The Evolution and Diversity of Fungi 22. Chapter 22 The Evolution and Diversity of Fungi Key...

The Evolutionand Diversity of Fungi

22

Chapter 22 The Evolution and Diversity of Fungi

Key Concepts

• 22.1 Fungi Live by Absorptive Heterotrophy

• 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or Mutualistic

• 22.3 Major Groups of Fungi Differ in Their Life Cycles

• 22.4 Fungi Can Be Sensitive Indicators of Environmental Change

Chapter 22 Opening Question

Have antibiotics derived from fungi eliminated the danger of bacterial diseases in human populations?

Concept 22.1 Fungi Live by Absorptive Heterotrophy

Fungi live by absorptive heterotrophy:

Digestive enzymes are secreted to break down large food molecules in the environment.

The smaller molecules are absorbed into the cells.


Saprobes absorb nutrients from dead organic matter.

Parasites absorb nutrients from living hosts.

Mutualists live in intimate associations with other organisms that benefit both partners.


Modern fungi probably evolved from a unicellular protist with a flagellum.

Current evidence from gene sequencing suggests that fungi, choanoflagellates, and animals share a common ancestor.

Collectively called opisthokonts, if flagella are present, they are posterior.

Figure 22.1 Fungi in Evolutionary Context


Most fungi are multicellular, but single-celled species (yeasts) occur in most groups.

“Yeast” refers to a lifestyle that has evolved several times.

Yeasts are used in the laboratory as model organisms for eukaryotes.

Figure 22.2 Yeasts


Multicellular fungi:

Body is a mycelium, a mass of individual tubular filaments called hyphae.

The cell walls are strengthened by the polysaccharide chitin.

Figure 22.3 Mycelia Are Made Up of Hyphae (Part 1)


Septate species—hyphae are subdivided by incomplete crosswalls called septa. Organelles can move between compartments.

Some species are coenocytic—no septa, but many nuclei (from mitosis without cytokinesis).


Mycelia can grow very fast, and may cover a wide area to forage for nutrients.

Some species produce sexual spores in fruiting structures (e.g., mushrooms).

The mycelial mass is often far larger than the mushroom.

Rhizoids—modified hyphae; anchor some fungi to their substrate.


Mycelia have very large surface area-to-volume ratio—excellent for absorptive heterotrophy.

But they can dry out rapidly. Fungi are more common in moist areas.

Some fungi can live in hypertonic environments, and some can tolerate temperature extremes.

Concept 22.2 Fungi Can Be Saprobic, Parasitic, Predatory, or Mutualistic

Fungi are very important to ecosystem functioning.

They decompose dead organisms and wastes and recycle mineral nutrients.

Fungi are the main decomposers of cellulose and keratin.

Without fungi, the carbon cycle would fail. Most carbon would be buried instead of being returned to the atmosphere as CO2.


During the Carboniferous period, saprobic fungi declined dramatically.

Dead plants in the swamps built up into peat, which eventually formed coal deposits.

But fungi flourished through the extinctions that marked the end of the Permian.


When food becomes scarce, fungi produce spores that can remain dormant until conditions improve or be dispersed.

Spores are extremely tiny and easily spread by wind or water.

They can spread over great distances, and at least some will find conditions suitable for growth.

Figure 22.4 Spores Galore


Parasitic fungi:

Facultative parasites can grow on living organisms, or by themselves.

Obligate parasites can grow only on their specific living host.


Hyphae are well suited to absorbing nutrients from living plants.

Hyphae can enter through stomata, wounds, or by direct penetration of epidermal cell walls.

Some produce haustoria, branching projections that push through cell walls, invaginate into the cell membrane, and absorb nutrients.

Figure 22.5 Invading a Leaf (Part 1)

Figure 22.5 Invading a Leaf (Part 2)


Pathogens:

Fungi are especially lethal to people with comprised immune systems, such as AIDS patients.

Fungi also cause less threatening problems such as ringworm and athlete’s foot.


Amphibian species around the world have been attacked by a chytrid fungus.

Originating in South Africa, it may have spread worldwide with the African clawed frog, which was once used in human pregnancy tests.


Fungi are the most important plant pathogens, causing crop losses amounting to billions of dollars.

Example: black stem rust of wheat


Predatory fungi:

Some fungi trap microscopic protists or animals.

They secrete sticky substances and hyphae quickly grow into trapped prey.

Some soil fungi can form a ring that a nematode enters, then the cells of the ring swell and trap the nematode.

Figure 22.6 Fungus as Predator


Symbiotic relationships—the partners live in close, permanent contact with each other.

Mutualistic—the relationship benefits both partners.


Lichens: associations of a fungus with a cyanobacterium, a photosynthetic alga, or both.

“Species” are assigned the name of the fungal component. Most have never been found in nature without the photosynthetic partner.

Lichens can grow on exposed surfaces such as rocks and can live in harsh environments.

Figure 22.7 Lichen Body Forms (Part 1)

Figure 22.7 Lichen Body Forms (Part 2)


Fungal hyphae of the lichen absorb mineral nutrients and provide a moist environment for the photosynthetic cells.

The fungi receive fixed carbon.

If cultured, the algal cells grow more quickly on their own, but in the environments where lichens are found, the algae would not survive on their own.


Lichens can reproduce by fragmentation of the vegetative body;

Or by soredia—one or a few photosynthetic cells surrounded by hyphae—that disperse on air currents.

The fungal partner may undergo sexual reproduction; the spores disperse alone.

Figure 22.8 Lichen Anatomy


Lichens are often the first colonists on bare rock.

Lichens grow very slowly.

They acidify the environment slightly, which contributes to rock weathering.

When dry, they become highly insensitive to extremes of temperature.


Mycorrhizae—associations of fungi and plant roots.

Ectomycorrhizae—the fungus wraps around individual cells in the root but doesn’t penetrate the cells.

An extensive web of hyphae penetrates the soil around the root.

The hyphae expand surface area for absorption of water and minerals.


Arbuscular mycorrhizae penetrate root cell walls forming arbuscular (treelike) structures inside the cell wall but outside the plasma membrane.

The web of hyphae in the soil around the root increases the absorptive area.

Figure 22.9 Mycorrhizal Associations (Part 1)

Figure 22.9 Mycorrhizal Associations (Part 2)


The mycorrhizal fungus obtains organic compounds from the plant.

The plant’s ability to absorb water and mineral nutrients is enhanced.

The fungus may also provide some growth hormones and protect roots from pathogenic microorganisms.

Many plants grow poorly or not at all without their mycorrhizal partners.


Endophytic fungi live in aboveground parts of plants, but don’t harm the plant.

The fungi produce alkaloid compounds that are toxic to animals, which helps protect plant from herbivores.

Some may not benefit the plant or harm it.

Concept 22.3 Major Groups of Fungi Differ in Their Life Cycles

There are six major groups of fungi.

Chytrids and zygospore fungi may not be monophyletic.

Figure 22.10 A Phylogeny of the Fungi

Table 22.1 Classification of the Fungi


Forms of asexual reproduction:

• Haploid spores produced in sporangia

• Haploid spores (conidia) form at tips of hyphae

• Cell division or budding by yeasts

• Simple breakage of the mycelium


Sexual reproduction is rare or unknown in some groups, common in others.

There is no morphological distinction between female and male individuals, but mating types are genetically distinct.

Figure 22.11 A Fungal Life Cycle


Microsporidia:

• Unicellular; obligate intracellular parasites of animals

• No true mitochondria, but have mitosomes derived from mitochondria

• Infect insects, crustaceans, fishes, and mammals, including humans.

• A polar tube grows from the spore, and the contents of the spore are injected into the host cell.

Figure 22.12 Invasion of the Microsporidia Spores


Chytrids:

• Only fungi with flagella at any life stage

• Reproduce both sexually and asexually; some species have alternation of generations.

• Flagellated spores and flagellated gametes

• May be parasitic or saprobic

Figure 22.13 Sexual Life Cycles of Chytrids and Zygospore Fungi (Part 1)


The other four fungal groups are mostly terrestrial.

No motile gametes.

Cytoplasm of individuals of different mating types fuse (plasmogamy) before their nuclei fuse (karyogamy).

Liquid water is not required for fertilization.


Zygospore fungi:

• Sexual reproduction occurs when adjacent hyphae of different mating types release chemical signals (pheromones) and grow toward each other.

• Fusion results in a unicellular zygospore with diploid nuclei (a resting stage).

• Zygospore is the only diploid cell in the life cycle.


Zygospore nuclei undergo meiosis; a stalked sporangiophore sprouts, bearing haploid spores.

Hyphae are coenocytic.

Rhizopus stolonifer, black bread mold, produces many stalked sporangiophores.

Figure 22.13 Sexual Life Cycles of Chytrids and Zygospore Fungi (Part 2)

Figure 22.14 Zygospore Fungi Produce Sporangiophores


Arbuscular mycorrhizal fungi:

• Have symbiotic, mutualistic relationship with 80–90% of all plants.

• Hyphae are coenocytic

• Use glucose from plant partners as primary energy source

• Asexual reproduction only


The two remaining fungal clades have a life stage called a dikaryon.

Plasmogamy (fusion of cytoplasm) results in the dikaryon stage with 2 genetically different haploid nuclei within each cell (n + n).

Karyogamy (fusion of nuclei) occurs long after plasmogamy, in fruiting structures, to give rise to zygotes.


Zygote is the only true diploid stage, but genes in both nuclei of the dikaryon stage can be expressed.

Dikaryotic hyphae often have characteristics that are different from their n or 2n products.


Sac fungi (Ascomycota):

• Many are the fungal partners in lichens

• Hyphae with septa

• Produce haploid spores in sacs called asci

In some species, asci are in a fruiting structure (ascoma)

Figure 22.16 Sac Fungi (Part 1)

Figure 22.16 Sac Fungi (Part 2)


Some sac fungi are unicellular yeasts, including baker’s, or brewer’s, yeast (Saccharomyces cerevisiae).

They metabolize glucose into ethanol and CO2 by fermentation.

Reproduce by budding and sexual reproduction; but have lost the dikaryon stage.


Cup fungi:

Ascomata are cup-shaped

Many are edible; truffles are underground ascomata—the odor attracts pigs, which eat them and disperse the fungus.


Molds:

Many are parasites of flowering plants (e.g., Chestnut blight, Dutch elm disease, and powdery mildew).


Aspergillus species:

A. tamarii acts on soybeans in production of soy sauce.

A. oryzae is used in brewing the Japanese alcoholic beverage sake.

Species that grow on grains and nuts produce extremely carcinogenic aflatoxins.


Penicillium species:

Produce the antibiotic penicillin

P. camembertii and P. roquefortii are responsible for the characteristic flavors of Camembert and Roquefort cheeses.


Filamentous sac fungi reproduce asexually by conidia that form at the tips of specialized hyphae.

Conidia give molds their characteristic colors.

Figure 22.17 Conidia


Sexual reproduction includes formation of a dikaryon.

The dikaryotic mycelium typically forms the cup-shaped ascoma, which bears the asci.

Figure 22.15 Sexual Life Cycles among the Dikarya (Part 1)


Club fungi (Basidiomycota):

The fruiting structures (basidiomata) include puffballs and mushrooms.

Bracket fungi are saprobic and are important in the carbon cycle.

Some are plant pathogens, including rusts and smuts.

Others are fungal partners in ectomycorrhizae.

Figure 22.18 Club Fungus Basidiomata (Part 1)

Figure 22.18 Club Fungus Basidiomata (Part 2)


Hyphae have septa.

The basidium is a cell at the tip of a specialized hypha; site of nuclear fusion and meiosis.

The dikaryon stage may persist for years. Some club fungi live for decades or even centuries.

Figure 22.15 Sexual Life Cycles among the Dikarya (Part 2)

Concept 22.4 Fungi Can Be Sensitive Indicators of Environmental Change

Lichens are highly sensitive to air pollution—they can’t excrete toxic substances they absorb.

Lichens are not found in large cities or heavily industrialized areas.

They can be used to gauge air pollution around cities and to track pollutants and their effects.

Figure 22.19 More Lichens, Better Air (Part 1)

Figure 22.19 More Lichens, Better Air (Part 2)


Museum collections of fungi provide a record of air pollutants over decades or centuries.

They can provide information on pollutants that existed before people were taking direct measurements.

Also, effectiveness of cleanup efforts and regulatory programs for controlling air pollutants can be monitored.


When deforestation removes trees, the populations of mycorrhizal fungi decline quickly.

Reforestation projects must also restore the mycorrhizal community.

A planned succession of plant growth and soil improvement is often necessary before forest trees can be replanted.

Answer to Opening Question

Many antibiotics are losing their effectiveness as pathogenic bacteria evolve resistance.

Mutations that allow bacteria to survive antibiotics are favored by selection whenever an antibiotic is used.

To reduce rate of evolution of resistance, antibiotics should be used only for the treatment of appropriate bacterial diseases.

Figure 22.20 Penicillin Resistance

The Evolution and Diversity of Fungi 22. Chapter 22 The Evolution and Diversity of Fungi Key...

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