UNTIL MAY 15, 2 p.m. PST (5 p.m. ET)
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SUCCESS OF INTRODUCED ARGENTINE
ANTS TIED TO REDUCED GENETIC VARIATION
Biologists at the University of
California, San Diego have discovered that the proliferation in
California of the introduced Argentine ant, a major pest that has
invaded homes and displaced native species of ants in much of the
coastal regions of the state, is due to the lack of genetic diversity
among individuals up and down the coast.
In paper published in the
May 16 issue of the Proceedings of the National Academy
of Sciences, the scientists conclude that this reduced genetic
variation has essentially allowed a giant "supercolony"
of closely related ants to grow unchecked from San Diego to
Ukiah, 100 miles north of San Francisco. In Argentina, by contrast,
fighting among the more genetically dissimilar, territorial
ants has managed to keep these insects in check and in smaller,
much more sharply defined colonies than those in California.
"When we did our field
work in Argentina, it was surprisingly difficult to find Argentine
ants, compared to our experience in California," says Ted J.
Case, a professor of biology at UCSD who headed the research team.
"They are a relatively inconspicuous feature, both in the urban
and in the natural environment."
Not so in California,
particularly in the southern part of the state, where the ant is
regarded as the number one pest extermination companies are called
upon to remove from people’s homes. "If you live in urban San
Diego or Los Angeles, this is the ant that’s in your house and is
invading your garden and office," says Neil D. Tsutsui, a
graduate student and the first author of the study.
infested by Argentine ants in red
dark-brown and black ants, which are about two millimeters in length,
are thought to have entered the United States aboard ships carrying
coffee or sugar from Argentina during the 1890s, then expanded
throughout California and the southern parts of the United States. In
the Southeast and much of the South, their proliferation is now
limited to some extent by the introduction of fire ants.
But in California, where those
competitors are largely absent, the ants thrive in the temperate and
damp coastal regions, killing and displacing native ants, many of
which are 10 times larger in size. "The striking thing about
Argentine ants in California is that as long as the habitat is
favorable, they completely dominate," says David A. Holway, a
postdoctoral researcher at UCSD and a co-author of the paper.
Their growth has not only been
a problem for native ant species, but for species that feed on the
larger native ants. One such creature is the coastal horned lizard,
which has declined by 50 percent in recent years, according to Andrew
V. Suarez, a graduate student and a co-author of the paper who has
been studying the impact of the introduced ants on the lizards.
discovered two years ago that native Argentine ant colonies
living in close proximity were territorial and aggressive toward
one another, literally tearing one another apart whenever they
came into contact. The puzzling question to the researchers
was, Why weren’t the introduced ants in California the same
way? Why would an Argentine ant from San Diego dropped into
a colony in San Francisco be welcomed, while an Argentine ant
dropped into a colony two hundred meters away in its native
country be torn apart?
Tsutsui set out to answer that
question by comparing the genetic differences among the ants in
Argentina with those in various parts of California, from San Diego to
Ukiah. Using some of the same DNA fingerprinting techniques employed
by criminologists, he discovered that the native Argentine ants were
twice as diverse as the native California ants, explaining why the
California ants regard individuals up and down the coast as close kin,
while those different nearby colonies in Argentina do not.
"They have an innate
ability to recognize other members of their colony based on how
genetically similar they are to themselves," says Tsutsui.
"Since they evolved in their native range, where colonies are set
up as family structures, they tend to recognize other members of their
colony by how closely related they are."
Tsutsui and his colleagues
conclude that the low genetic diversity among the California ants is
the result of the introduced ants having gone through a "genetic
bottleneck." In other words, the genes are similar among
California ants because they are descended from a relatively small
founding population of genetically similar ants from one single or
closely related colonies in Argentina.
"Because they’ve gone
through a genetic bottleneck, everybody’s genetically similar and
everybody recognizes everybody else as a member of their own
colony," says Tsutsui. "In essence, the supercolony that we
see in California is in fact one big colony."
"The thing that’s
surprising about this result is that, typically, reduced genetic
variation or diversity are considered bad for populations," he
adds. "With Argentine ants, it really looks like this is
beneficial for the species, at least for the short term."
The UCSD researchers are
extending their investigations to other parts of the United States and
the world where Argentine ants have been introduced to determine if
the genetic similarities found in California are found in other
regions and are typical of other introduced populations that have been
expanding rapidly. They also hope to examine whether genetic
bottlenecks are a common phenomenon in the introduction and success of
other invasive ant species. Their study was financed by the Canon
National Parks Science Scholars Program, the National Science
Foundation and the U.S. Department of Agriculture.
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An Argentine ant worker, shown on a pencil for scale. Credit: Andrew
Andrew Suarez collecting native ants in Argentina. Credit: Neil