Fungi

But first, a digression:

Brief History of the Earth

The point is, we are not in an infinite, unchanging Universe. Instead, time is finite, and many changes have occurred since the beginning, and will continue to occur. Evolution is how living things deal with these changes.

The Big Bang• The Universe is thought to be 13.7 billion years old. • The Universe came into existence in an event called Big

Bang. • At that point in time, everything in the entire Universe was

contained in a single, very tiny, very hot and very dense object. This "singularity" exploded outward and continues to explode outward to this day. We are all part of that explosion, moving apart from everything else in the Universe.

– How this happened, and what came before, is a matter of complete speculation. Our history began then, and we have no scientific evidence of anything before then.

• Then, lots of interesting things happened…..• The Sun, Earth, and the rest of the Solar System

formed about 4.6 billion years ago. They condensed out of a cloud of gas, by the force of gravity.

Early Earth and the Origin of Life • The first part of Earth's existence, from 4.6 to

about 3.8 billion years ago, was the Hadean Era. Lots of large objects hit Earth and everything else: cosmic bombardment. This is the source of craters on the Moon. Earth's crust was probably molten much of this time.

• Life seems to have arisen on Earth quite early, maybe 3.5 billion years ago. This seems to have happened shortly after the surface became solid and a permanent atmosphere developed. – We have rocks that appear to have fossils in them

from over 3 billion years ago. These organism were prokaryotes.

– The era after the cosmic bombardment stopped and during which life appeared on Earth is the Archaean Era.

A few early fossils

• Stromatolite formed from mats of cyanobacteria, living on the Australian coast. Next to it a section of a 3.4 billion year old stromatolite

Microfossils from 3.4 billion year chert found in Greenland.

Oxygen in the Atmosphere

• The original atmosphere of Earth was mostly composed of nitrogen and carbon dioxide, like Mars today.

– Oxygen, which is very reactive, was not present.

• Roughly 2.5 billion years ago, cyanobacteria developed a form of photosynthesis that generates oxygen as a waste product. This adaptation was very successful: the cyanobacteria multiplied and spread everywhere, spewing their toxic waste (the oxygen).

– This event marks the beginning of the Proterozoic Era: 2.5 -0.6 billion years ago.

• As the concentration of oxygen in the atmosphere rose, many forms of life died out, killed by the poison gas.

• Some adapted by developing ways of tolerating oxygen, or hiding from it.• Others developed a way to use oxygen in their metabolism: you get 15 times as much

energy from glucose with oxygen as without it. This is called aerobic respiration.• This led to the first eukaryotes, which are able to be as large and active as they are

because of their aerobic metabolism. We could call the Proterozic Era the "Age of Protists".

Our Story So Far• We have come 5/6 of the way through the

history of the Earth• Earth forms 4.6 billion years ago

– Solid surface forms 4 billion years ago• Life starts (?) 3.8 billion years ago

– Archaean Era (Age of Prokaryotes)• Oxygen atmosphere develops 2 billion years ago.

– Eukaryotes develop.– Proterozoic Era (Age of Protists)

• We are now at the Cambrian explosion: many new forms of animal life appear in the fossil record. 550 million years ago.

• Next: Large plants and animals move on to the land in the Paleozoic, Mesozoic, and Cenozoic Eras.

The Cambrian Explosion• About 550 million years ago, there is a

sudden appearance of large numbers of multicellular organisms in the fossil record. Rocks older than this appear almost devoid of fossils (because they are too small to see, and because the multicellular organisms didn’t have hard parts to preserve.

• This sudden appearance of fossils is called the Cambrian explosion.

– This is the end of the Proterozoic Era and the beginning of the Paleozoic Era.

– It is also the end of the long period called "Pre-Cambrian"

• Most of the higher level taxa present today appeared at this time.

• Cause: a matter of speculation. Maybe triggered by a mass extinction? Maybe because of an arms race between predator and prey, with the development of hard parts: teeth, claws, shells?

Paleozoic Era• At the beginning of the Paleozoic Era,

most life was in the sea. Fishes, crustaceans, molluscs.

• The land was invaded during this time: tetrapod vertebrates developed from fish.– Plants and fungi also invaded the land,

developing characteristics that allowed them to survive out of the water.

• The Paleozoic era ended with the Permian Extinction, about 250 million years ago. This is the largest mass extinction event that has happened on Earth: up to 95% of all species died. The cause is unknown.

Mesozoic Era• The Mesozoic Era began with the Permian Extinction

250 million years ago, and ended at the Cretaceous Mass Extinction 65 million years ago.

• The Mesozoic was the age of the dinosaurs. Very few mammals around.– From the plant point of view, it was the age of cycads,

which are a kind of gymnosperm (like conifers).

• The Cretaceous Mass Extinction occurred when a large object (10 km diameter) collided with the Earth at the Yucatan Peninsula 65 million years ago. (It was undoubtedly a very bad day). – This triggered the extinction of the dinosaurs (except

their relatives the birds) and many other groups. Probably by generating so much dust in the atmosphere that photosynthesis shut down, causing a collapse in the food chain.

Cenozoic Era• The Cenozoic Era started at the Cretaceous

Extinction 65 million years ago. We are still in this era.

• Mammals and flowering plants dominate the land. The loss of dinosaurs opened up a lot of niches for mammals to fill.

– And insects too

• The hominid lineage split off from the chimpanzee lineage about 6 million years ago, in Africa.

• Modern humans arose in Africa about 100,000 years ago, and migrated out to populate the Old World (Europe, Asia, and Africa) maybe 60,000 years ago.

• The New World (North and South America) was populated 12,000 years ago, with maybe some people here 30,000 years ago (controversial).

Back to

Fungi

Fungus Classification• Linnaeus classified fungi as plants, part of his basic

division of the world into the Plant Kingdom and the Animal Kingdom. If is wasn’t obviously an animal, it must be a plant.

– Linnaeus invented the Genus species binomial naming system and the basic classification scheme we still use.

– He didn’t draw phylogenetic trees: the first one was done by Darwin.

• The “Five Kingdom” model of Whitaker (1969) put the fungi into its own separate kingdom.

• Modern DNA-based techniques show that the fungi are a monophyletic group that is clearly different from plants or animals.

– fungi are closer to animals than to plants. – However, oomycetes and slime molds are no longer

considered fungi: only distantly related.

What is a Fungus?

• Basic characteristics:– Eukaryotes– Heterotrophs that feed by absorption– Cell wall composed of chitin – Can be unicellular or multicellular. – Most don’t move under their own power– Can be decomposers, parasites, or symbiotes.– Lots of asexual reproduction, with some sexual as well.– Haploid or dikaryotic cells. Diploid only as a zygote.

• "-mycete" and "-mycota" both mean fungi. However, in former times, any organism that grew in the fungal form (hyphae) was considered a fungus. This includes the oomycetes (a stramenopile protist) and the actinomycetes (gram positive bacteria).

Fossil Fungi• Fungi don't fossilize very well: no hard parts.• Estimated divergence time from animals based on

DNA: 1.6 billion years. Proterozoic– Earliest fossils with fungal features: about 1.4 billion

years ago

• Fungus fossils become common starting in the Devonian epoch (in the Paleozoic Era, 400 million years ago.

• It is thought that fungi invaded the land during the Cambrian period, at the same time as animals, and well before land plants got past the bryophyte (moss) stage. Fungi may have been the dominant stationary organism on land in the early Paleozoic Era.

• There seems to have been a large spike in fungal spores just after the Permian extinction event. (It's controversial, and we don't know what caused the Permian Extinction.)

Rhynie chert, showing fungal hyphae invading plant cells. Tissues are very well preserved. A sudden rise in highly mineralized hot springs waters fossilized a plant community almost instantaneously.

How Many Species of Fungus? • Most fungi don't look very different from each other, or very much like

anything at all. Thus, our estimates of species number could be way low.• Most fungi live on land. The chytrids are the only major group that lives

primarily in the sea. – But there are a lot of microorganisms in the sea that we know nothing about.

• After all that hedging: let's say there about 100,000 known species, with estimates of 1.5 million species total.

Fungal Body Forms• Fungi only use a few basic body types. The different

phyla of fungi often contain members with different body plans.

• Multicellular fungi are usually in filaments called hyphae, and a mass of hyphae make up a mycellium. A mycellium is also called a mold.

• Some fungi are unicellular throughout their life cycle. These are called yeasts.– A single celled form with a flagellum is a zoospore.

• Some fungi form symbiotic relationships with roots. These are mycorrhizae.

• Some fungi form symbiotic relationships with photosynthetic green algae or with cyanobacteria. These are lichens.

The white halo around the roots is mycorrhizae.

Yeasts, Molds, Mushrooms• These are all “classifications of convenience”.

They are not monophyletic groups and don’t have much use as scientific terms.

• A yeast is a single-celled fungus. By far the most important yeast for humans in Saccharomyces cerevesiae, which is used to make most of our alcoholic beverages as well as to make bread rise. But, there are many other species that can be called yeasts.

• A mold is a fungus growing as a mass of hyphae (a mycellium), generally someplace we don’t want it.

• A mushroom is the fruiting body of a fungus that is mostly living underground. Mushrooms are produced by two of the major fungal groups, the Ascomycetes and the Basidomycetes.

Hyphae• Multicellular fungi are composed of filaments called hyphae

(sing. = hypha) that form a mat called a mycellium. Hyphae are just 1 or 2 cells thick, so very thin compared to a plant root (5-10 μm in diameter).– A mushroom is composed of hyphae whose growth has

been regulated to produce the characteristic shape. • The hyphae grow from the tip, branching frequently. They

can thoroughly explore a volume of space and find all available resources in it.

– http://www.youtube.com/watch?v=X77ae6xhK3s

• Cell nuclei in the hyphae are sometimes divided into separate cells, or they can be all together in one big mass of cytoplasm with no divisions between cells.– Septate fungi have separate cells. The dividers are

called septa, and they still allow a lot of material flow between them.

– Coenocytic (or aseptate) fungi have all the nuclei in an undivided mass.

Cell Walls• Fungal cells are surrounded by a cell wall

made of chitin.• Cell walls are very common in many

groups: fungi, prokaryotes, plants, many protists. They are used to resist osmotic pressure.

• The cell membrane is very flexible, like a balloon. Water diffuses into the cell, creating osmotic pressure. This can cause a cell to swell up and burst.

• The cell wall is a rigid box outside the membrane that allows the cell to resist hydrostatic pressure by preventing it from swelling too much.– The cell wall is porous, so nutrients and

other chemicals can be in contact with the membrane.

Hydrostatic Pressure and Fungal Growth

• Because of the cell wall, hydrostatic pressure can build up inside the hyphae. This allows them to exert force, which is how they penetrate living tissues.

• A special part of a hypha forms, called as appresorium. It has a thick cell wall (to resist pressure) around all but the side facing the host. Hydrostatic pressure builds up, and an outgrowth of the appresorium penetrates the host’s surface layer.

• The fungus can then grow inside and feed on the host.

From Bastmeyer et al., 2002. Ann Rev. Biophys. Biomol. Struct. 31: 321-341.

Chitin• Chitin is a polysaccharide: a large molecule

composed of many sugar subunits. It is similar to cellulose, and to the peptidoglycan cell wall of bacteria. – The sugars are rings containing 6 carbon atoms. – Chitin cells walls are a major defining

characteristic of the monophyletic Fungi kingdom. Water molds (oomycetes) and slime molds have cellulose cell walls and are considered protists instead.

• Chitin is also used for the exoskeleton of insects and other arthropods.

• Cell walls are rigid and not easily digested.

Peptidoglycan (without the connecting peptides). Two different sugars alternating

Cellulose. Each sugar is a glucose.

Chitin. Each sugar is a glucose derivative called N-acetyl glucosamine

Fungal Feeding• Fungi feed by secreting digestive enzymes into

the environment and then absorbing the resulting nutrients.– So, no internal digestive system.

• The main difference between living and dead food is whether the food is actively resisting your efforts.

• Fungi produce enzymes that digest compounds other groups can't digest: cellulose, lignin and keratin (hair), for example.

• Enzymes are very specific as to what type of molecule they work on (their substrate), so you need a lot of different enzymes to digest everything in your environment.– Enzyme names all end with “-ase”: protease

digests protein, lipase digest lipids, etc.

• Sequencing of the Aspergillus niger genome showed that it secretes over 200 different enzymes into the environment. – A. niger is used to produce industrial

enzymes– From Wikipedia: “In the heyday of the

opium trade, chandoo opium, which was meant to be smoked, was made by long term fermentation of A. niger and other molds on raw opium.”

• Types of enzyme: – carbohydrate (=polysaccharide); – pectin, cellulose, and lignin

(components of the plant cell wall); – Proteins (proteases)– Lipids (lipases)– other complex compounds.

Genomics of Fungal Secretion

From Braaksma et al. (2010) BMC Genomics 11:584.

Reproduction• Reproduction can be either sexual or asexual.• Asexual reproduction produces offspring that are genetically identical to

the parents. The offspring are produced by the cell division mechanism of mitosis. – Asexual reproduction can be as simple as breaking off a piece of the parent

organism and moving it to a new location. – More frequently, the parent forms spores, which are single cells packaged in a

tough coat. Spores are dispersed to new locations. They are able to survive harsh conditions, and then germinate (sprout) when they find favorable conditions.

• Sexual reproduction produces offspring that are genetically different from their parents and each other. Sexual reproduction uses the cell division of meiosis and the joining of cells from two parents (fertilization).– The basic idea is that by mixing genes from two different parents, better

combinations are formed. That is, some of the offspring are more fit, better adapted to their environment, than the parents. This is the process of natural selection.

• Almost all eukaryotes reproduce sexually some of the time.

• Cells can be either haploid (one copy of each chromosome) or diploid (two copies of each chromosome, one from each parent).

– Humans are diploid, except for the sperm and eggs (the gametes), which are haploid.

• Fertilization is when two haploid gametes fuse together to form a diploid zygote. The zygote is the first cell of a new individual.

– In humans, the zygote grows into an embryo and eventually into an adult diploid person.

• Meiosis is when a diploid cell divides into 4 haploid cells. These cells become sperm or eggs, the gametes.

• The sexual reproduction cycle: diploid cells undergo meiosis to become haploid, and haploid cells undergo fertilization to become diploid.

Review of Sexual Reproduction

Reproduction in Fungi• Fungi spend most of their life cycle as

haploids. Fertilization is followed immediately by meiosis, so the diploid stage is just one cell.– This is the opposite of animals like humans: we

are diploid most of our life cycle, and are only haploid for the 1 cell gamete stage.

– Most plants are similar to us; diploid except for a very short haploid phase.

• All fungi produce asexual spores: this is the main way fungi reproduce. There can be more than one type of asexual spore produced at different stages of the life cycle, especially in the rusts.

• Like all eukaryotes, sexual reproduction is common in the fungi, but not all do it. – Some species have never been seen to

reproduce sexually. They are called imperfect fungi.

Plasmogamy and Karyogamy• When they mate, one cell from each individual

merges together to make a single cell. This is called plasmogamy, and the result is a dikaryon: two cells in the same nucleus. That is, the cytoplasm of the cells fuse together, but the nuclei remain separate.– This mean the cells are not diploid, which means

having 1 nucleus with two copies of each chromosome.

– In human fertilization, the sperm and egg nuclei fuse almost immediately to form a diploid nucleus.

• The fungus can live and grow for a long time as a dikaryon. Eventually, something triggers the process of karyogamy: the nuceli fuse into a diploid cell. This cell then undergoes meiosis to produce offspring.

Nuclei in a dikaryon fuse to form a diploid nucleus, which then undergoes meiosis to form 4 haploid nuclei.

Mating in Fungi• Rather than having 2 distinct

sexes, fungi have different mating types within the species. There can be two mating types, or several, or hundreds. – When two individuals meet, they

can mate successfully only if they are of different mating types.

– Different mating types look identical. When hyphae from different individuals meet and touch, they can determine each other’s mating type by chemicals on their surface. Some even use pheromones to attract each other.

Imperfect Fungi• Some fungi have r been seen to reproduce sexually.

These have been classed as “imperfect fungi”, and even been put into their own phylum. However, the imperfect fungi are not monophyletic, and aren’t recognized as a distinct phylum. – They don’t have the reproductive structures used to

classify other fungi.

• Some examples: the athlete’s foot fungus and the Penicillium species that produce penicllin and also are used to make cheese

• DNA sequencing can be used to determine their close relatives among the sexual fungi. However, this hasn’t been done with many of them.

• In some cases, sexual reproduction hasn’t been seen because no one had looked hard enough. In several cases, two different imperfect fungi were found to be the same species in different stages.– Asperigillus and Emericella = same species

Secondary Metabolites• All living cells contain the same four basic groups of chemical

compounds: – 1. Nucleic acids (DNA and RNA). Genetic information– 2. Lipids. (fatty acids) Cell membranes and food storage– 3. Carbohydrates (sugars, polysaccharides). Structure (like cellulose and

chitin) and food storage (starch)– 4. Proteins. (made of amino acids). Act as enzymes to convert chemical

compounds and generate energy. Also structure, transport, and almost anything else that needs doing in the cell.

• Secondary metabolites are compounds other than the four basics that are not involved in the primary processes of growth and reproduction. They are made by specific groups of organisms, with lots of variation between species and larger groups. Many are for defense (poisonous) or attracting mates or pollinators (as in flowers). – Especially in the fungi and plants: they can't move out of the way of a

predator. – Source of antibiotics, food flavors, medicinal (and recreational) drugs, etc.

Fungal Diversity• The fungi are opisthokonts, and closely

related to animals. The nucleariids are a sister group to the fungi.

• DNA sequencing has rearranged parts of fungal phylogeny, mostly by putting the oomycetes and the slime molds into the protists and not the fungi.

• We are going to talk about 5 groups of fungi:– chytrids, which are a basal group that

has a spore with a flagellum (a zoospore)– zygomycetes, a fairly primitive group– glomerulomycetes, which form intimate

interactions with plant roots.– Ascomycetes, sac fungi. A very large,

advanced group– Basidomycetes, club fungi including most

of the mushrooms

Chytrids• Pronounced “kit- trid”• The earliest branch of the fungi.• Mostly live in water, especially fresh water. They are

attracted to dead organisms in the water. They can degrade keratin (from vertebrate hair and scales) and chitin (insect exoskeleton).

• They produce zoospores, which have flagella (zoo- means "animal").

– Most fungi do not have flagella: this is a primitive characteristic that has been lost in most groups.

• One common form of chytrid life: a spherical cell attached to the host by thread-like projections.

– Others form hyphae.– All are very small

• Most reproduction in asexual, but sexual reproduction also occurs. Both male and female gametes are flagellated zoospores, and fertilization produces a diploid zoospore as well.

Chytridomycosis

• One species of chytrid (Batrachochytrium dendrobatidis) has produced a worldwide decline in amphibians. It attacks the keratin in their skin. Amphibians breathe and drink through their skin. This alters the balance of sodium and potassium ions in their body, which causes heart failure.– One bad side effect of bulimia in

humans (I.e. vomiting to get rid of food) is that it decreases your internal potassium ion level, which can cause a heart attack.

Zygomycetes• The zygospore is the defining characeristic of this phylum. Zygospores are the result of sexual

reproduction: – Hyphae of two different mating types emit pheromones (sexual attractant chemicals) and grow

towards each other. – When they meet, a large cell with many nuclei from each strain is formed by walling off a section

of the two hyphae growing together.– The nuclei fuse in pairs (one of each mating type) to form diploid nuclei. – The zygospore grows from this cell. It has a thick, pigmented cell wall and can become dormant

for many months. The pigment is melanin, and it helps resist UV light. Animals also use melanin pigment.

– When conditions are right for germination, each diploid nucleus undergoes meiosis and to form new haploid individuals, genetically different from both parents.

– The zygospore grows a sporangium: a fruiting body on a stalk. The haploid nuclei get packaged into individual spores, which get dispersed to new locations by air currents.

Zygomycete Sexual Reproduction • Zygomycetes are coenocytic: no divisions between

cells, just a mass of cytoplasm with many nuclei in it.• The hyphae are haploid. • When two cells of different mating type meet, they

fuse into a single cell: this is plasmogamy. This cell contains many nuclei from both parents.

• Then, the nuclei pair up and fuse together: this is karyogamy, which is also the moment of fertilization, considering the general eukaryotic sexual life cycle.

• As the zygospore germinates, meiosis occurs, converting each diploid nucleus into 4 haploid nuclei. Note that fertilization and meiosis both happened in the same cell, with no intervening diploid phase.

• The spores resulting from sexual reproduction are all genetically different.

More Zygomycetes

• Zygomycetes also reproduce by asexual reproduction. Some hyphae grow upward from the mycelium and produce globe-shaped sporangia, which contain many spores. These disperse, and start growing new hyphae when they find good conditions. – Note: all spores resulting from asexual

reproduction are genetically identical to the parent.

• Black bread mold is a common example: the stalks with little black spheres at the ends are asexual sporangia.

• The zygospore lineage split from other fungi after chytrids but before ascomycetes or basidomycetes.

Glomeromycetes• A small group, recently identified as different

from zygomycetes. They are obligate symbiotes: they can't live without the plant.

• They form arbuscular mycorrhizae: intimate association with plant roots. A specialized part of the hypha called a haustorium forms a highly branched structure inside plant root cells. Lots of surface area to exchange nutrients.– The plant supplies sugars from photosynthesis,

and the fungus supplies mineral nutrients (especially phosphate) from the soil.

• Fungal hyphae are much thinner than roots, so they are much more efficient at extracting nutrients from soil.

2.5 µm

(b) Haustoria

Plantcellwall

Haustorium

Plant cellplasmamembrane

Plant cell

Fungal hypha

Haustorium inside a plant cell

Glomerulomycetes• Most plants (like 90%) form these

mycorrhizal associations, and studies show they clearly improve plant growth.– Some plants can't grow at all without

them: the vanilla orchid (source of the spice vanilla), for example.

• Glomeromycetes produce asexual spores; they are not known to reproduce sexually.

• Some other fungi form mychorrhizae as well, but these are on the root surface or between the root cells, not inside the cells. Called ectomychorrhizae.

- +Plant growth without (-) or with (+) mycorrhizal fungi.

Ascomycetes• Ascomycetes are one of the two largest and

most advanced fungal phyla.• Ascomycetes= "sac fungi". The sexual spores

develop inside a sack (called an ascus) that contains 4 or 8 spores produced by meiosis. The asci are usually borne on a fruiting body that can be quite large.

• Some Ascomycetes:– we eat: truffles, morels. – bad plant diseases: ergot, Dutch elm disease,

chestnut blight,– useful organisms: bread yeast, penicillin– things we don't like: Candida (thrush, vaginal

yeast infection), most food molds.

• Many ascomycetes live in symbiosis with photosynthetic organisms: lichens.

Sexual Reproduction in Ascomycetes • Ascomycetes are septate: the hyphae are divided

into separate cells. This is the opposite of coenocytic (which is what zygomycetes are).

• When cells of opposite mating types meet, they form dikaryon hyphae.

– Ascomycete hyphae can grow quite large, and for a long time, as dikaryons. That is, plasmogamy occurs long before karyogamy.

• The dikaryon forms a fruiting body (e.g. the truffle or morel) made of hyphae. At the tips of each hypha in the fruiting body, the ascus (sac) forms.

• In the ascus cell, the two haploid nuclei fuse to form a diploid, which then undergoes meiosis.

– The 4 haploid cells from meiosis often form a nice line; in some species each nucleus divides to form a total of 8 haploid cells (geneticists love this).

• The haploid nuclei are then packaged into sexual spores , which can resist harsh conditions, and expelled from the fruiting body.

Asexual Reproduction• Most ascomycete reproduction is asexual. The

cells involved in asexual reproduction are all generated by mitosis: no meiosis or fertilization occurs.

• The haploid hyphae make specialized tips that produce chains of pigmented asexual spores, which are called conidia. These spores disperse to new locations, and germinate into new hyphae when conditions are right.

• Note that the asexual spores are NOT in a sac, like the sexual spores are, and they are not enclosed in a fruiting body the way zygomycete asexual spores are.

Lichens• Symbiosis between a photosynthetic microorganism

and a fungus. Can be cyanobacteria or green algae. Usually ascomycetes, but a few other fungal types are known to be part of some lichens.

– This is an external symbiosis: the cells of the two organisms remain separate.

• Algae or cyanobacteria are held below the surface in a tangle of hyphae. They provide fixed carbon (sugar) and fixed nitrogen, while the fungi provide protection, minerals, water.

• The fungal part secretes acids that release minerals from the rocks and also help with attachment. Lichens are often the first organism to colonize newly formed rocks.

• Asexual reproduction is common: small clumps of hyphae with embedded algae are released. Sexual reproduction also occurs, with the production of a fruiting body containing asci.

Dutch Elm Disease• Elm trees used to be very popular in cities. They

tolerate the urban environment, and they grow in a way that provides good shade while not blocking the streets.

• In the 1920's, an ascomycete from Asia was accidentally imported: Ophiostoma ulmi.

• It is spread by native bark beetles, which burrow under the bark.

• The tree tries to block the spread of the fungus by producing a sticky gum in its xylem (tubes that conduct water up from the roots). This prevents water from reaching the top of the tree, which kills it.

• It was first identified in the Netherlands, hence the name, but it is not native there.

• Spread has been slow: from New England in the 1920's, it only reached Chicago in the 1960's.

• Lots of work breeding resistant trees.

Yeast• Probably the most useful fungus for humans is a single

celled ascomycete, Saccharomyces cerevisiae. This is the yeast used to make bread and alcoholic beverages: baker's yeast and brewer's yeast.– Different strains are used for different purposes. Each

brewery and vinyard has its own special strain.

• Making alcohol means growing the yeast anaerobically (no oxygen). The yeast converts sugar into alcohol under these conditions: this is fermentation.

• Getting bread to rise is done aerobically (oxygen present), so the yeast converts sugar into carbon dioxide, which is trapped in the bread by gluten proteins.– Bread rising: http://www.youtube.com/watch?

v=ajWr98ZcySM

More Yeast• S. cerevisiae is usually haploid, but after it mates

it can live as a stable diploid for many generations.– The mating types are called a and α (alpha). Each

secretes a pheromone that attracts the other.

• The diploid cell undergoes meiosis to produce 4 haploid cells.

• S. cerevisiae is a very important model organism in biological research. It is the simplest eukaryote that is widely studied. – About 23% of yeast genes are homologous with

human genes.– The genes involved in mitosis were first studied in

yeast.

• Lots of other yeasts. Candida albicans is usually what infects humans.

Basidomycetes• Most mushrooms are basidomycetes,

– including all mushrooms with gills.– Puffballs, shelf fungi

• Also some mycorrhizae (but not inside the plant root cells: that’s glomeromycetes only)

• Bad plant diseases: rusts and smuts

Basidomycete Reproduction • Unlike other fungi, most basidomycetes only reproduce sexually.• The characteristic feature of the basidomycetes: club-shaped reproductive

cells called basidia (which means “little pedestal”), on the outside of the organism (i.e. not in a sac).

• Hyphae of different mating types fuse to form a dikaryon. Just like in ascomycetes, the basidomycete dikaryon can grow for a long time. Plasmgamy and karyogamy are separate events.

• When conditions are right, the mycelium sends up a fruiting body (mushroom).

• The gills of the fruiting body are lined with basidia. The nuclei fuse here, then undergo meiosis and produce 4 new haploid offspring nuclei (they combine genes form both parents). The nuclei migrate into spores at the end of the basidium. When spore development is finished, they are expelled into the air, where they are dispersed by the wind to new locations. Each spore can develop into a new haploid mycelium. Huge numbers: trillions of spores in a puffball.

Mushrooms• Some are good to eat, some are

deadly poisons, some are hallucinogenic.

• Mostly it's hard to tell one from another: there is no easy way to recognize a poisonous one.

– So don't eat mushrooms you find in the woods.

• Corn smut is good to eat! Called cuitlacoche in Mexico.

Recipe for Smut

• Cooked by the following method, cuitlacoche can be used for crepas, quesadillas, budin, or in plain tacos.

• 3 tablespoons safflower oil• 2 tablespoons finely chopped white onion• 2 small garlic cloves, peeled and finely chopped rajas of 4 chiles polbanos• 1 ½ pounds (about 6 cups) cuitlacoche• sea salt to taste• 2 tablespoons roughly chopped epazote leaves

• Heat the oil in a frying pan. Add the onion and garlic and fry gently until translucent - about 3 minutes. Add the chile strips and fry for 1 minute more. Add the cuitlacoche and salt, cover the pan and cook over medium heat, shaking the pan from time to time for about 15 minutes. The fungus should be tender, retaining some moisture, but not soft and mushy. Stir in the epazote and cook, uncovered, for another 2 minutes.

More Mushrooms• Most of the basidiomycete lives

underground, as a mass of hyphae.• The mycelium expands outward slowly,

if conditions are good. This can lead to a "fairy ring": a ring of mushrooms that are being sent up by the growing edge of the mycelium.

• The largest known organism at this time is thought to be a honey mushroom (Armillaria ostoyae) in Oregon whose mycelium covers 4 square miles. Genetic tests show that the mycelium is identical everywhere, so it is not a collection of different individuals.

Rusts

• Rusts are a group of basidomycetes that are obligate parasites of plants. They need a host to reproduce.

• The life cycle is often very complex: rusts have up to 4 different asexual spore types, plus a sexual spore (i.e. the result of meiosis). Rusts often require two different plant hosts at different stages of their life cycle.

• One way of controlling rust is to remove the alternate host. For example, wheat stem rust spends part of its life cycle in barberry. Removing nearby barberry plants prevents the sexual cycle from completing.