THE
INGENUITY OF THE LEAF-CUTTING ANTS
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A
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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.
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B
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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 –the 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.
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C
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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 leaf-cutters 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.
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D
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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. The pure strain of fungus
grown by the leaf-cutters, 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 ant fungus 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.
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E
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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.”
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F
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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.
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G
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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.
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H
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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.
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Questions 14-19
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 14-19 on your answer sheet
NB you may use any letter more
than once
A
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Leaf-cutting
ants
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B
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Lower attines
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C
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Both leaft-cutting ants and
lower attine ants
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14
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can use toxic leaves to
feed fungus
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15
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build small
nests and live with different foreign fungus
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16
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use dead
vegetation to feed fungus
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17
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raise a
single fungus which do not live with other variety of foreigners
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18
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normally keep
a highly dangerous parasite under control
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19
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use special
strategies to fight against Escovopsis
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Questions 20-24
The reading Passage has eight
paragraphs A-H.
Which paragraph contains the
following information?
Write the correct letter A-H,
in boxes 20-24 on your answer sheet.
20
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Dangerous outcome of
Escovopsis.
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21
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Risk of
growing single fungus.
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22
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Comparison of
features of two different nests for feeding gardens.
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23
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Discovery of
significant achievements made by ants earlier than human.
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24
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Advantage of
growing new breed of fungus in the ant farm.
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Questions 12-13
Choose the correct letter, A,
B, C or D.
Write your answers in boxes 25-26
on your answer sheet.
25
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How does author think of Currie’s opinion on
the saying “ants keep their gardens free of parasites”?
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A
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his viewpoint
was verified later.
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B
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his earlier study has
sufficient evidence immediately.
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C
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there is no
details mentioned in the article.
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D
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his opinion was proved to be
wrong later on.
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26
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What did scientists
find on the skin of ants under microscope?
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A
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some white cloud mold embed in
their skin
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B
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that wax is
all over their skin
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C
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a substance which is useful to
humans
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D
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a substance
which suppresses growth of fungus.
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