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Leaf-Cutting Ants and Fungus
The ants and their agriculture have been extensively
studied over the years, but the recent research has uncovered intriguing new
findings about the fungus they cultivate, how they domesticated it and how they
cultivate it and preserve it from pathogens. For example, the fungus farms,
which the ants were thought to keep free of pathogens, turn out to be
vulnerable to a devastating mold, found nowhere else but in ants’ nests. To
keep the mold in check, the ants long ago made a discovery that would do credit
to any pharmaceutical laboratory.
Leaf-cutting ants and their fungus farms are a marvel of
nature and perhaps the best known example of symbiosis, the mutual dependence
of two species. The ants’ achievement is remarkable -he biologist Edward O.
Wilson has called it “one of the major breakthroughs in animal evolution”
-because it allows them to eat, courtesy of their mushroom’s digestive powers,
the otherwise poisoned harvest of tropical forests whose leaves are laden with
terpenoids, alkaloids and other chemicals designed to sicken browsers
Fungus growing seems to have originated only once in
evolution, because all gardening ants belong to a single tribe, the descendants
of the first fungus farmer. There are more than 200 known species of the attine
ant tribe, divided into 12 groups, or genera. The leafcutters use fresh
vegetation; the other groups, known as the lower attines because their nests
are smaller and their techniques more primitive, feed their gardens with
detritus like dead leaves, insects and feces. In 1994 a team of four
biologists, Ulrich G. Mueller and Ted R. Schultz from Cornell University and
Ignacio H. Chapela and Stephen A. Rehner from the United States Department of
Agriculture, nanlyzed the DNA of ant funguses. The common assumption that the
funguses are all derived from a single strain, they found, was only half true.
The leaf-cutters’ fungus was indeed descended from a single
strain, propagated clonally, or just by budding, for at least 23 million years.
But the lower attine ants used different varieties of the fungus, and in one
case a quite separate species, the four biologists discovered. Cameron R.
Currie, a Ph.D. student in the University of Toronto, it seemed to Mr. Currie,
resembled the monocultures of various human crops, that are very productive for
a while and then succumb to some disastrous pathogen, such as the Irish potato
blight. Monocultures, which lack the genetic diversity to respond to changing
environmental threats, are sitting ducks for parasites. Mr. Currie felt there had
to be a parasite in the antfungus system. But a century of ant research offered
no support for the idea. Textbooks describe how leaf-cutter ants scrupulously
weed their gardens of all foreign organisms. “People kept telling me, ‘You know
the ants keep their gardens free of parasites, don’t you?’ “ Mr. Currie said of
his efforts to find a hidden interloper.
But after three years of sifting
through attine ant gardens, Mr. Currie discovered they are far from free of
infections. In last month’s issue of the Proceedings of the National Academy of
Sciences, he and two colleagues, Dr. Mueller and David Mairoch, isolated
several alien organisms, particularly a family of parasitic molds called
Escovopsis.
Escovopsis turns out to be a highly virulent pathogen that
can devastate a fungus garden in a couple of days. It blooms like a white
cloud, with the garden dimly visible underneath. In a day or two the whole
garden is enveloped. “Other ants won’t go near it and the ants associated with
the garden just starve to death,” Dr. Rehner said. “They just seem to give up,
except for those that have rescued their larvae.”
Evidently the ants usually manage to keep Escovopsis and
other parasites under control.
But with any lapse in control, or if the ants are removed,
Escovopsis will quickly burst forth. Although new leaf-cutter gardens start off
free of Escovopsis, within two years some 60 percent become infected. The
discovery of Escovopsis’s role brings a new level of understanding to the
evolution of the attine ants. “In the last decade, evolutionary biologists have
been increasingly aware of the role of parasites as driving forces in
evolution,” Dr. Schultz said. There is now a possible reason to explain why the
lower attine species keep changing the variety of fungus in their mushroom
gardens, and occasionally domesticating new ones— to stay one step ahead of the
relentless Escovopsis.
Interestingly, Mr. Currie found that the leaf-cutters had
in general fewer alien molds in their gardens than the lower attines, yet they
had more Escovopsis infections. It seems that the price they pay for
cultivating a pure variety of fungus is a higher risk from Escovopsis. But the
leaf-cutters may have little alternative: they cultivate a special variety of
fungus which, unlike those grown by the lower attines, produces nutritious
swollen tips for the ants to eat.
Discovery of a third partner in the ant-fungus symbiosis
raises the question of how the attine ants, especially the leaf-cutters, keep
this dangerous interloper under control. Amazingly enough, Mr. Currie has again
provided the answer. “People have known for a hundred years that ants have a
whitish growth on the cuticle,” said Dr. Mueller, referring to the insects’
body surface. “People would say this is like a cuticular wax. But Cameron was
the first one in a hundred years to put these things under a microscope. He saw
it was not inert wax. It is alive.” Mr. Currie discovered a specialized patch
on the ants’ cuticle that harbors a particular kind of bacterium, one well
known to the pharmaceutical industry, because it is the source of half the
antibiotics used in medicine. From each of 22 species of attine ant studied,
Mr. Cameron and colleagues isolated a species of Streptomyces bacterium, they
reported in Nature in April. The Streptomyces does not have much effect on
ordinary laboratory funguses. But it is a potent poisoner of Escovopsis,
inhibiting its growth and suppressing spore formation. Because both the
leaf-cutters and the lower attines use Streptomyces, the bacterium may have
been part of their symbiosis for almost as long as theEscovopsis mold. If so,
some Alexander Fleming of an ant discovered antibiotics millions of years
before people did. Even now, the ants are accomplishing two feats beyond the
powers of human technology. The leaf-cutters are growing a monocultural crop
year after year without disaster, and they are using an antibiotic apparently
so wisely and prudently that, unlike people, they are not provoking antibiotic
resistance in the target pathogen.
Questions 1-6
Use
the information in the passage to match the options (listed A-C) with activities or features of
ants below.
Write
the appropriate letters A-C in boxes
1-6 on your answer sheet
NB
you may use any letter more than once
A
Leaf-cutting ants
B
Lower attines
C
Both leaft-cutting ants and lower attine
ants
1.....................
can use toxic leaves to feed fungus
2.....................
build small nests and live with different foreign fungus
3.....................
use dead vegetation to feed fungus
4.....................
raise a single fungus which do not live with other variety of foreigners
5.....................
normally keep a highly dangerous parasite under control
6.....................
use special strategies to fight against Escovopsis
Questions 7-11
The
reading Passage has ten paragraphs A-J.
Which
paragraph contains the following information?
Write
the correct letter A-J, in boxes 7-11 on your answer sheet.
7.....................
Dangerous outcome of Escovopsis.
8.....................
Risk of growing single fungus.
9.....................
Comparison of features of two different nests for feeding gardens.
10.....................
Discovery of significant achievements made by ants earlier than human.
11.....................
Advantage of growing new breed of fungus in the ant farm.
Questions 12-13
Choose
the correct letter, A, B, C or D.
Write
your answers in boxes 12-13 on your
answer sheet.
12. How does author think of Currie’s opinion
on the saying “ants keep theirgardens free of parasites”?
A his
viewpoint was verified later.
B his
earlier study has sufficient evidence immediately.
C there
is no details mentioned in the article.
D his
opinion was proved to be wrong later on.
13. What did scientists find on the skin of
ants under microscope?
A some
white cloud mold embed in their skin
B that
wax is all over their skin
C a
substance which is useful to humans
D a
substance which suppresses growth of fungus.
Solution:
|
1. A |
8. D |
|
2. B |
9. C |
|
3. B |
10. H |
|
4. A |
11. F |
|
5. A |
12. A |
|
6. C |
13. C |
7. E
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