Ch

Biology 2 AP Ch. 28 Lecture Notes

I. Structural and life history adaptation equip fungi for an absorptive mode of nutrition.

Fungi are eukaryotes, nearly all multicellular (although yeasts are unicellular). Their nutrition, structural organization, growth and reproduction distinguish them from organisms in the other four kingdoms.

A. Nutrition and Habitats

Fungi are herotrophs that acquire nutrients by absorption

· They secrete hydrolytic enzymes and acids to decompose complex molecules into simpler ones that can be absorbed.

· Are specialized into three main types:

1. Saprophytes, which absorb nutrients from dead organic material (decomposers).

2. Parasitic fungi, which absorb nutrients from body fluids of living hosts. Some are pathogenic.

3. Mutualistic fungi, which absorb nutrients from a host but reciprocate to benefit the host.

4. Predatory, which is oyster fungus, it eats round worms.

Fungi exist in diverse habitats and form symbioses with many organisms. For example fungi are found:

· commonly in terrestrial habitats.

· in aquatic habitats –both freshwater and marine.

· in symbiotic relationships with algae to form lichens.

B. Structure

The basic structural unit of a fungal vegetative body (mycelium) is the hypha. Except for yeast, fungal bodies are diffuse, intertwining mats of hyphae that are organized around and within their food source. These hyphae:

· Are composed of tubular walls containing chitin, a strong , flexible nitrogen containing polysaccharide similar to that found in arthropod exoskeletons.

· Provide enormous surface area for the absorptive mose of nutrition. Parasitic fungi have modified hyphae called haustoria – nutrient absorbing hyphae that penetrate host tissue, but remain outside the host cell membrane.

Fungal hyphae may be aseptate or septate.

· Hypea of aseptat fungi lack cross-walls and are coenocytic, formed from repeated nuclear division without cytokinesis.

· Hyphae of septate fungi are divided into cells by crosswalls called septa. Pores in the septa allow organelles to move from cell to cell.

True fungi have no flagella stages in their life cycle. This characteristic is partially why the Chytridiomycota and Oomycota have been moved to the Protista.

C. Growth and Reproduction

Mycelial growth is adapted to the absorptive node of nutrition.

· Mycelia grow in length, not girth, which maximizes the surface area for absorption.

· Mycelia grow rapidly, as much as a kilometer of hyphae each day. Fast growt can occur because cytoplasmic streaming carries molecules synthesized by the mycelium to the growing hyphal tips.

· Since fungi are nonmotile, they cannot search for food or mates. Instead, they grow in hyphal length to reach new food sources and territory.

Fungal chromosomes and nuclei are relatively small, and the nuclei divide differently from most other eukaryotes.

· During mitosis, the nuclear envelope remains intact from prophase to anaphase; the spindle is inside the nucleat envelope.

· After anaphase, the nuclear envelope pinches in two, and the spindle disappears.

Fungi reproduce by releasing spores that are:

· Usually unicellular, haploid, and of various shapes and sizes.

· Produce either sexually or asexually. In favorable conditions, fungi generally produce enormous numbers of spores asexually. For many fungi, sexual reproduction occurs only as a contingency for stressful environmental conditions.

· The agent of dispersal responsible for the wide geographic distribution of fungi. Carried by wind or water, spores germinate if they land in a moist place with an appropriate substratum.

Except for transient diploid stages in sexual life cycles, fungal hyphea and spores are haploid. Some mycelia may, however, be genetically heterogeneous resulting from fusion of hyphae with different nuclei.

· The different nuclei may stay in separate parts of the same mycelium.

· Alternatively , the different nuclei may mingle and even exchange genes in a process similar to crossing over.

The sexual cycle in fungi differs from other eukaryotic organisms in that syngamy occurs in two stages that are separated in time.

Syngamy is the sexual union of haploid cells from two individuals. In fungi, syngamy occurs in two stages:

1. Plasmogamy, the fusion of cytoplasm.

2. Karyogamy, the fusion of nuclei.

After plasmogamy, haploid nuclei from each parent pair up but do not fuse, forming a dikaryon.

· Nuclear pairs in dikaryons may exist and divide synchronously for months or years.

· They dikaryotic condition has some advantages of diploidy; one haploid genome may compensate for harmful mutations in the other nucleus.

· Eventually, the haploid nuclei fuse forming a diploid cell that immediately undergoes meiosis.

II. The three major division of fungi differ in details of reproduction

Fungi in the three divisions differ in the:

· structures involved in plasmogamy

· length of time spent as a dikaryon

· Location of karyogamy. (Note that the fungal division are named for the sexual structures in which karyogamy occurs.)

A. Division Zygomycota (Zygote Fungi)

Fungi in this division are characterized by the presence of dikaryotic zygosporangia, resistant structures formed during sexual reproduction.

· Zygomycetes are mostly terrestrial and live in soil or on decaying organic material.

· Some form mycorrhizae, mutualistic associations with plant roots.

1. Endomycorrhizae

2. Ectomycorrhizae

* Zygomycete hyphae are coenocytic; septa are found only in reproductive cells.

3 types of hyphae

1. rhizord – anchor

2. sporangiophones – hold spores

3. stolons – horizontal

· The mycelion consists of horizontal hyphae that sprea out and penetrate the food source.

· Under favorable environmental conditions, Rhizopus reproduce asexually:

è Sporangia develop at the tips of upright hyphae

è Mitosis produces hundreds of haploid spores that are dispersed through the air.

è If they land in a moist, favorable environment, spores germinate into new mycelia.

· In unfavorable conditions, Rhizopus begins its sexual cycle of reproduction:

è Mycelia of opposite mating types ( + and - ) form gametangia tht contain several haploid muclie walled off by the septum.

è Plasmogamy of the + and – gametangia occurs, and the haploid nuclei pair up forming a dikaryotic zygosporangium that is metabolically inactive and resistant to desiccation and freezing.

è When conditions become favorable, karyogamy occurs between paired nuclei; the resulting diploid nuclei immediately undergo meiosis producing genetically diverse haploid spores.

è The zygosporangium germinates a sporangium that releases the genetically recombined haploid spores.

è If they land in a moist, favorable environment, spores germinate into new mycelia.

Even though air currents are not a very precise way to disperse spores, Rhizopus releases so many that enough land in hospitable places. Some zygomycetes, however, can actually aim their spores.

· For example, Pilobolus, a fungus that decomposes animal dung, bends sporangium-bearing hyphae toward light, where grass is likely to be growing.

· The sporangium is hot out of the hypha, dispersing spores away from the dung and onto surrounding grass. If an herbicore eats the grass and comsumes the spores, the asexual life cycle is complete when the animal disperses the spores in its feces.

B. Division Ascomycota (Sac Fungi)

30,000 species- largest division of fungi

Ascomycetes include unicellular yeast and complex multicelluar cup fungi.

· Hyphae are septate.

· In asexual reproduction, the tips of specialized hyphae form conidia, which are chains of haploid, asexual spores that are usually wind dispersed.

· In sexual reproduction, haploid mycelia of opposite mating strains fuse. One acts of ‘female’ and produces and ascogonium which receives haploid nuclei from the antheridium of the ‘male.

The ascogonium grows hyphae with

dikaryotic cells. Syngamy is delayed.

In terminal cells of didaryotic hyphae –

syngamy occurs.

Meiosis forms 4 haploid nuclei which

undergo mitotic division to yield eight

haploid nuclei.

The nuclei form walls and become ascospores

within an ascus, the sac of sexually produced

spores. Multiple asci may form an ascocarp.

Ascacarp: Fruiting structures of many asci packed together.

· The ascospores of each ascus are lined up in a row in the order in which they formed from a single zygote, allowing geneticists to study genetic recombination.

· Unicellular yeasts appear dissimilar, but produce the equivalent of an ascus during sexual reproduction and bud during asexual reproduction in a manner similar to the formation of conidia. Thus, they are classified as ascomycetes.

· Includes important decomposers and both mutualistic and parasitic symbionts.

· Many lives symbiotically with algae as lichens.

C. Division Basidiomycota (Club Fungi)

The division Basidiomycota, or club fungi, includes mushrooms, shelf fungi, puffballs, and stinkhorms. Basidiomyceyes:

· Are named for a transient diploid stage called the basidium, a club- shaped spre producing structure.

· Are important decomposers of wood and other palnt material. Saprobic basidiomycetes can decompose the complex polymer lignin, an abundant component of wood.

· Include mycorrhiza-forming mutualisms and plants parasites. Many shelf fungi are tree parasites that function later as saprobes after the trees die.

· Include mushroom forming fungi, only few of which are strictly parasitic. About half are saprobic and the other half form mycorrhizae.

· Include the rusts and smuts, which are plant parasites.

Basidiomycete life cycles are characterized by a long-lived dikaryotic mycelium that reproduces sexually by producing fruiting bodies called basidiocarps. Refer to Campbell,Figure 28.7 for the life cycle of a mushroom-forming basidiomycete.

· Haploid basidiospres grow into short-lived haploid mycelia. Under certain environmental conditions, plasmogamy occurs between two haploid mycelia of opposite mating types.

· The resulting dikaryotic mycelion grows; depending upon the species, it may form mycorrhizea with tress. Certain environmental cues stimulate the mycelium to produce mushrooms. A ‘fairy ring’ is an expanding ring of living mycelium that produces mushrooms above it; it slowly increases in diameter, about 30 cm per year.

· The mushroom cap supports and protects a large surface area of gills; kayogamy in the terminal, dikaryotic cells lining the gills produces diploid basidia.

· Each basidium immediately undergoes meiosis producing four haploid basidiospores. When mature, these sexual spores drop from the cap and are dispersed by wind.

A sexual reproduction occurs less often than in ascomycetes, but also results in conidia formation.

III. Molds, yeasts, lichens, and mycorrhizea represent unique lifestyles that evolved independently in all fungal divisions.

A. Molds

Mold: A rapidly growing, asexually reproducing fungus.

· May be saprobes or parasites on a great variety of substrates.

· Molds only include asexual stages; they may be zygomycetes, ascomycetes, basidiomycetes or fungi with no known sexual stages.

Since molds are classified by their sexual stages (zygosporangium, ascogomiun or basidium), molds with no known sexual stage cannot be classified as zygomycetes, ascomycetes or basidiomycetes.

· Molds with no known sexual stages are classified as Deuteronmycota or imperfect fungi.

· Imperfect fungi reproduce asexually by producing conidia on specialized hypae called conidiphores. Their asexual formation of conidia suggests that many imperfect fungi are related to ascomycetes.

· Deuteromycetes are sources of antibiotics. For example, Penicillium produced penicillin.

· Othe commercial uses of imperfect fungi include flavoring for chesses, such as blue cheese, Brie, Camembert and Roquefort; fermenting food products such as soybeans; and providing pharmaceuticals such a cyclosporine.

· Some deutermycetes are predatory soil fungi that kill small animals such as soil menatodes.

B. Yeasts

Yeasts are unicellular fungi that inhabit liquid or moist habitats; some can alternate between mycelium or yeast, depending on the amount of liquid in the environment.

Yeast reproduce;

· Asexually by simple cell division or by budding off from a parent cell. Some are classified as Deuteromycota, if no sexual stages are known.

· Sexually by forming asci (ascomycota) or basidia (basidiomycota).

Though humans have used yeast to raise bread and ferment alcoholic beverages for thousands of years, only recently have they been separated into pure culture for more controlled human use.

· Saccharmyces cerevisiae is the most important of all domesticated fungi. Highly active metabolically, this ascomycete is available as bakers and brewers yeast.

· In an aerobic environment, bakers yeast respires, releasing small bubbles of carbon dioxide that leaven dough; cultured anaerobically, Saccharomyces ferments sugar to alcohol.

· Researchers use Saccharomyces to study eukaryotic molecular genetics because it is easy to culture and manipulate.

Some yeast couse problems for humans.

· Rhodotorula, a pink yeast, grow on showers curtains

and other moist surfaces.

· Candida, a normal inhabitant of moist human tissue, can become pathogenic when there is a change in pH or other environmental factors; or when someones’ immune system is sompromised.

C. Lichens

Lichen: Highly integrated symbiotic association of algal cells (usually filamentous green algae or blue-green algae) with fungal hyphae (usually ascomycetes.)

Though lichens vary in shape and physiology, some shared general fearutes characterize the symbiotic relationship.

The alga, which is below the lichen’s surface.

· Always provides the fungus with food.

· May fix nitrogen (i.e. Cyanobacteria).

The fungus provides a suitable environment for algal growth:

· The hyphal mass:

à Absorbs needed minerals from airborne dust or rain.

à Retains water and minerals.

à Allows gas exchange.

à Protects the algae.

· The fungus produces unique organic compounds with several functions.

à Fungal pigment shade the algae from intense sunlight.

à Toxic fungal compounds prevent lichens from being eaten by consumers.

à Fungal acid secretion aids the uptake of minerals.

Most of the lichens mass is hyphal tissue which gives the lichen its shape and strucute. Named for theur fungal component, lichens are informally catergorized as:

· Foliose (leafy)

· Fruticose (shrubby)

· Crustose (crusty)

Lichen reproduction occurs as a combined unit or as independent reproduction of the symbionts.

· Many lichen fungi reproduce sexually by forming ascocarps or rarely, basidiocarps.

· Lichen algae reproduce independently by asexual cell division.

· Symbiotic units commonly reproduce asexually by:

à Fragmentation of the parental lichen.

à Formation of soredia – specialized reproductive structures that are small clusters of hyphae with embedded algae.

Though most evidence points to a mutualistic symbiosis, some debate that the relationship may actually by parasitic.

· The argument for mutualism is that fungi benefit the algae and that lichens can survive in habitats that are inhospitable to either organism alone.

· The argument for ‘controlled parasitism’ is based on the fact that the fungus actually kills some algal cells, though not as fast as the algae replenishes itself.

Lichens are important pioneer, breaking down rock and allowing for colonization by other plants.

· Some can tolerate severe cold.

· Photosynthesis occurs when lichen water content is 65-90%.

Lichens are sensitive to air pollution due to their mode of mineral uptake.

D. Mycorrhizea

Mycorrhizae are specific, mutualistic associations of plant roots and fungi.

· The fungi increase the absorptive surface of rotts and exchange soil minerals.

· Mycorrhizae are seen in 90% of trees (fungus is usually a basidiomycete) and the majority of small vascular plants.

· Are necessary for optimal plant growth.

IV. Fungi have a tremendous ecological impact

A. Fungi as Decomposers

Fungi and bacteria are the principal decomposers that reduce complex polysaccharides and proteins to simple organic compounds that plant can assimilate as raw material for photosynthesis.

B. Fungi as Spoilers

Fungi decompose food, wood and even certain plastics.

C. Pathogenic Fungi

Many fungi are pathogenic (e.g. athletes foot, ring worm and yeast infections.)

Plants are particularly susceptible. For example, Dutch elm disease, caused by an ascomycete, dractically changed the landscape of mortheastern U.S.

Ergots: Purple structure on rye caused by an ascomycete.

· Causes gangrene, hallucinations and burning sensations (St. Anthony’s fire).

· Produves lysergic acid, from which LSD is made.

Toxins from fungi may be used in weak doses for medical purposes such as treating high blood pressure.

D. Edible Fungi

Fungi are comsumed as food by a variety of animals, including humans.

· In the U.S., mushroom (basidiomycete) comsumption is usually restricted to one species of Agaricus, which is culticated commercially on compost in the dark.

· In many other countries, however, people eat a variety of cultivated and wild mushrooms.

· Truffle prized by gourmets are underground ascocarps of mycelia that are mycorrhizal on tree roots. The fruiting bodies (ascorcarps) release strong odors that attract mammals and insects – consumers that excavate that truffles and disperse their spores.

· Since it is difficult for novices to distinguish between poisonous and edible mushrooms, only qualified experts at identification should collect wild mushrooms for eating.

V. Fungi and animals probably evolved from a common protistan ancestor.

The Chytridiomycota and Fungi may share a protistan ancestor.

· Chytrids were placed in the Kingdom Protista because they form flagellated zoospores and gametes – a protistan characteristic.

· However, Chytrids and fungi share many characteristics, such as:

à An absorptive mode of nutrition.

à Cell walls of chitin.

à Most form hyphae.

à Key enzymes and metabolic pathways that are not found in the other fungus – like protests (slime molds and water molds.)

à Similar sequences of proteins and nucleic acids.

· This evidence lends support for

à Combing the chytrids with fungi as a monophyletic group.

à The hypothesis that chytrids are the most primitive fungi, diverging earliest in fungal phyloengy.

à The hypothesis that fungi evolved from protests with flagella, a feature retained by the chytrids.

The is compelling evidence that animals and fungi diverged from a common protistan ancestor.

· Animal probably evolved also from flagellated protests.

· Proteins and rRNA comparisons indicate that fungi and animals are more closely related to each other than to plants. Molecular systematists believe the most likely protistan ancestor common to fungi and animals was a choanoflagellate.

Perhaps the fungi are a consequence of adaptive radiation when life began to colonize land.

· The oldest undisputed fossils are 450-500 million years old.

· All major groups of fungi evolved by the end of the Carboniferous period (approximately 300 million years ago.)

· Plants and fungi moved from water to land together. Fossils of the first vascular plants have mycorrhizae.

1) Structural and life history adaptations equip fungi for an absorption mode of nutrition.

Absorption à Small organic molecules are absorbed from the surrounding environment.

· The fungi are a eukaryotic, primarily multicellular group.

· Most have cell walls made of chitin.

· All fungi are heterotrophs, acquiring their nutrients by absorption.

· There are saprobic decomposers, parasitic fungi, and mutualistic forms.

Saprobic decomposer à absorbs nutrients from nonliving organic material (fallen logs, animal corpses, wastes of live organisms)

Parasitic fungi à Absorbs nutrients from the cells of living hosts. Some are pathogenic.

Mutualistic Fungi àAbsorb nutrients from another organism, but reciprocate with functions beneficial to their partners in some way.

· Fungi inhabit diverse environments : terrestrial, aquatic, inside the tissues of plants, in insects, and in some of the most inhospitable habitats on Earth.

· The fungal vegetative (nutritionally active) bodies consist of mycelia, netlike collections of branched hyphae adapted for absorption.

Hyphae (hypha)à Minute threads composed of tubular walls surrounding plasma membranes and cytoplasm.

Mycelium à The ‘feeding’ network of a fungus formed by interwoven hyphae.

Septa (septum) à crosswalls that divide the hyphaw into cells.

Chitin à strong but flexible nitrogen-containing polysaccharide similar to the chitin found in the external skeletons of insects and other arthropods.

Haustoria à Nutrients absorbing hyphal tips that penetrate the tissue of the host.

· It digests food outside by secreting powerful hydrolytic enzymes into the food.

· Parasitic fungi penetrate their hosts with specialized hyphae called haustoria.

· Although aseptate forms occur, most fungi have their hyphae partitioned into cells by septa , with pores allowing cell-to-cell continuity.

· Fungi repruce by dispering enormous numbers of spores.

· Fungal life cycles vary by division asexual and later reproduction is sexual.

· The sexual cycle involves cell fusion and nuclear fusion with an intervening dikaryotic stage. The diploid phase is hort-lived and rapidly undergoes meiosis to produce haploid spores.

Plasmogamy à the fusion of cytoplasm.

Karyogamy à The fusion of nuclei.

2) The three major divisions of fungi differ in details of reproduction.

· The zygote fungi of the Zygomycota division are fungi that live in soil or decaying organic matter. Asexual spores develop in aerial sporangia. The division is named for its sexually produced zygosporangia, which are dikaryotic structures capable of persisting through unfavorable conditions.

Zygote Fungi à Baby fungus

Mycorrhizae à Mutualistic associations with the roots of plants.

· The sac fungi of the division Ascomycota include plant parasites, fungal components of lichens, and saprobes. Asexual reproduction by conidia is common, Sexual reproduction involves the formation of spores in sacs, or asci, at the ends of dikaryotic hypae, usually in ascocarps.

Sac Fungi à Ascomucetes

Asci (ascus) à Saclike place where sexual spores are produced

Ascocarps à Macroscopic fruiting bodies.

Conidia à Naked dust spores.

3) The club fungi of the division Basidiomycota include mushrooms, shelf fungi, puff balls and rust.

Basidium à a transient diploid stage in the organism’s life cycle.

Club fungus à common name of Basidiomycota

Basidiocarp à elaborate fruiting bodies

· Mycelia of club fungi can last years as dikaryons. Sexual reproduction involves the formation of spores on club-shaped basidia at the end of hyphae in fruiting bodies, such as mushrooms.

Dikaryon à nuclei form each parent pair up but do not fuse, forming a pair of nuclei.

4) Molds, yeast, lichen, and mycorrhiae represent unique lifestyles that evolved independently in all fungal divisions.

· Molds are either asexual stages of fungi classified as zygonycetes, ascomycetes or basidiomycetes, or conidiaproducing frungi not known to reproduce sexually. Molds are important in the commercial production of antibiotics, such as penicillin.

Moldà rapidly growing asexually reproducing fungus.

Imperfect fungià reproduce asexually by producing conidia on specialized hyphae called conidiophores.

Yeastsà Unicellular fungi adapted to life in liquid such as plant saps. They may be classified as ascomydetes, basidiomycetes, or deutermycetes.

· Lichens are such highly intergrated symbiotic associations of algae and fungi that they are classified as single organisms. Lichens are rugged organisms that set the stage for plant growth by slowly breaking down bare rocks.

Lichenà symbiotic association of millions of photosynthetic microorganisms tangled in a mesh of fungal hyphae.

Sorediaà small clusters of hyphae with embedded algae.

· Mycorrhizae are mutualistic assouciations of fungi with the roots of vascular plants. The plant is more efficiently able to absorb minerals and water with the aid of the fungus, which is provided with its carbohydrate needed in return.

5) Fungi have tremendous ecological impact.

· Without fungi and bacteria as decomposter, biological communities would be deprived of the essential recycleing of chemical elements between the biological and non biological world.

· They are particularly important decomposers of wood.

· Fungi also decompose food and other useful objects.

· Some fungi cause disease, harming humans with a variety of ills. Plants are especially vulnerable to fungal infections.

· Many fungi are food for humans and other animals.

Kingdom Fungi

Division Zygomycota – (sporangium fungi)

Division Ascomycota – ( Cup fungi, yeasts)

Division Basidiomycota – (Club Fungi)

Division Deutermycota – ( Imperfect fungi)

Division Mycophycota – ( Lichens

Division Chytridiomycote – (Chytrids)

à Mold, Mildew, Rust

à Most are microscopic and heterotrophic

à Some pathogens

à Some are economically important

à Eukaryotic, primarily multicellular

à Most have cell walls made of chitin (like shells)

** 4 Types of Fungi:

1) Saprobic Decomposers

- absorbs nutrients form nonliving organic material (fallen logs, animal corpes)

2) Parasitic Fungi

.- absorb nutrients from the cells of living hosts

3) Mutualistic Fungi

- (commensalisms) absorb nutrients from another organism, as well as helping the other organism

4) Predatory

ex. Oyster fungus – eats roundworms

*** Fossils Found!!***

à 900 million years ago in Precambrian times – resemble

à 500 million years ago in Ordovician period – earliest

à 300 million years ago in Carboniferous period – modern

à Fungi reproduce by dispersing enormous amounts of spores.

à Early reproduction is usually asexual and later sexual is used.

** 3 Types of Fungal reproduction:

1) cytoplasmic fusion ( sexual )

2) spore production (asexual )

3) fragmentation ( asexual )

à Fungi have an absorptive mode of nutrition

Absorption à absorb particles of dead or licing decomposing junk.

1) Division Zygomycota

- sporangium fungi

zygote producing

- bread mold

- ex. Rhizopus

- terrestrial

> found in soil and dung

* 3 types of hyphae:

Rhizoids à anchor

Sporangiophores à stick up – hold spores

Stolons à grow across (horizontally)

2) Division Ascomycota

- 30,000 species – largest division of fungi

- sac fungi

- morels

- powdery mildew

-yeasts

-cup fungi

- most recent to evolve

- variety of habitats

- cam be plant parasites/sexual

ascià sac-like spore holder

ascocarpà fruit-like body – holds seed after it is made

3) Division Basidiomycota

- 25,000 species

- club fungi

- mushroom

- puffballs

- toadstools

-rust/smuts

-Basidiocarp – Fruiting part of the ‘shroom

-dikaryon à The parent nuclei don’t fuse, but instead form a pair of nuclei

4) Division Deuteromycota

- Fungi imperfecti

- asexual on hyphae called conidiophores

- ringworm, athletes foot

5) Division Mycophycota

-Lichen , millions of photsynthestic microorganisms tangle in the mesh of fungal hyphae.

- highly intergrated symbiotic associations of algae – classifies as single organisms.

- fungi and green algae

- controlled parasitism

- live on rocks