Off topic - NYT article : Leaf-cutter ants "farming" fungus.
August 3, 1999
For Leaf-Cutter Ants, Farm Life Isn't So Simple
By NICHOLAS WADE
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.
But the textbook accounts, it turns out, do not tell even half the story. From
research in the past five years the ants' symbiosis has emerged as far more
intricate than it appears, involving not two but at least four species, their lives
knotted together in a ruthless yet highly successful struggle for survival.
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.
The fact that there was still so much to
learn about leaf-cutter ants and their
agriculture, biologists say, only
underlines how much remains to be
discovered about the world's plants and
animals.
Ants invented agriculture 50 million
years before people did, and the
leaf-cutters, members of a large family called the attine ants, practice the
most sophisticated example of it.
They grow their fungus, a kind of mushroom, in underground chambers that
can reach the size of a football.
A single leaf-cutter nest may contain a thousand such chambers, embedded
in an underground metropolis up to 18 feet deep, and support a society of
more than a million ants.
These ant communities are the dominant plant-eaters of the Neotropics, the
region comprising South and Central America, Mexico and the Caribbean.
Biologists believe some 15 percent of the leaf production of tropical forests
disappears down the nests of leaf-cutter ants.
In the nest the leaves are shredded and inoculated with the fungus, which
digests them and is in turn eaten by the ants.
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.
So precious is their particular strain of fungus that the ants' virgin queens,
before their nuptial flight, secrete a mouthful with which to seed the garden
of their new nest. The worker castes they produce are so tailored to the craft
of fungus gardening that they come in made-to-fit sizes -- large ants to saw
off leaves, medium ones to shred them and miniature workers to seed them
with fungus and clean off all alien growths.
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.
A question that has long perplexed ant biologists is whether the funguses
cultivated by attine ants are all descended from a single ancestor, just as the
ants are. The issue was hard to settle because the ants' gardening habits
prevent the fungus from forming mushrooms, the spore-bearing stage by
which mycologists tell one fungus from another.
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, analyzed 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.
After further study, three of the biologists, Dr. Mueller, Dr. Rehner and Dr.
Schultz, reported last year that funguses grown by lower attine ants fell into
four groups of varieties, as if the ants had domesticated wild funguses at
least four times in evolutionary history. Two of those occasions must have
been quite recent because the biologists identified free-living counterparts for
two of the four fungus groups they found in the ants' gardens. A single
variety of fungus is grown in each nest, but most lower attine species
cultivate at least two of the four fungal lineages, suggesting that varieties are
exchanged among species every so often, the biologists concluded.
What evolutionary force could be driving these two patterns of fungus
gardening, the pure clone cultivation of the leaf-cutters and fungus exchange
program of the lower attines? The answer, or part of it, has been divined by
Cameron R. Currie, a Ph.D. student in a climate no wild attine ever reached,
the University of Toronto. Trained as an entomologist, Currie was attracted
to the ants because of his interest in symbiosis and in the cheaters who take
advantage of that mutualism.
The pure strain of fungus grown by the leaf-cutters, it seemed to him,
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.
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?' " Currie said of his efforts to find a hidden interloper.
But after three years of sifting through attine ant gardens, 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." The deadly mold then turns
greenish-brown as it enters its spore-forming stage.
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, 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, 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."
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, 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. It also stimulates growth of the ants'
mushroom fungus. The bacterium is carried by virgin queens when they
leave to establish new nests, but is not found on male ants, playboys who
take no responsibility in nest-making or gardening.
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 the
Escovopsis 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.
In a loose team, the four biologists involved in the new findings are seeking
to understand the deeper intricacies of the extraordinary system. Dr. Schultz,
now at the Smithsonian Institution in Washington, D.C., is a specialist in the
evolutionary relationships of ants, and Dr. Rehner, of the University of Puerto
Rico, is an expert on fungi. Dr. Mueller, now at the University of Texas at
Austin, is an ecologist Together with Dr. Currie, they hope to unravel the
relationships woven between the four members of the symbiosis -- the attine
ants, their mushroom, Escovopsis and the Streptomyces bacterium. There
are doubtless other members to be discovered.
The four researchers are pleased but not much surprised to have discovered
so much new about an already well studied system.
"It may be one of the best studied symbioses in biology but that is a sad
reflection on how little we know in general," Dr. Schultz said.
Discovery of the deadly Escovopsis fungus could change the correlation of
forces between ants and people. In the 10,000 years since the ants have had
to deal with human agriculture, a blink of an eye in their 50 million year
farming history, the ants have generally prevailed. They thrive in disturbed
ground, and so have usually benefited from the clearing of forests.
For the most part, people and the attine empire have co-existed peacefully:
the two species live on different scales and in separate spaces, people above
ground and attines below it. But attines are a serious agricultural pest in much
of the tropics. Currie, having shown that nests in the lab are devastated with
a squirt of Escovopsis spores, suggests the fungus might prove a useful way
of controlling attine nests.
There seems little objection to applying Escovopsis nest by nest. But Dr.
Schultz expressed horror at indiscriminately wiping out all fungus-growing
ants, many species of which are harmless.
That an Escovopsis-like discovery could tip the scales too far toward the
human side was foreseen by the Dr. Wilson and his colleague Bert Holldöbler.
In their book "The Ants," published in 1990, they wrote that biologists need
to search for "the weak points" in the ants' social system: "The goal,
however, should be intelligent management of their populations and never
their complete eradication. Our advantage -- and responsibility -- lies in the
fact that we can think about these matters and they cannot."
Copyright 1999 The New York Times Company |
