Unit 3. Fungi

Every man carries a parasite somewhere.

Japanese proverb

Exercise 1. What do you know about fungi?

1. What is a fungus? Why are fungi classified into a separate Kingdom?

2. Are fungi unicellular or multicellular organisms?

3. Are fungi stationary or moving organisms?

4. What is the difference between a fungus and a mushroom? Do all fungi have a fruiting body?

5. Are fungi hetero- or autotrophs?

6. What ecological functions are performed by fungi?

7. What cases of symbiotic relationship between fungi and other organisms do you know? Give examples of mutualism and parasitism.

8. What is the significance of fungi for humans?

Exercise 2. Explain the following terms in English:

chitin yeast mold hypha (hyphae) mycelium

extracellular digestion saprophyte mutualist parasite haustoria

asexual reproduction sexual reproduction spore fragmentation budding

Exercise 3. Can you believe that fungi are the largest and fastest organisms on Earth? Why? Why not?

Read the following two texts (Text A and Text B) to find out.

Text A. The Largest Organism on Earth Is a Fungus

The blue whale is big, but nowhere near as huge as a sprawling fungus in eastern Oregon

By Anne Casselman   

Next time you purchase white button mushrooms at the grocery store, just remember, they may be cute and bite-size but they have a relative out west that occupies some 2,384 acres (965 hectares) of soil in Oregon's Blue Mountains. Put another way, this humongous fungus would cover 1,665 football fields, or nearly four square miles (10 square kilometers) of turf. The discovery of this giant Armillaria ostoyae in 1998 heralded a new record holder for the title of the world's largest known organism, believed by most to be the 110-foot- (33.5-meter-) long, 200-ton blue whale. Based on its current growth rate, the fungus is estimated to be 2,400 years old but could be as ancient as 8,650 years, which would earn it a place among the oldest living organisms as well.

A team of forestry scientists discovered the giant after setting out to map the population of this pathogenic fungus in eastern Oregon. The team paired fungal samples in petri dishes to see if they fused, a sign that they were from the same genetic individual, and used DNA fingerprinting to determine where one individual fungus ended.

This one, A. ostoyae, causes Armillaria root disease, which kills swaths of conifers in many parts of the U.S. and Canada. The fungus primarily grows along tree roots via hyphae, fine filaments that mat together and excrete digestive enzymes. But Armillaria has the unique ability to extend rhizomorphs, flat shoestringlike structures, that bridge gaps between food sources and expand the fungus's sweeping perimeter ever more. A combination of good genes and a stable environment has allowed this particularly ginormous fungus to continue its creeping existence over the past millennia. "These are very strange organisms to our anthropocentric way of thinking," says biochemist Myron Smith of Carleton University in Ottawa, Ontario.

All fungi in the Armillaria genus are known as honey mushrooms, for the yellow-capped and sweet fruiting bodies they produce. Some varieties share this tendency for monstrosity but are more benign in nature. In fact the very first massive fungus discovered in 1992—a 37-acre (15-hectare) Armillaria bulbosa, which was later renamed Armillaria gallica—is annually celebrated at a "fungus fest" in the nearby town of Crystal Falls, Mich. Myron Smith was a PhD candidate in botany at the University of Toronto when he and colleagues discovered this exclusive fungus in the hardwood forests near Crystal Falls. "This was kind of a side project," Smith recalls. "We were looking at the boundaries of [fungal] individuals using genetic tests and the first year we didn't find the edge."

Next, the microbiologists developed a new way to tell an individual apart from a group of closely related siblings using a battery of molecular genetic techniques. The major test compared fungal genes for signs of inbreeding, where heterozygous strips of DNA become homozygous. That's when they realized they had struck it big. The individual Armillaria bulbosa they found weighed over 100 tons (90.7 metric tons) and was roughly 1,500 years old.

Ironically, the discovery of such huge fungi specimens rekindled the debate of what constitutes an individual organism. "It's one set of genetically identical cells that are in communication with one another that have a sort of common purpose or at least can coordinate themselves to do something," explains Tom Volk, a biology professor at the University of Wisconsin–La Crosse. Both the giant blue whale and the humongous fungus fit comfortably within this definition. So does the 6,615-ton (six-million-kilogram) colony of a male quaking aspen tree and his clones that covers 107 acres (43 hectares) of a Utah mountainside.

And, at second glance, even those button mushrooms aren't so tiny. A large mushroom farm can produce as much as one million pounds (454 metric tons) of them in a year. "The mushrooms that people grow in the mushroom houses - they're nearly genetically identical from one grower to another," Smith says. "So in a large mushroom-growing facility that would be a genetic individual—and it's massive!" In fact, humongous may be in the nature of things for a fungus. "We think that these things are not very rare," Volk says. "We think that they're in fact normal." (From Scientific American Online, October 4, 2007)

Text B. Seedy but Speedy: Fungus Spews Spores at 55 Mph

By Susannah F. Locke

In a finding that could help control harmful fungus, researchers have discovered a high-speed mechanism the germs use to project their spores into the air. Scientists from Miami University (M.U.) in Oxford, Ohio, and the College of Mount St. Joseph in Cincinnati report in the journal PLoS ONE that fungi may be one of the fastest land species, clocking speeds of up to 55 miles (88 kilometers) per hour and producing accelerations 180,000 times greater than gravity.

Fungi are the most common crop pathogens in the world. Most are fairly harmless to people, although like other allergens they sometimes exacerbate allergies and asthma. But certain varieties such as Stachybotrys chartarum, commonly referred to as black mold, that thrive in damp places like basements may also infect the lungs of people who have compromised immune systems or chronic bronchitis. Biologists once believed that mild air currents were enough to release fungi's spores, but are increasingly finding that molds employ elaborate methods to spew their seeds away from the nest. Using ultrahigh-speed video, the researchers calculated that some fungi use their own natural water pressure like squirt guns to eject their spores.

Lead study author Nicholas Money, a fungus biologist at M.U., studied fungi that grow on cow patties and other herbivore dung. These species play a critical role in the ecosystem by breaking down waste to recycle its nutrients into the soil. The fungi project their spores away from the resident dung because cows will not eat near feces. By shooting them up to eight feet (2.5 meters) away, a grazing animal will be more inclined to eat them, thereby spreading the fungal spawn via its own manure.

The research video camera shot 250,000 frames per second to capture fungi spurting their spores into the air, trailing glistening liquid behind them. The researchers used the video to clock the spores speeding along at 55 mph. The team also identified how several fungi build up water pressure to power a spore launch. First, the fungi accumulate sugars and other small molecules in their cells, which, in turn, brings in more water. Targeting the first step of this process could be a key to developing new fungicides. "By understanding the basic mechanism," he says, "you might find ways to remediate a mold-damaged home." (From Scientific American Online, September 17, 2008)

Exercise 4. Answer the following questions using the information from the texts:

1. What animal is generally considered the biggest organism on earth?

2. What extraordinary characteristics does the discovered Armillaria ostoyae fungus possess?

3. Why is Armillaria ostoyae characterized as a pathogenic fungus?

4. What conditions allowed the fungus to grow to such enormous size?

5. How can scientists distinguish an individual from a group of closely related siblings?

6. What effect do fungi produce on people and the environment?

7. What mechanisms does Stachybotrys chartarum black mold use to spurt its spores into the air?

8. Why do fungi have to employ such complicated techniques?

Exercise 5. Earlier you discussed parasitic fungi. Now read the article which describes one example of multi-species relationship in detail.

Fungus genome boosts fight to save North American forests

DNA sequence could advance efforts to control pine beetle infestations.

By Elie Dolgin

Canadian researchers have decoded the DNA of the tree-killing fungus found in the mouths of mountain pine beetles, the destructive bugs that wipe out entire North American forests. Further genome sequencing of the beetle and pine tree species should help forest managers design better pest-control tactics, the authors say. "It's really getting to a systems-level understanding of the mountain pine beetle epidemic," says study co-author Jörg Bohlmann, a chemical ecologist at the University of British Columbia in Vancouver, Canada, who is leading the multi-species genome initiative. "What really happens in nature is not confined to one species, but is happening at the intersection when one species interacts with another."

Mountain pine beetles (Dendroctonus ponderosae) have eaten their way through vast swathes of western North American pine forests, including around 15 million hectares in British Columbia alone. As the burrowing beetles tunnel under the bark to feed and lay eggs, they release spores of the blue-stain fungus (Grosmannia clavigera), which stops the production of a protective toxic resin released by the tree and allows the beetles to continue to infest.

Bohlmann and his colleagues assembled the fungus's 32.5-million-base-pair genome, which is around a hundredth the size of the human genome, using a combination of next-generation and traditional sequencing technologies — the first time that a complex eukaryotic organism has been sequenced from scratch using such a hybrid approach. The genome was reported online this month in the journal Genome Biology.

For the other two species — the beetle and the tree — the researchers are concentrating mainly on expressed gene sequences, fragments of the complete DNA sequence, rather than the genomes in their entirety. They've already amassed one of the largest insect libraries of gene transcripts for the bark beetle from more than a dozen beetle life stages and body parts. The lodgepole pine (Pinus contorta) and jack pine (Pinus banksiana) are still at a much earlier stage of sequencing.

The goal, says Dezene Huber, a chemical ecologist at the University of Northern British Columbia in Prince George, Canada, is to predict the dynamics between the organisms under various climatic conditions. "We should be able to look at particular genes and say which population of trees is interacting with which population of fungus and which population of beetles," he says. "It's really going to push science in a big way," says Brian Aukema of the Canadian Forestry Service in Prince George, who plans to incorporate the genomic data into landscape ecological models. "Once you have that information, you can hopefully feed that into models and understand where these beetle populations might be the most susceptible to treatments, intervention strategies and mitigation."

Multi-species genomic interactions have been studied for some human diseases, including malaria, and a few symbiotic ecological relationships such as leaf-cutter ants and their microbial partners, but the approach has never before been applied on this scale for an outbreaking forest nuisance.

Already, the University of British Columbia researchers, led by mycologist Colette Breuil, have taken the fungus genome, pinpointed the gene responsible for staining the pine wood blue and created a knockout strain that does not produce any pigment. The blue staining reduces the commercial value of affected timber, but it is not clear what role the colouring plays in driving infestation. The researchers are now testing this strain to tease that apart.

But the full utility of the fungus genome might only be realized after other related species are also sequenced, says Diana Six, who studies the interaction between bark beetles and fungi at the University of Montana in Missoula. Comparing the blue-stain fungus with free-living or pathogenic fungi will shed light on how the beneficial fungus helps the beetles thrive, she says.

Using genomics to stop the bark beetles is a "bit of a long shot, for sure", admits Chris Keeling, a research associate in Bohlmann's lab. But it might offer the best strategy for containing the forest pests, which have already started to jump host species from lodgepole pine, which is found only west of the Rockies, to jack pine, which stretches east across the entire continent. "We might be able to tweak the system to reduce the beetle populations or prevent them from spreading further east," Keeling says. (From Nature online, 18 Sept. 2009)

Exercise 6. Speak in detail on the multi-species interaction described in this article according to the following plan:

1. What species are involved in the relationship? What is the role of each?

2. Why do scientists study this phenomenon on the genetic level?

3. What main steps have researchers taken so far? What results have they obtained?

4. What are the perspectives of such genomic investigations?

Exercise 7. Put the words given in the brackets in the correct word-form.

Gene Study Suggests Early Evolution of Land Plants and Fungi Changed Earth's Climate

By Harald Franzen   

Land plants and fungi may have arisen far earlier than previously thought, according to a study published today in the journal Science. The gene-based research suggests that the (1)_________ (to emerge) of these organisms may have led to major climate and animal (2)_________ (to evolve) events.

The Pennsylvania State University research team, led by (3)__________ (to evolve) biologist Blair Hedges, based their (4)___________ (to find) on the so-called molecular clock. Genes (5)___________ (to know) to accumulate (6)____________ (to mutate) at a constant rate, much like a ticking clock, can be used to determine when a species originated. To figure out when land plants and fungi originated, the team analyzed 119 such molecular clock genes common to (7)___________ (to live) species of animals, plants and fungi. Their (8)_________ (to result) proved startling. In contrast to fossil-based studies that place the (9)__________ (to appear) of land plants and fungi at around 480 million years ago, the genetic findings indicate that land plants and fungi evolved approximately 700 million and 1,300 million years ago, respectively.

The early (10)______________ (to present) of plants and fungi on land would have reduced the amount of carbon dioxide in the atmosphere, producing a (11)____________ (to cool) effect, the authors note. At the same time, the plants boosted atmospheric oxygen levels, thus paving the way for the (12)__________ (to evolve) of complex animals. (From Scientific American Online, August 10, 2001)

Exercise 8. Speak about fungi and their role on our planet. Summarize all the facts which have been discussed in this unit.