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.
•
•
•
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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:
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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. Each sugar is a glucose
derivative called
N-acetyl glucosamine
• 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.
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.
Genomics of Fungal Secretion
• 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.
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.
Review of Sexual Reproduction
•
•
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.
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 (zoomeans "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
Haustorium inside a plant cell
Plant
cell
wall
Fungal hypha
Plant cell
Haustorium
(b) Haustoria
Plant cell
plasma
membrane
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
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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
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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
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Cooked by the following method, cuitlacoche can be used for crepas, quesadillas, budin, or in
plain tacos.
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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
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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
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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.