Bacterial diseases

Despite their vast diversity, all fungi share some key traits—

most importantly, the way they derive nutrition. Another

key characteristic of many fungi is that they grow by forming

multicellular filaments, a body structure that plays an important

role in how they obtain food.

Nutrition and Ecology

Like animals, fungi are heterotrophs:

They cannot make their own food as

plants and algae can. But unlike animals,

fungi do not ingest (eat) their

food. Instead, a fungus absorbs nutrients

from the environment outside of

its body. Many fungi do this by secreting

hydrolytic enzymes into their surroundings.

These enzymes break down

complex molecules to smaller organic

compounds that the fungi can absorb

into their cells and use. Other fungi use

enzymes to penetrate the walls of cells,

enabling the fungi to absorb nutrients

from the cells. Collectively, the different

enzymes found in various fungal

species can digest compounds from a

wide range of sources, living or dead.

This diversity of food sources corresponds

to the varied roles of fungi in

ecological communities: Different species

live as decomposers, parasites, or

mutualists. Fungi that are decomposers

break down and absorb nutrients

from nonliving organic material, such as fallen logs, animal corpses, and the wastes of organisms.

Parasitic fungi absorb nutrients from the cells of living hosts.

Some parasitic fungi are pathogenic, including many species

that cause diseases in plants and others that cause diseases in

animals. Mutualistic fungi also absorb nutrients from a host, but

they reciprocate with actions that benefit the host. For example,

mutualistic fungi that live within the digestive tracts of certain

termite species use their enzymes to break down wood, as do

mutualistic protists in other termites (see Figure 28.29).

The versatile enzymes that enable fungi to digest a wide

range of food sources are not the only reason for their ecological

success. Another important factor is how their body structure

increases the efficiency of nutrient absorption.

Body Structure

The most common fungal body structures are multicellular

filaments and single cells (yeasts). Many fungal species can

grow as both filaments and yeasts, but even more grow only

as filaments; relatively few species grow only as single-celled

yeasts. Yeasts often inhabit moist environments, including

plant sap and animal tissues, where there is a ready supply of

soluble nutrients, such as sugars and amino acids.

The morphology of multicellular fungi enhances their

ability to grow into and absorb nutrients from their surroundings

(Figure 31.2). The bodies of these fungi typically form a 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

strengthened by chitin, a strong but flexible polysaccharide.

Chitin-rich walls can enhance feeding by absorption. As a fungus

absorbs nutrients from its environment, the concentrations

of those nutrients in its cells increases, causing water to move

into the cells by osmosis. The movement of water into fungal

cells creates pressure that could cause their cells to burst if they

were not surrounded by a chitin-strengthened, rigid cell wall.

Another important structural feature of most fungi is

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 organisms

consist of a continuous cytoplasmic mass having hundreds

or thousands of nuclei. The coenocytic condition results

from the repeated division of nuclei without cytokinesis.

Fungal hyphae form an interwoven mass called a

mycelium (plural, mycelia) that infiltrates the material on

which the fungus feeds (see Figure 31.2). The structure of a

mycelium maximizes its surface-to-volume ratio, making feeding

very efficient. Just 1 cm3 of rich soil may contain as much

as 1 km of hyphae with a total surface area of 300 cm2 in contact

with the soil. A fungal mycelium grows rapidly, as proteins

and other materials synthesized by the fungus move through

cytoplasmic streaming to the tips of the extending hyphae. The

fungus concentrates its energy and resources on adding hyphal

length and thus overall absorptive surface area, rather than on

increasing hyphal girth. Multicellular fungi are not motile in

the typical sense—they cannot run, swim, or fly in search of

food or mates. However, as they grow, such fungi can move

into new territory, swiftly extending the tips of their hyphae.

Specialized Hyphae in Mycorrhizal Fungi

Some fungi have specialized hyphae that allow them to feed on

living animals (Figure 31.4a), while others have modified hyphae

called haustoria that enable them to extract nutrients from plants.

Our focus here, however, will be on fungi that have specialized

branching hyphae such as arbuscules (Figure 31.4b) through

which fungi exchange nutrients with their plant hosts. Such

mutually beneficial relationships between fungi and plant roots

are called mycorrhizae (the term means “fungus roots”). Mycorrhizal fungi (fungi that form mycorrhizae) can

improve delivery of phosphate ions and other minerals to

plants because the vast mycelial networks of the fungi are

more efficient than the plants’ roots at acquiring these minerals

from the soil. In exchange, the plants supply the fungi

with organic nutrients such as carbohydrates.

There are two main types of mycorrhizal fungi (see

Figure 37.14). Ectomycorrhizal fungi (from the Greek ektos,

out) form sheaths of hyphae over the surface of a root and

typically grow into the extracellular spaces of the root cortex.

Arbuscular mycorrhizal fungi extend arbuscules through

the root cell wall and into tubes formed by invagination (pushing

inward, as in Figure 31.4b) of the root cell plasma membrane.

In the Scientific Skills Exercise, you’ll compare genomic

data from fungi that form mycorrhizae and fungi that do not.

Mycorrhizae are enormously important both in natural

ecosystems and in agriculture. Almost all vascular plants

have mycorrhizae and rely on their fungal partners for essential

nutrients. Foresters commonly inoculate pine seedlings with mycorrhizal fungi to promote growth. In the absence

of human intervention, mycorrhizal fungi colonize soils by

dispersing haploid cells called spores that form new mycelia

after germinating. Spore dispersal is a key component of how

fungi reproduce and spread to new areas, as we discuss next.

Fungi produce spores through

sexual or asexual life cycles

Most fungi propagate themselves by producing vast numbers

of spores, either sexually or asexually. For example, puffballs,

the reproductive structures of certain fungal species, may

release trillions of spores (see Figure 31.17). Spores can be carried

long distances by wind or water. If they land in a moist

place where there is food, they germinate, producing a new

mycelium. To appreciate how effective spores are at dispersing,

leave a slice of melon exposed to the air. Even without a

visible source of spores nearby, within a week, you will likely

observe fuzzy mycelia growing from microscopic spores that

have fallen onto the melon.

Sexual Reproduction

The nuclei of fungal hyphae and the spores of most fungi

are haploid, although many species have transient diploid

stages that form during sexual life cycles. Sexual reproduction

often begins when hyphae from two mycelia release signaling

molecules called pheromones. If the mycelia are of different

mating types, the pheromones from each partner bind

to receptors on the other, and the hyphae extend toward the

source of the pheromones. When the hyphae meet, they fuse.

In species with such a “compatibility test,” this process contributes

to genetic variation by preventing hyphae from fusing

with other hyphae from the same mycelium or another

genetically identical mycelium.

The union of the cytoplasms of two parent mycelia is

known as plasmogamy (see Figure 31.5). In most fungi, the

haploid nuclei contributed by each parent do not fuse right

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 separate

haploid nuclei, they differ from diploid cells, which have

pairs of homologous chromosomes within a single nucleus. Hours, days, or (in some fungi) even

centuries may pass between plasmogamy

and the next stage in the sexual cycle,

karyogamy. During karyogamy, the

haploid nuclei contributed by the two parents

fuse, producing diploid cells. Zygotes

and other transient structures form during

karyogamy, the only diploid stage in

most fungi. Meiosis then restores the haploid

condition, ultimately leading to the

formation of genetically diverse spores.

Meiosis is a key step in sexual reproduction,

so spores produced in this way are

sometimes referred to as “sexual spores.”

The sexual processes of karyogamy

and meiosis generate extensive genetic

variation, a prerequisite for natural selection.

(See Concepts 13.2 and 23.1 to review

how sex can increase genetic diversity.) The

heterokaryotic condition also offers some of

the advantages of diploidy in that one haploid

genome may compensate for harmful mutations

in the other.

Asexual Reproduction

Many fungi reproduce both sexually and asexually, as shown

in Figure 31.5; others, however, reproduce only sexually or

only asexually. As with sexual reproduction, the processes of

asexual reproduction vary widely among fungi.

Many fungi reproduce asexually by growing as filamentous

fungi that produce (haploid) spores by mitosis; such

species are informally referred to as molds if they form visible

mycelia. Depending on your housekeeping habits, you

may have observed molds in your kitchen, forming furry

carpets on bread or fruit (Figure 31.6). Molds typically grow

rapidly and produce many spores asexually, enabling the

fungi to colonize new sources of food. Many species that produce such spores can

also reproduce sexually if

they happen to contact a

member of their species of a

different mating type.

Other fungi reproduce

asexually by growing as

single-celled yeasts. Instead

of producing spores, asexual

reproduction in yeasts

occurs by ordinary cell division

or by the pinching of

small “bud cells” off a parent

cell (Figure 31.7). As

already mentioned, some fungi that grow as yeasts can also

grow as filamentous mycelia.

Many yeasts and filamentous fungi have no known sexual

stage in their life cycle. Since early mycologists (biologists

who study fungi) classified fungi based mainly on their type

of sexual structure, this posed a problem. Mycologists have

traditionally lumped all fungi lacking sexual reproduction

into a group called deuteromycetes (from the Greek deutero,

second, and mycete, fungus). Whenever a sexual stage is

discovered for a so-called deuteromycete, the species is reclassified

in a particular phylum, depending on the type of sexual

structures it forms. In addition to searching for sexual stages

of such unassigned fungi, mycologists can now use genomic

techniques to classify them.

The ancestor of fungi was an aquatic,

single-celled, flagellated protist

Data from molecular systematics offer insights into the early

evolution of fungi. As a result, systematists now recognize

that fungi and animals are more closely related to each other

than either group is to plants or to most other eukaryotes.

The Origin of Fungi

Phylogenetic analyses suggest that fungi evolved from a flagellated

ancestor. While the majority of fungi lack flagella, two

basal lineages of fungi (the cryptomycetes and the chytrids,

as we’ll discuss shortly) do have flagella. Moreover, most of the protists that share a close common ancestor with animals

and fungi also have flagella. DNA sequence data indicate that

these three groups of eukaryotes—the fungi, the animals,

and their protistan relatives—form a monophyletic group, or

clade (Figure 31.8). As discussed in Concept 28.5, members

of this clade are called opisthokonts, a name that refers

to the posterior (opistho-) location of the flagellum in these

organisms.

Within the opisthokont clade, fungi are more closely

related to several groups of single-celled protists than they

are to animals, suggesting that the ancestor of fungi was

unicellular. One such group of unicellular protists, the

nucleariids, consists of amoebas that feed on algae and

bacteria. DNA evidence further indicates that animals are

more closely related to a different group of protists (the

choanoflagellates) than they are to either fungi or nucleariids.

Together, these results suggest that multicellularity evolved

in animals and fungi independently, from different singlecelled

ancestors.

Using molecular clock analyses, scientists

have estimated that the ancestors of

animals and fungi diverged into separate

lineages more than a billion years ago.

Fossils of certain unicellular, marine

eukaryotes that lived as early as

1.5 billion years ago have been

interpreted as fungi, but those claims

remain controversial. Furthermore,

although fungi probably originated in

aquatic environments, the oldest fossils

that are widely accepted as fungi are of

terrestrial species that lived 440 million

years ago (Figure 31.9). Fungi may

have colonized land as early as

505 million years ago: Soils of that age have a chemical “signature” similar to that found in soils

where fungi are active today. Overall, more fossils are needed

to help clarify when fungi originated and what features were

present in their earliest lineages.

The Move to Land

Plants colonized land about 470 million years ago (see

Concept 29.1), and fungi may well have colonized land

before plants. Indeed, some researchers have described life

on land before the arrival of plants as a “green slime” that

consisted of cyanobacteria, algae, and a variety of small, heterotrophic

species, including fungi. With their capacity for

extracellular digestion, fungi would have been well suited for

feeding on other early terrestrial organisms (or their remains).

Once on land, some fungi formed symbiotic associations

with early plants. For example, 405-million-year-old fossils of

the early plant Aglaophyton contain evidence of mycorrhizal

relationships between plants and fungi (see Figure 25.13). This

evidence includes fossils of hyphae that have penetrated within

plant cells and formed structures that resemble the arbuscules

formed today by arbuscular mycorrhizae. Similar structures

have been found in a variety of other early plants, suggesting

that plants probably existed in beneficial relationships with

fungi from the earliest periods of colonization of land. The earliest

plants lacked roots, limiting their ability to extract nutrients

from the soil. As occurs in mycorrhizal associations today, it is

likely that soil nutrients were transferred to early plants via the

extensive mycelia formed by their symbiotic fungal partners.

Support for the antiquity of mycorrhizal associations has

also come from molecular studies. For a mycorrhizal fungus and

its plant partner to establish a symbiotic relationship, certain

genes must be expressed by the fungus and other genes must

be expressed by the plant. Researchers focused on three plant

genes (called sym genes) whose expression is required for the

formation of mycorrhizae in flowering plants. They found that

these genes were present in all major plant lineages, including

basal lineages such as liverworts (see Figure 29.13). Furthermore,

after they transferred a liverwort sym gene to a flowering plant

mutant that could not form mycorrhizae, the mutant recovered

its ability to form mycorrhizae. These results suggest that

mycorrhizal sym genes were present in early plants—and that

the function of these genes has been conserved for hundreds of

millions of years as plants continued to adapt to life on land.

Fungi have radiated into a

diverse set of lineages

In the past decade, molecular analyses have reshaped our

understanding of the evolutionary relationships between

fungal groups. In addition, metagenomic studies have led

to the discovery of entirely new groups of fungi. As a result,

the phylogeny of fungi is undergoing dramatic change. For

example, one traditional group, the Zygomycota, has been

abandoned because it was paraphyletic, and its members have

been reassigned to other groups. Recent studies also indicate

that the microsporidians, an enigmatic group of unicellular

parasites, should be classified as fungi and may belong to a

basal fungal lineage (one that diverged from other fungi early

in the history of the group).

Figure 31.10 presents a current hypothesis of the relationships

among fungal groups. In this section, we’ll survey the

groups identified in this phylogenetic tree. However, the

groups shown in Figure 31.10 may represent only a small

fraction of the diversity of extant fungal groups. (Extant lineages

are those that have surviving members.) While there

are about 145,000 known species of fungi, in recent years

more than 2,000 new species have been discovered annually.

By some estimates the actual number of fungal species lies

between 2.2 and 3.8 million—more than all of the 1.9 million

species of organisms (of every type) that biologists have currently

identified and named.

Cryptomycetes and Microsporidians

Genomic studies indicate that

cryptomycetes (fungi in the

phylum Cryptomycota) and

microsporidians (fungi in the

phylum Microsporidia) form a

sister group and are a basal fungal

lineage (see Figure 31.10). While most molecular comparisons

support the placement of cryptomycetes and microsporidians

at the base of the fungal tree, more data are needed to help

resolve this phylogeny.

Cryptomycetes

Although only 30 species have been identified to date,

genetic data suggest that the cryptomycetes are a large and

diverse group. DNA sequences from members of this group

have been found in marine and freshwater communities, as

well as soils. Cryptomycetes also have been found in aerobic

and anaerobic environments, and in geographical locations

across the globe. Like the species shown in Figure 31.11,

Rozella allomycis, many of the cryptomycetes identified to

date are parasites of protists and other fungi.

Cryptomycetes are unicellular and have flagellated spores.

Cryptomycetes also can synthesize a chitin-rich cell wall, a

key structural feature of the fungi (see Concept 31.1).

Microsporidians

The 1,300 species of microsporidians are unicellular parasites

of protists and animals, including humans (Figure 31.12).

Infections in humans can cause reduced longevity and

weight loss. The microsporidian Nosema ceranae is a parasite

of honeybees and may contribute to Colony Collapse Disorder,

a devastating outbreak that has led to the loss of honeybee

colonies throughout the world.

Like all fungi, microsporidians can synthesize a chitin-rich

cell wall. Other aspects of their biology are unusual. For example,

microsporidians have highly reduced mitochondria and small genomes, with only 2,000 genes in some species. The

genome of one microsporidian, Encephalitozoon intestinalis,

has just 2.3 Mb of DNA—the smallest genome of any eukaryote

sequenced to date. Unlike other basal fungi, microsporidians

lack flagellated spores; instead, they produce unique

spores that infect host cells via a harpoon-like organelle.

Chytrids

The fungi classified in phylum

Chytridiomycota, called chytrids,

are ubiquitous in lakes and soil;

recent metagenomic studies have

uncovered new clades of chytrids

in hydrothermal vent and other

marine communities. Some of the approximately 1,000 chytrid

species are decomposers, while others are parasites of protists,

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 mutualists.

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

(Figure 31.13). Like other fungi, chytrids have cell walls made of

chitin, and they also share certain key enzymes and metabolic

pathways with other fungal groups. Some chytrids form colonies

with hyphae, while others exist as single spherical cells.

Most of the 900 species of

zoopagomycetes, fungi in the

phylum Zoopagomycota, live as

parasites or as commensal (neutral)

symbionts of animals; some are

parasites of other fungi or protists.

Zoopagomycetes form filamentous hyphae and reproduce

asexually by producing nonflagellated spores. Some zoopagomycetes

induce insects that they parasitize to perch near the

top of plants; the insects subsequently die and fungal spores

are released to infect new victims (Figure 31.14). Sexual

reproduction, where known, involves the formation of a

durable structure called a zygosporangium, which houses and

protects the zygote.

The loss of flagellated spores in the zoopagomycetes

transition to life on land. Basal fungal lineages had flagellated

spores, enabling dispersal through water. In contrast, zoopagomycetes

and all of their closest fungal relatives (the clade

consisting of the mucoromycetes, ascomycetes, and basidiomycetes; see Figure 31.10) have nonflagellated spores,

which are dispersed by wind in terrestrial fungi.

Mucoromycetes

There are approximately

750 known species of

mucoromycetes, fungi in the

phylum Mucoromycota. This

phylum includes species of fastgrowing

molds responsible for

causing foods such as bread, peaches, strawberries, and sweet

potatoes to rot during storage. Although some mucoromycetes

are decomposers, most are associated with plants. Many

mucoromycetes live as parasites or pathogens of plants, while

others live as mutualists (including some mycorrhizae).

The life cycle of Rhizopus stolonifer (black bread mold) is

fairly typical of mucoromycete species (Figure 31.15). Its

hyphae spread out over the food surface, penetrate it, and

absorb nutrients. The hyphae are coenocytic, with septa

found only where reproductive cells are formed. In the

asexual phase, bulbous black sporangia develop at the tips

of upright hyphae. Within each sporangium, hundreds of

genetically identical haploid spores develop and are dispersed

through the air. Spores that happen to land on moist food

germinate, growing into new mycelia.

If environmental conditions deteriorate—for instance,

if the mold consumes all its food—Rhizopus may reproduce

sexually. The parents in a sexual union are mycelia of different

mating types, which possess different chemical markers but

may appear identical. Plasmogamy produces a sturdy structure

called a zygosporangium (plural, zygosporangia), in which

karyogamy and then meiosis occur. Note that while a zygosporangium

represents the zygote (2n) stage in the life cycle, it is

not a zygote in the usual sense (that is, a cell with one diploid

nucleus). Rather, a zygosporangium is a multinucleate structure,

first heterokaryotic with many haploid nuclei from the

two parents, then with many diploid nuclei after karyogamy.

Zygosporangia are resistant to freezing and drying and are

metabolically inactive. When conditions improve, the nuclei

of the zygosporangium undergo meiosis, the zygosporangium

germinates into a sporangium, and the sporangium releases

genetically diverse haploid spores that may colonize a new

substrate. Some mucoromycetes can actually “aim” and then

shoot their sporangia toward bright light. Figure 31.16 shows

one example, Pilobolus, which decomposes animal dung.

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

ingest the fungi with the grass and then scatter the spores in

feces, thereby enabling the next generation of fungi to grow.

Finally, the phylum Mucoromycota also includes the

glomeromycetes, a clade of fungi that form arbuscular mycorrhizae (see Figure 31.4b and Figure 37.14). The tips of

the hyphae that push into plant root cells branch into tiny

treelike arbuscules. About 85% of all plant species have mutualistic

partnerships with arbuscular mycorrhizae.

Ascomycetes

Mycologists have described 90,000

species of ascomycetes, fungi in the

phylum Ascomycota, from a wide

variety of marine, freshwater, and terrestrial

habitats. The defining feature

of ascomycetes is the production of

spores (called ascospores) in saclike asci (singular, ascus); thus,

they are commonly called sac fungi. During their sexual stage,

most ascomycetes develop fruiting bodies, called ascocarps,

which range in size from microscopic to macroscopic

(Figure 31.17). The ascocarps contain the spore-forming asci.

Ascomycetes vary in size and complexity from unicellular

yeasts to elaborate cup fungi and morels (see Figure 31.17).

They include some of the most devastating plant pathogens,

which we will discuss later. However, many ascomycetes are

important decomposers, particularly of plant material. More

than 25% of all ascomycete species live with green algae or

cyanobacteria in beneficial symbiotic associations called

lichens. Some ascomycetes form mycorrhizae with plants

Many others live between mesophyll cells in leaves; some of

these species release toxic compounds that help protect the

plant from insects.

Although the life cycles of various ascomycete groups

differ in the details of their reproductive structures and processes,

we’ll illustrate some common elements using the

bread mold Neurospora crassa (Figure 31.18). Ascomycetes

reproduce asexually by producing enormous numbers of

asexual spores called conidia (singular, conidium). Unlike the

asexual spores of most mucoromycetes, in most ascomycetes,

conidia are not formed inside sporangia. Rather, they are

produced externally at the tips of specialized hyphae called conidiophores, often in clusters or long chains, from which

they may be dispersed by the wind.

Conidia may also be involved in sexual reproduction,

fusing with hyphae from a mycelium of a different mating

type, as occurs in Neurospora. Fusion of two different mating

types is followed by plasmogamy, resulting in the formation

of dikaryotic cells, each with two haploid nuclei representing

the two parents. The cells at the tips of these dikaryotic

hyphae develop into many asci. Within each ascus, karyogamy

combines the two parental genomes, and then meiosis

forms four genetically different nuclei. This is usually followed

by a mitotic division, forming eight ascospores. The ascospores develop in and are eventually discharged from

the ascocarp.

Compared to the life cycle of mucoromycetes, the

extended dikaryotic stage of ascomycetes (and also basidiomycetes)

provides additional opportunities for genetic recombination.

In Neurospora, for example, many dikaryotic cells

can develop into asci. The haploid nuclei in these asci fuse,

and their genomes then recombine during meiosis, resulting

in a multitude of genetically different offspring from one

mating event (see steps 3–5 in Figure 31.18).

As described in Figure 17.2, biologists in the 1930s used

Neurospora in research that led to the one gene–one enzyme

hypothesis. Today, this ascomycete continues to serve

as a model research organism. In 2003, its entire genome

was published. This tiny fungus has about three-fourths

as many genes as the fruit fly Drosophila and about half as

many as a human (Table 31.1). The Neurospora genome

is relatively compact, having few of the stretches of noncoding

DNA that occupy so much space in the genomes

of humans and many other eukaryotes. In fact, there is

evidence that Neurospora has a genomic defense system

that prevents noncoding DNA such as transposons from

accumulating.

Basidiomycetes

About 50,000 species, including

mushrooms, puffballs, and shelf

fungi, are called basidiomycetes

and are classified in the phylum

Basidiomycota (Figure 31.19). This

phylum also includes mutualists

that form mycorrhizae and two groups of destructive plant

parasites: rusts and smuts. The name of the phylum derives

from the basidium (plural, basidia; Latin for “little pedestals”),

a cell in which karyogamy occurs, followed immediately

by meiosis. The club-like shape of the basidium also

gives rise to the common name club fungus. Basidiomycetes are important decomposers of wood and

other plant material. Of all the fungi, certain basidiomycetes

are the best at decomposing the complex polymer lignin, an

abundant component of wood. Many shelf fungi break down

the wood of weak or damaged trees and continue to decompose

the wood after the tree dies.

The life cycle of a basidiomycete usually includes a longlived

dikaryotic mycelium. As in ascomycetes, this extended

dikaryotic stage provides many opportunities for genetic

recombination events, in effect multiplying the result of a

single mating. Periodically, in response to environmental

stimuli, the mycelium reproduces sexually by producing elaborate fruiting bodies called basidiocarps (Figure 31.20).

The common white mushrooms in the supermarket are familiar

examples of a basidiocarp.

By concentrating growth in the hyphae of mushrooms,

a basidiomycete mycelium can erect its fruiting structures

in just a few hours; a mushroom pops up as it absorbs water

and as cytoplasm streams in from the dikaryotic mycelium.

By this process, in some species a ring of mushrooms, popularly

called a “fairy ring,” may appear literally overnight

(Figure 31.21). The mycelium below the fairy ring expands

outward at a rate of about 30 cm per year, decomposing

organic matter in the soil as it grows. Some giant fairy rings

are produced by mycelia that are centuries old.

After a mushroom forms, its cap supports and protects a

large surface area of dikaryotic basidia on gills. During karyogamy,

the two nuclei in each basidium fuse, producing a

diploid nucleus (see Figure 31.20). This nucleus then undergoes

meiosis, yielding four haploid nuclei, each of which

ultimately develops into a basidiospore. Large numbers of

basidiospores are produced: The gills of a common white

mushroom have a surface area of about 200 cm2 and may

drop a billion basidiospores, which blow away.

Fungi play key roles in nutrient

cycling, ecological interactions,

and human welfare

In our survey of fungal classification, we’ve touched on some

of the ways fungi influence other organisms. We will now

look more closely at these impacts, focusing on how fungi act

as decomposers, mutualists, and pathogens.

Fungi as Decomposers

Fungi are well adapted as decomposers of organic material,

including the cellulose and lignin of plant cell walls. In fact,

almost any carbon-containing substrate—even jet fuel and

house paint—can be consumed by at least some fungi. The

same is true of bacteria. As a result, fungi and bacteria are primarily

responsible for keeping ecosystems stocked with the

inorganic nutrients essential for plant growth. Without these

decomposers, carbon, nitrogen, and other elements would

remain tied up in organic matter. If that were to happen,

plants and the animals that eat them could not exist because

elements taken from the soil would not be returned. Without

decomposers, life as we know it would cease.

Fungi as Mutualists

Fungi may form mutualistic relationships with plants,

algae, cyanobacteria, and animals. Mutualistic fungi absorb nutrients from a host organism, but they reciprocate with

actions that benefit the host—as we already saw for the key

mycorrhizal associations that fungi form with most vascular

plants. We turn now to other examples of mutualistic fungi.

Fungus-Plant Mutualisms

All plant species studied to date appear to harbor symbiotic

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 herbivores

or by increasing host plant tolerance of heat, drought,

or heavy metals. As described in Figure 31.22, researchers 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 endophytes

of woody flowering plants can play an important role

in defending against pathogens.

Fungus-Animal Mutualisms

As mentioned earlier, some fungi share their digestive services

with animals, helping break down plant material in

the guts of cattle and other grazing mammals. Many species

of ants take advantage of the digestive power of fungi

by raising them in “farms.” Leaf-cutter ants, for example,

scour tropical forests in search of leaves, which they cannot

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

proteins and carbohydrates. The ants feed primarily on

these nutrient-rich tips. Not only do the fungi break down

plant leaves into substances the insects can digest, but

they also detoxify plant defensive compounds that would

otherwise kill or harm the ants. In some tropical forests,

the fungi have helped these insects become the major

consumers of leaves.

The evolution of such farmer ants and that of their fungal

“crops” have been tightly linked for over 50 million years.

The fungi have become so dependent on their caretakers that

in many cases they can no longer survive without the ants,

and vice versa.

Lichens

A lichen is a symbiotic association between a photosynthetic

microorganism and a fungus in which millions of photosynthetic

cells are held in a mass of fungal hyphae. Lichens grow

on the surfaces of rocks, rotting logs, trees, and roofs in various

forms (Figure 31.24). The photosynthetic partners are

unicellular or filamentous green algae or cyanobacteria. The

fungal component is most often an ascomycete, but some glomeromycete and basidiomycete lichens are known. Recent

studies have found that many lichens also have a basidiomycete

yeast as a second fungal component. As the role of these

yeasts remains unknown, our discussion will focus on the

primary fungal partner.

The fungus usually gives a lichen its overall shape and

structure, and tissues formed by hyphae account for most

of the lichen’s mass. The cells of the alga or cyanobacterium

generally occupy an inner layer below the lichen surface

(Figure 31.25). The merger of fungus and alga or cyanobacterium

is so complete that lichens are given scientific

names as though they were single organisms. As might be

expected of such “dual organisms,” asexual reproduction

as a symbiotic unit is common. This can occur either by

fragmentation of the parental lichen or by the formation

of soredia (singular, soredium), small clusters of hyphae

with embedded algae (see Figure 31.25). The fungi of many

lichens also reproduce sexually.

In most lichens, each partner provides something the

other could not obtain on its own. The alga or cyanobacterium

provides carbon compounds; a cyanobacterium

also fixes nitrogen (see Concept 27.3) and provides organic nitrogen compounds. The fungus provides its photosynthetic

partner with a suitable environment for growth. The physical

arrangement of hyphae allows for gas exchange, protects

the photosynthetic partner, and retains water and minerals,

most of which are absorbed from airborne dust or from rain.

The fungus also secretes acids, which aid in the uptake of

minerals.

Lichens are important pioneers on cleared rock and

soil surfaces, such as volcanic flows and burned forests.

They break down the surface by physically penetrating

and chemically attacking it, and they trap windblown soil.

Nitrogen-fixing lichens also add organic nitrogen to some

ecosystems. These processes make it possible for a succession

of plants to grow. Fossils show that lichens were on

land 420 million years ago. These early lichens may have

modified rocks and soil much as they do today, helping

pave the way for plants.

Fungi as Parasites

Like mutualistic fungi, parasitic fungi absorb nutrients

from the cells of living hosts, but they provide no benefits

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

Cryphonectria parasitica, the ascomycete fungus that causes

chestnut blight, which dramatically changed the landscape

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

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 fruit harvest is lost annually due

to fungi, and grain crops also suffer

major losses each year.

Some fungi that attack food

crops produce compounds that

are toxic to humans. One example

is the ascomycete Claviceps

purpurea, which grows on rye

plants, forming purple structures

called ergots (see Figure 31.26c).

If infected rye is milled into

flour, toxins from the ergots can

cause ergotism, characterized by

gangrene, nervous spasms, burning

sensations, hallucinations,

and temporary insanity. An epidemic

of ergotism around 944 ce

killed up to 40,000 people in

France. One compound that has

been isolated from ergots is lysergic acid, the raw material

from which the hallucinogen LSD is made.

Although animals are less susceptible to parasitic fungi

than are plants, about 1,000 fungi are known to parasitize

animals. Two such parasites, the chytrids Batrachochytrium

dendrobatidis (discovered in 1998) and B. salamandrivorans

(discovered in 2013; this species primarily attacks salamanders),

have been implicated in the recent decline or

extinction of 500 species of frogs and other amphibians.

These chytrids can cause severe skin infections, leading to massive die-offs (Figure 31.27). Field observations and

studies of museum specimens show that B. dendrobatidis

and B. salamandrivorans first appeared in amphibian populations

shortly before their declines in Australia, Costa

Rica, Germany, the United States, and other countries.

Genetic analyses indicate that both B. dendrobatidis and

B. salamandrivorans originated in Asia and spread from there

via the commercial trade of frogs and salamanders.

The general term for an infection in an animal by a fungal

parasite is mycosis. In humans, skin mycoses include

the disease ringworm, so named because it appears as

circular red areas on the skin. Most commonly, the ascomycetes

that cause ringworm grow on the feet, causing

the intense itching and blisters known as athlete’s foot.

Though highly contagious, athlete’s foot and other ringworm

infections can be treated with fungicidal lotions

and powders.

Systemic mycoses, by contrast, spread through the body

and usually cause very serious illnesses. They are typically

caused by inhaled spores. For example, coccidioidomycosis is

a systemic mycosis that produces tuberculosis-like symptoms in the lungs. Each year, hundreds of cases in North America

require treatment with antifungal drugs, without which the

disease could be fatal.

Some mycoses are opportunistic, occurring only when

a change in the body’s microorganisms, chemical environment,

or immune system allows fungi to grow unchecked.

Candida albicans, for example, is one of the normal inhabitants

of moist epithelia, such as the vaginal lining. Under

certain circumstances, C. albicans can grow too rapidly and

become pathogenic, leading to so-called “yeast infections.”

A related species, C. auris, has emerged as a global threat,

often in healthcare facilities. Resistant to multiple antifungal

drugs, this species can infect the bloodstream and cause

life-threatening infections.

Practical Uses of Fungi

The dangers posed by fungi should not overshadow their

immense benefits. We depend on their ecological services

as decomposers and recyclers of organic matter. In addition,

mushrooms are not the only fungi of interest for human

consumption. Fungi are used to ripen Roquefort and other

blue cheeses. Morels and truffles, the edible fruiting bodies

of various ascomycetes, are highly prized for their complex

flavors (see Figure 31.17). These fungi can sell for hundreds

to thousands of dollars a pound. Truffles release strong

odors that attract mammals and insects, which in nature

feed on them and disperse their spores. In some cases, the

odors mimic the pheromones (sex attractants) of certain

mammals. For example, the odors of several European

truffles mimic the pheromones released by male pigs, which

explains why truffle hunters sometimes use female pigs to

help find these delicacies.

Humans have used yeasts to produce alcoholic beverages

and bread for thousands of years. Under anaerobic conditions,

yeasts ferment sugars to alcohol and CO2, which

causes dough to rise. Only relatively recently have the yeasts

involved been separated into pure cultures for more controlled

use. The yeast Saccharomyces cerevisiae is the most

important of all cultured fungi (see Figure 31.7). It is available

as many strains of baker’s yeast and brewer’s yeast.

Many fungi have great medical value as well. For example,

a compound extracted from ergots is used to reduce

high blood pressure and to stop maternal bleeding after

childbirth. Some fungi produce antibiotics that are effective

in treating bacterial infections. In fact, the first antibiotic

discovered was penicillin, made by the ascomycete mold

Penicillium. Other examples of pharmaceuticals derived

from fungi include cholesterol-lowering drugs and cyclosporine,

a drug used to suppress the immune system after

organ transplants.

Fungi also figure prominently in basic research. For example,

the yeast Saccharomyces cerevisiae is used to study the molecular genetics of eukaryotes because its cells are easy to

culture and manipulate. Scientists are gaining insight into the

genes involved in Parkinson’s disease by examining the functions

of homologous genes in S. cerevisiae.

Genetically modified fungi also hold much promise. For

example, scientists have succeeded in engineering a strain of

S. cerevisiae that produces human glycoproteins, including

insulin-like growth factor. Such fungus-produced glycoproteins

have the potential to treat people with medical conditions

that prevent them from producing these compounds.

Meanwhile, other researchers are sequencing the genome of

Gliocladium roseum, an ascomycete that can grow on wood or

agricultural waste and that naturally produces hydrocarbons

similar to those in diesel fuel (Figure 31.28). They hope to decipher

the metabolic pathways by which G. roseum synthesizes

hydrocarbons, with the goal of harnessing those pathways to

produce biofuels without reducing land area for growing food

crops (as occurs when ethanol is produced from corn).

Having now completed our survey of the kingdom Fungi,

we will turn in the rest of this unit to the closely related kingdom

Animalia, to which we humans belong.