, but
are exclusively single-celled, though most have complex multi- they reciprocate with actions that benefit the host. For example,
cellular bodies. These diverse organisms are found in just about mutualistic fungi that live within the digestive tracts of certain
every imaginable terrestrial and aquatic habitat. termite species use their enzymes to break down wood, as do
Fungi are not only diverse and widespread but also essen- mutualistic protists in other termites (see Figure 28.29).
tial for the well-being of most ecosystems. They break down The versatile enzymes that enable fungi to digest a wide
organic material and recycle nutrients, allowing other organ- range of food sources are not the only reason for their ecologi-
isms to assimilate essential chemical elements. In this chap- cal success. Another important factor is how their body struc-
ter, we will investigate the structure and evolutionary history ture increases the efficiency of nutrient absorption.
of fungi, survey the major groups of fungi, and discuss their
ecological and commercial significance. Body Structure
The most common fungal body structures are multicellular
filaments and single cells (yeasts). Many fungal species can
CONCEPT 31.1
grow as both filaments and yeasts, but even more grow only
Fungi are heterotrophs that feed as filaments; relatively few species grow only as single-celled
yeasts. Yeasts often inhabit moist environments, including
by absorption plant sap and animal tissues, where there is a ready supply of
Despite their vast diversity, all fungi share some key traits— soluble nutrients, such as sugars and amino acids.
most importantly, the way they derive nutrition. Another The morphology of multicellular fungi enhances their
key characteristic of many fungi is that they grow by forming ability to grow into and absorb nutrients from their surround-
multicellular filaments, a body structure that plays an impor- ings (Figure 31.2). The bodies of these fungi typically form a
tant role in how they obtain food.
. Figure 31.2 Structure of a multicellular fungus. The top photograph shows the sexual
structures, in this case called mushrooms, of the penny bun fungus (Boletus edulis). The bottom
Nutrition and Ecology photograph shows a mycelium growing on fallen conifer needles. The inset SEM shows hyphae.
Like animals, fungi are heterotrophs: Reproductive structure.
They cannot make their own food as Tiny haploid cells called spores are
produced inside the mushroom.
plants and algae can. But unlike ani-
mals, fungi do not ingest (eat) their Hyphae. The mushroom and its
food. Instead, a fungus absorbs nutri- subterranean mycelium are a
ents from the environment outside of continuous network of hyphae.
its body. Many fungi do this by secret-
ing hydrolytic enzymes into their sur-
roundings. These enzymes break down
complex molecules to smaller organic
compounds that the fungi can absorb
into their cells and use. Other fungi use Spore-producing
enzymes to penetrate the walls of cells, structures
enabling the fungi to absorb nutrients
from the cells. Collectively, the differ-
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ent enzymes found in various fungal
species can digest compounds from a
wide range of sources, living or dead.
This diversity of food sources cor-
responds to the varied roles of fungi in
ecological communities: Different spe-
Mycelium
cies live as decomposers, parasites, or
mutualists. Fungi that are decompos- ? Although the mushrooms in the top photograph appear to be different individuals, could their
ers break down and absorb nutrients DNA be identical? Explain.
from nonliving organic material, such Mastering Biology Animation: Fungal Growth and Nutrition
Pore Nematode
Septum Nuclei
Nuclei
Hyphae
network of tiny filaments called hyphae (singular, hypha).
Hyphae consist of tubular cell walls surrounding the plasma
membrane and cytoplasm of the cells. The cell walls are
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strengthened by chitin, a strong but flexible polysaccharide.
Chitin-rich walls can enhance feeding by absorption. As a fun-
gus absorbs nutrients from its environment, the concentrations (a) Hyphae adapted for trapping and killing prey. In Arthrobotrys,
of those nutrients in its cells increases, causing water to move a soil fungus, portions of the hyphae are modified as hoops that
can constrict around a nematode (roundworm) in less than a
into the cells by osmosis. The movement of water into fungal second. The growing hyphae then penetrate the worm’s body, and
cells creates pressure that could cause their cells to burst if they the fungus digests its prey’s inner tissues (SEM).
were not surrounded by a chitin-strengthened, rigid cell wall. Plant
Another important structural feature of most fungi is Fungal hypha cell
wall
that their hyphae are divided into cells by cross-walls, or
septa (singular, septum) (Figure 31.3a). Septa generally have
pores large enough to allow ribosomes, mitochondria, and
even nuclei to flow from cell to cell. Some fungi lack septa
(Figure 31.3b). Known as coenocytic fungi, these organ-
isms consist of a continuous cytoplasmic mass having hun-
Plant cell
dreds or thousands of nuclei. The coenocytic condition results
from the repeated division of nuclei without cytokinesis. Plant cell
plasma
Fungal hyphae form an interwoven mass called a
Arbuscule membrane
mycelium (plural, mycelia) that infiltrates the material on
(b) Arbuscules. Some mutualistic fungi have specialized hyphae called
which the fungus feeds (see Figure 31.2). The structure of a arbuscules that can exchange nutrients with living plant cells.
mycelium maximizes its surface-to-volume ratio, making feed- Arbuscules remain separated from a plant cell’s cytoplasm by the
ing very efficient. Just 1 cm3 of rich soil may contain as much plasma membrane of the plant cell (orange).
as 1 km of hyphae with a total surface area of 300 cm2 in con-
tact with the soil. A fungal mycelium grows rapidly, as proteins Mycorrhizal fungi (fungi that form mycorrhizae) can
and other materials synthesized by the fungus move through improve delivery of phosphate ions and other minerals to
cytoplasmic streaming to the tips of the extending hyphae. The plants because the vast mycelial networks of the fungi are
fungus concentrates its energy and resources on adding hyphal more efficient than the plants’ roots at acquiring these min-
length and thus overall absorptive surface area, rather than on erals from the soil. In exchange, the plants supply the fungi
increasing hyphal girth. Multicellular fungi are not motile in with organic nutrients such as carbohydrates.
the typical sense—they cannot run, swim, or fly in search of There are two main types of mycorrhizal fungi (see
food or mates. However, as they grow, such fungi can move Figure 37.14). Ectomycorrhizal fungi (from the Greek ektos,
into new territory, swiftly extending the tips of their hyphae. out) form sheaths of hyphae over the surface of a root and
typically grow into the extracellular spaces of the root cortex.
Specialized Hyphae in Mycorrhizal Fungi Arbuscular mycorrhizal fungi extend arbuscules through
Some fungi have specialized hyphae that allow them to feed on the root cell wall and into tubes formed by invagination (push-
living animals (Figure 31.4a), while others have modified hyphae ing inward, as in Figure 31.4b) of the root cell plasma mem-
called haustoria that enable them to extract nutrients from plants. brane. In the Scientific Skills Exercise, you’ll compare genomic
Our focus here, however, will be on fungi that have specialized data from fungi that form mycorrhizae and fungi that do not.
branching hyphae such as arbuscules (Figure 31.4b) through Mycorrhizae are enormously important both in natural
which fungi exchange nutrients with their plant hosts. Such ecosystems and in agriculture. Almost all vascular plants
mutually beneficial relationships between fungi and plant roots have mycorrhizae and rely on their fungal partners for essen-
are called mycorrhizae (the term means “fungus roots”). tial nutrients. Foresters commonly inoculate pine seedlings
Figure 31.5 generalizes the many different life cycles that Asexual Reproduction
can produce fungal spores. In this section, we will survey the
Many fungi reproduce both sexually and asexually, as shown
main aspects of sexual and asexual reproduction in fungi.
in Figure 31.5; others, however, reproduce only sexually or
only asexually. As with sexual reproduction, the processes of
Sexual Reproduction asexual reproduction vary widely among fungi.
Many fungi reproduce asexually by growing as filamen-
The nuclei of fungal hyphae and the spores of most fungi
tous fungi that produce (haploid) spores by mitosis; such
are haploid, although many species have transient diploid
species are informally referred to as molds if they form vis-
stages that form during sexual life cycles. Sexual reproduction
ible mycelia. Depending on your housekeeping habits, you
often begins when hyphae from two mycelia release signaling
may have observed molds in your kitchen, forming furry
molecules called pheromones. If the mycelia are of differ-
carpets on bread or fruit (Figure 31.6). Molds typically grow
ent mating types, the pheromones from each partner bind
rapidly and produce many spores asexually, enabling the
to receptors on the other, and the hyphae extend toward the
fungi to colonize new sources of food. Many species that
source of the pheromones. When the hyphae meet, they fuse.
In species with such a “compatibility test,” this process con-
tributes to genetic variation by preventing hyphae from fus- . Figure 31.6 Penicillium, a
ing with other hyphae from the same mycelium or another mold commonly encountered
genetically identical mycelium. as a decomposer of food.
The bead-like clusters in the
The union of the cytoplasms of two parent mycelia is
colorized SEM are conidia,
known as plasmogamy (see Figure 31.5). In most fungi, the structures involved in
haploid nuclei contributed by each parent do not fuse right asexual reproduction.
away. Instead, parts of the fused mycelium contain coexisting,
genetically different nuclei. Such a mycelium is said to be a
heterokaryon (meaning “different nuclei”). In some species,
the haploid nuclei pair off two to a cell, one from each parent.
Such a mycelium is dikaryotic (meaning “two nuclei”). As a
dikaryotic mycelium grows, the two nuclei in each cell divide
in tandem without fusing. Because these cells retain two sepa-
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rate haploid nuclei, they differ from diploid cells, which have
pairs of homologous chromosomes within a single nucleus.
Opisthokonts
different mating type.
UNICELLULAR,
Other fungi reproduce FLAGELLATED Nucleariids
asexually by growing as ANCESTOR
single-celled yeasts. Instead
of producing spores, asexual Fungi
reproduction in yeasts
occurs by ordinary cell divi-
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sion or by the pinching of Parent the protists that share a close common ancestor with animals
cell
small “bud cells” off a par- and fungi also have flagella. DNA sequence data indicate that
ent cell (Figure 31.7). As these three groups of eukaryotes—the fungi, the animals,
already mentioned, some fungi that grow as yeasts can also and their protistan relatives—form a monophyletic group, or
grow as filamentous mycelia. clade (Figure 31.8). As discussed in Concept 28.5, members
Many yeasts and filamentous fungi have no known sexual of this clade are called opisthokonts, a name that refers
stage in their life cycle. Since early mycologists (biologists to the posterior (opistho-) location of the flagellum in these
who study fungi) classified fungi based mainly on their type organisms.
of sexual structure, this posed a problem. Mycologists have Within the opisthokont clade, fungi are more closely
traditionally lumped all fungi lacking sexual reproduction related to several groups of single-celled protists than they
into a group called deuteromycetes (from the Greek deu- are to animals, suggesting that the ancestor of fungi was
tero, second, and mycete, fungus). Whenever a sexual stage is unicellular. One such group of unicellular protists, the
discovered for a so-called deuteromycete, the species is reclas- nucleariids, consists of amoebas that feed on algae and
sified in a particular phylum, depending on the type of sexual bacteria. DNA evidence further indicates that animals are
structures it forms. In addition to searching for sexual stages more closely related to a different group of protists (the
of such unassigned fungi, mycologists can now use genomic choanoflagellates) than they are to either fungi or nucleariids.
techniques to classify them. Together, these results suggest that multicellularity evolved
in animals and fungi independently, from different single-
CONCEPT CHECK 31.2 celled ancestors.
1. MAKE CONNECTIONS Compare Figure 31.5 with Figure Using molecular clock analyses, scien-
13.6. In terms of haploidy versus diploidy, how do the life tists have estimated that the ancestors of
cycles of fungi and humans differ?
animals and fungi diverged into separate
2. WHAT IF? Suppose that you sample the DNA of two mush-
rooms on opposite sides of your yard and find that they are lineages more than a billion years ago.
identical. Propose two hypotheses that could reasonably Fossils of certain unicellular, marine
account for this result. eukaryotes that lived as early as Septa
For suggested answers, see Appendix A. 1.5 billion years ago have been
interpreted as fungi, but those claims
CONCEPT 31.3 remain controversial. Furthermore,
although fungi probably originated in
The ancestor of fungi was an aquatic, aquatic environments, the oldest fossils
that are widely accepted as fungi are of
single-celled, flagellated protist terrestrial species that lived 440 million
Data from molecular systematics offer insights into the early years ago (Figure 31.9). Fungi may
evolution of fungi. As a result, systematists now recognize have colonized land as early as
that fungi and animals are more closely related to each other 505 million years ago: Soils of that age
than either group is to plants or to most other eukaryotes.
Central
filament
The Origin of Fungi
Phylogenetic analyses suggest that fungi evolved from a flag-
c Figure 31.9 Fossil hyphae from the
ellated ancestor. While the majority of fungi lack flagella, two
fungus Tortotubus (440 million years ago).
basal lineages of fungi (the cryptomycetes and the chytrids, The central filament is surrounded by two
as we’ll discuss shortly) do have flagella. Moreover, most of partially overlapping filaments (LM).
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Cryptomycetes
Although only 30 species have been identified to date,
genetic data suggest that the cryptomycetes are a large and small genomes, with only 2,000 genes in some species. The
diverse group. DNA sequences from members of this group genome of one microsporidian, Encephalitozoon intestinalis,
have been found in marine and freshwater communities, as has just 2.3 Mb of DNA—the smallest genome of any eukary-
well as soils. Cryptomycetes also have been found in aerobic ote sequenced to date. Unlike other basal fungi, microspo-
and anaerobic environments, and in geographical locations ridians lack flagellated spores; instead, they produce unique
across the globe. Like the species shown in Figure 31.11, spores that infect host cells via a harpoon-like organelle.
Rozella allomycis, many of the cryptomycetes identified to
date are parasites of protists and other fungi. Chytrids
Cryptomycetes The fungi classified in phylum
Microsporidians
. Figure 31.11 The cryptomycete Rozella allomycis Chytridiomycota, called chytrids,
Chytrids
parasitizing another fungus. are ubiquitous in lakes and soil;
Zoopagomycetes
Mucoromycetes recent metagenomic studies have
Hypha of Ascomycetes
host fungus uncovered new clades of chytrids
Basidiomycetes
in hydrothermal vent and other
marine communities. Some of the approximately 1,000 chytrid
species are decomposers, while others are parasites of protists,
Rozella spores
other fungi, plants, or animals; as we’ll see later in the chapter,
two chytrid parasites have contributed to the global decline of
amphibian populations. Still other chytrids are important mutu-
alists. For example, anaerobic chytrids that live in the digestive
tracts of sheep and cattle help to break down plant matter,
thereby contributing significantly to the animal’s growth.
Nearly all chytrids have flagellated spores, called zoospores
Cryptomycetes are unicellular and have flagellated spores. (Figure 31.13). Like other fungi, chytrids have cell walls made of
Cryptomycetes also can synthesize a chitin-rich cell wall, a chitin, and they also share certain key enzymes and metabolic
key structural feature of the fungi (see Concept 31.1). pathways with other fungal groups. Some chytrids form colo-
nies with hyphae, while others exist as single spherical cells.
Microsporidians Mastering Biology Video: Phlyctochytrium Zoospore Release
The 1,300 species of microsporidians are unicellular parasites
of protists and animals, including humans (Figure 31.12).
c Figure 31.13
Infections in humans can cause reduced longevity and Flagellated
weight loss. The microsporidian Nosema ceranae is a parasite chytrid
of honeybees and may contribute to Colony Collapse Disorder, zoospore.
a devastating outbreak that has led to the loss of honeybee (TEM) Flagellum
colonies throughout the world.
Like all fungi, microsporidians can synthesize a chitin-rich
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cell wall. Other aspects of their biology are unusual. For exam-
ple, microsporidians have highly reduced mitochondria and
. Figure 31.15 The life cycle of the mucoromycete Rhizopus stolonifer (black bread mold).
3 A zygosporangium
PLASMOGAMY forms, containing
multiple haploid nuclei
Mating Gametangia with from the two parents.
type (–) haploid nuclei
Mating
type (+)
Rhizopus 100 om
growing Young
on bread zygosporangium
(heterokaryotic)
8 The spores
germinate and
SEXUAL
grow into new
REPRODUCTION
9 Mycelia can also reproduce mycelia.
asexually by forming sporangia
that produce genetically
identical haploid spores. Dispersal and Zygosporangium
germination
KARYOGAMY
Sporangia
7 The sporangium 4 The zygosporangium
disperses genetically develops a rough,
diverse haploid spores. Diploid thick-walled coating
nuclei that can resist harsh
Sporangium conditions for months.
ASEXUAL
REPRODUCTION MEIOSIS
5 When conditions are
Dispersal and favorable, karyogamy
germination occurs, then meiosis.
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6 The zygosporangium
Mycelium germinates into a
sporangium on a short stalk.
Mucoromycetes
Cryptomycetes There are approximately
Microsporidians
750 known species of
Chytrids
Zoopagomycetes mucoromycetes, fungi in the
Mucoromycetes phylum Mucoromycota. This
Ascomycetes phylum includes species of fast- 1 mm
Basidiomycetes
growing molds responsible for
causing foods such as bread, peaches, strawberries, and sweet mycorrhizae (see Figure 31.4b and Figure 37.14). The tips of
potatoes to rot during storage. Although some mucoromy- the hyphae that push into plant root cells branch into tiny
cetes are decomposers, most are associated with plants. Many treelike arbuscules. About 85% of all plant species have mutu-
mucoromycetes live as parasites or pathogens of plants, while alistic partnerships with arbuscular mycorrhizae.
others live as mutualists (including some mycorrhizae).
The life cycle of Rhizopus stolonifer (black bread mold) is Ascomycetes
fairly typical of mucoromycete species (Figure 31.15). Its Cryptomycetes Mycologists have described 90,000
hyphae spread out over the food surface, penetrate it, and Microsporidians species of ascomycetes, fungi in the
Chytrids
absorb nutrients. The hyphae are coenocytic, with septa phylum Ascomycota, from a wide
Zoopagomycetes
found only where reproductive cells are formed. In the Mucoromycetes variety of marine, freshwater, and ter-
asexual phase, bulbous black sporangia develop at the tips Ascomycetes restrial habitats. The defining feature
of upright hyphae. Within each sporangium, hundreds of Basidiomycetes of ascomycetes is the production of
genetically identical haploid spores develop and are dispersed spores (called ascospores) in saclike asci (singular, ascus); thus,
through the air. Spores that happen to land on moist food they are commonly called sac fungi. During their sexual stage,
germinate, growing into new mycelia. most ascomycetes develop fruiting bodies, called ascocarps,
If environmental conditions deteriorate—for instance, which range in size from microscopic to macroscopic
if the mold consumes all its food—Rhizopus may reproduce (Figure 31.17). The ascocarps contain the spore-forming asci.
sexually. The parents in a sexual union are mycelia of different Ascomycetes vary in size and complexity from unicellular
mating types, which possess different chemical markers but yeasts to elaborate cup fungi and morels (see Figure 31.17).
may appear identical. Plasmogamy produces a sturdy structure They include some of the most devastating plant pathogens,
called a zygosporangium (plural, zygosporangia), in which which we will discuss later. However, many ascomycetes are
karyogamy and then meiosis occur. Note that while a zygospo- important decomposers, particularly of plant material. More
rangium represents the zygote (2n) stage in the life cycle, it is than 25% of all ascomycete species live with green algae or
not a zygote in the usual sense (that is, a cell with one diploid cyanobacteria in beneficial symbiotic associations called
nucleus). Rather, a zygosporangium is a multinucleate struc- lichens. Some ascomycetes form mycorrhizae with plants.
ture, first heterokaryotic with many haploid nuclei from the
two parents, then with many diploid nuclei after karyogamy. . Figure 31.17 Ascomycetes (sac fungi).
Zygosporangia are resistant to freezing and drying and are
c Tuber melanosporum is a truffle
metabolically inactive. When conditions improve, the nuclei species that forms ectomycorrhi-
of the zygosporangium undergo meiosis, the zygosporangium zae with trees. The ascocarp
germinates into a sporangium, and the sporangium releases grows underground and emits
a strong odor. These
genetically diverse haploid spores that may colonize a new
ascocarps have
substrate. Some mucoromycetes can actually “aim” and then been dug up
shoot their sporangia toward bright light. Figure 31.16 shows and the
one example, Pilobolus, which decomposes animal dung. middle one
sliced open.
Its sporebearing hyphae bend toward light, where there are
likely to be openings in the vegetation through which spores
may reach fresh grass. The fungus then launches its sporangia
in a jet of water that can travel up to 2.5 m. Grazing animals
b The edible ascocarp of Morchella
ingest the fungi with the grass and then scatter the spores in esculenta, the tasty morel, is often
feces, thereby enabling the next generation of fungi to grow. found under trees in orchards.
Finally, the phylum Mucoromycota also includes the ? Ascomycetes vary greatly in morphology (see also Figure 31.10). How
glomeromycetes, a clade of fungi that form arbuscular could you confirm that a fungus is an ascomycete?
. Figure 31.18 The life cycle of Neurospora crassa, an ascomycete. Neurospora is a bread mold
and research organism that also grows in the wild on burned vegetation.
1 Ascomycete mycelia 2 Neurospora can also reproduce Key
can reproduce asexually Conidia; sexually by producing specialized
by producing pigmented mating type (–) hyphae. Conidia of the opposite Haploid (n)
haploid spores (conidia). mating type fuse to these hyphae.
Dikaryotic (n + n)
Diploid (2n)
Dispersal
Germination Mating 3 The dikaryotic hyphae
type (+) that result from
ASEXUAL
plasmogamy produce
REPRODUCTION Hypha PLASMOGAMY many dikaryotic asci, two
of which are shown here.
Ascus
(dikaryotic)
Conidiophore
Mycelia
(n) Dikaryotic
hyphae
Mycelium (n + n)
Germination SEXUAL
REPRODUCTION KARYOGAMY
Ascospores (n) Diploid nucleus
Dispersal 4 Karyogamy
(zygote; 2n) occurs within each
Asci ascus, producing a
Ascocarp diploid nucleus.
7 The ascospores
Eight
are discharged forcibly
ascospores
from the asci through an Four
opening in the ascocarp. haploid
Germinating ascospores nuclei (n) MEIOSIS
give rise to new mycelia.
VISUAL SKILLS What is the ploidy of a cell in the specialized hypha shown in 2 ?
Basidiomycetes
Cryptomycetes About 50,000 species, including Basidiomycetes are important decomposers of wood and
Microsporidians mushrooms, puffballs, and shelf other plant material. Of all the fungi, certain basidiomycetes
Chytrids
Zoopagomycetes
fungi, are called basidiomycetes are the best at decomposing the complex polymer lignin, an
Mucoromycetes and are classified in the phylum abundant component of wood. Many shelf fungi break down
Ascomycetes Basidiomycota (Figure 31.19). This the wood of weak or damaged trees and continue to decom-
Basidiomycetes
phylum also includes mutualists pose the wood after the tree dies.
that form mycorrhizae and two groups of destructive plant The life cycle of a basidiomycete usually includes a long-
parasites: rusts and smuts. The name of the phylum derives lived dikaryotic mycelium. As in ascomycetes, this extended
from the basidium (plural, basidia; Latin for “little pedes- dikaryotic stage provides many opportunities for genetic
tals”), a cell in which karyogamy occurs, followed immedi- recombination events, in effect multiplying the result of a
ately by meiosis. The club-like shape of the basidium also single mating. Periodically, in response to environmental
gives rise to the common name club fungus. stimuli, the mycelium reproduces sexually by producing
Dikaryotic
PLASMOGAMY mycelium 3 Environmental cues
such as rain or change in
temperature induce the
dikaryotic mycelium to
Mating form compact masses
8 In a suitable type (–) that develop into
environment, the basidiocarps (mushrooms,
basidiospores in this case).
germinate and Mating
grow into type (+)
short-lived
Haploid
haploid mycelia.
mycelia Gills lined
SEXUAL with basidia
REPRODUCTION Basidiocarp
(n + n)
7 When mature,
the basidiospores Dispersal
are ejected and and
then dispersed germination
by the wind.
Basidiospores
(n)
MEIOSIS
Key
6 Each diploid nucleus
yields four haploid 5 Karyogamy in each
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Haploid (n)
nuclei, each of which Diploid basidium produces a
Basidiospore develops into a nuclei diploid nucleus, which Dikaryotic (n + n)
basidiospore (SEM). then undergoes meiosis. Diploid (2n)
VISUAL SKILLS Use the diagram to determine the ploidy of a cell in the aboveground stalk of a mushroom.
Mastering Biology Animation: Life Cycle of a Mushroom
elaborate fruiting bodies called basidiocarps (Figure 31.20). organic matter in the soil as it grows. Some giant fairy rings
The common white mushrooms in the supermarket are famil- are produced by mycelia that are centuries old.
iar examples of a basidiocarp. After a mushroom forms, its cap supports and protects a
By concentrating growth in the hyphae of mushrooms, large surface area of dikaryotic basidia on gills. During kary-
a basidiomycete mycelium can erect its fruiting structures ogamy, the two nuclei in each basidium fuse, producing a
in just a few hours; a mushroom pops up as it absorbs water diploid nucleus (see Figure 31.20). This nucleus then under-
and as cytoplasm streams in from the dikaryotic mycelium. goes meiosis, yielding four haploid nuclei, each of which
By this process, in some species a ring of mushrooms, popu- ultimately develops into a basidiospore. Large numbers of
larly called a “fairy ring,” may appear literally overnight basidiospores are produced: The gills of a common white
(Figure 31.21). The mycelium below the fairy ring expands mushroom have a surface area of about 200 cm2 and may
outward at a rate of about 30 cm per year, decomposing drop a billion basidiospores, which blow away.
Fungus-Plant Mutualisms
All plant species studied to date appear to harbor symbi-
otic endophytes, fungi (or bacteria) that live inside leaves
or other plant parts without causing harm. Most fungal
endophytes identified to date are ascomycetes but some are
mucoromycetes. Fungal endophytes benefit certain grasses
and other nonwoody plants by making toxins that deter her-
bivores or by increasing host plant tolerance of heat, drought,
or heavy metals. As described in Figure 31.22, researchers
Fungi as Decomposers 30 15
Leaf area damaged (%)
Leaf mortality (%)
c A foliose
(leaflike)
lichen
studying how fungal endophytes affect a woody plant tested
whether leaf endophytes benefit seedlings of the cacao tree,
Theobroma cacao. Their findings show that the fungal endo-
phytes of woody flowering plants can play an important role
in defending against pathogens.
Fungus-Animal Mutualisms
As mentioned earlier, some fungi share their digestive ser-
vices with animals, helping break down plant material in
the guts of cattle and other grazing mammals. Many spe-
b Crustose
cies of ants take advantage of the digestive power of fungi (encrusting)
by raising them in “farms.” Leaf-cutter ants, for example, lichens
scour tropical forests in search of leaves, which they can-
not digest on their own but carry back to their nests and
feed to the fungi (Figure 31.23). As the fungi grow, their
hyphae develop specialized swollen tips that are rich in glomeromycete and basidiomycete lichens are known. Recent
proteins and carbohydrates. The ants feed primarily on studies have found that many lichens also have a basidiomy-
these nutrient-rich tips. Not only do the fungi break down cete yeast as a second fungal component. As the role of these
plant leaves into substances the insects can digest, but yeasts remains unknown, our discussion will focus on the
they also detoxify plant defensive compounds that would primary fungal partner.
otherwise kill or harm the ants. In some tropical forests, The fungus usually gives a lichen its overall shape and
the fungi have helped these insects become the major structure, and tissues formed by hyphae account for most
consumers of leaves. of the lichen’s mass. The cells of the alga or cyanobacterium
The evolution of such farmer ants and that of their fungal generally occupy an inner layer below the lichen surface
“crops” have been tightly linked for over 50 million years. (Figure 31.25). The merger of fungus and alga or cyano-
The fungi have become so dependent on their caretakers that bacterium is so complete that lichens are given scientific
in many cases they can no longer survive without the ants, names as though they were single organisms. As might be
and vice versa. expected of such “dual organisms,” asexual reproduction
as a symbiotic unit is common. This can occur either by
Lichens fragmentation of the parental lichen or by the formation
A lichen is a symbiotic association between a photosynthetic of soredia (singular, soredium), small clusters of hyphae
microorganism and a fungus in which millions of photosyn- with embedded algae (see Figure 31.25). The fungi of many
thetic cells are held in a mass of fungal hyphae. Lichens grow lichens also reproduce sexually.
on the surfaces of rocks, rotting logs, trees, and roofs in vari- In most lichens, each partner provides something the
ous forms (Figure 31.24). The photosynthetic partners are other could not obtain on its own. The alga or cyanobac-
unicellular or filamentous green algae or cyanobacteria. The terium provides carbon compounds; a cyanobacterium
fungal component is most often an ascomycete, but some also fixes nitrogen (see Concept 27.3) and provides organic
Fungi as Parasites
Like mutualistic fungi, parasitic fungi absorb nutrients
from the cells of living hosts, but they provide no ben-
efits in return. About 30% of the 145,000 known species
of fungi make a living as parasites or pathogens, mostly of
plants (Figure 31.26). An example of a plant pathogen is
(b) Tar spot
Cryphonectria parasitica, the ascomycete fungus that causes fungus
chestnut blight, which dramatically changed the landscape on maple
leaves
of the northeastern United States. Accidentally introduced
via trees imported from Asia in the early 1900s, spores of the
fungus entered cracks in the bark of American chestnut trees
and produced hyphae, killing many trees. The once-common
chestnuts now survive mainly as sprouts from the stumps of (a) Corn smut on corn
former trees. Another ascomycete, Fusarium circinatum, causes
pine pitch canker, a disease that threatens pines throughout
the world. In addition, between 10% and 50% of the world’s (c) Ergots on rye