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2-Plants: Models show that gene flow from crops threatens wildplants

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SOURCE: University of Wisconsin-Madison, USA, Press Release
DATE:   Jul 23, 2003

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MADISON - In a river valley just southwest of Mexico City stands a small
patch of teosinte - a wild, weedy grass thought to be the ancient
ancestor of corn. As a gentle breeze blows gene-carrying pollen from a
nearby crop of maize to its wild relative, the genetic integrity and even
survival of this ancient plant and others could be jeopardized, according
to new mathematical models.

The models, described in the July 23 online edition of the Proceedings of
the Royal Society of London and developed by scientists at the University
of Wisconsin-Madison and the University of Minnesota-St. Paul, show that
genes from crops rapidly can take over those in related wild plants. The
end result, say the researchers, could be major changes in the genetic
make-up of wild plants, decreases in their population size and the
permanent loss of natural traits that could improve crop health.

Although gene flow from crops to wild relatives has occurred ever since
humans started farming, few studies before the 1980s examined the effects
of this evolutionary process in a scientific manner. Most of the people
concerned up until then were farmers, not researchers, says Ralph
Haygood, a UW-Madison postdoctoral fellow and lead author of the paper.

But, as genetic engineering developed and emerged as both a biological
and political issue, gene flow from crops containing transgenes - genetic
information from other species that's artificially inserted - to wild
plants gained more scientific attention.

"Most of the concern about crop-wild gene flow," says Haygood, "is driven
by concern about transgene escape," the idea that these artificially
inserted genes in a crop plant can leak into the genomes of wild
relatives. According to Haygood, growers around the world have planted
145 million acres of transgenic crops.

Conserving the genetic integrity of wild plants, explains Haygood, is
important for two reasons: protecting the survival of the plants
themselves and maintaining their repository of advantageous traits. These
traits, he adds, can be used to improve crop health: "The fact is that
most genes for crop improvement have come from wild relatives of those
same crops."

To begin to understand the effects of gene flow from crop to wild plant
populations, Haygood and his colleagues Anthony Ives from UW-Madison and
David Andow from UM-St. Paul, developed mathematical models based on
fundamental principles of population genetics.

"The key to the models," says Ives, "is that they summarize fundamental
properties of evolutionary change. They show what is likely to happen."

Specifically, the models examine how rates of pollen flow and how the
selective effects of crop genes on wild plants alter two evolutionary
processes: genetic assimilation, wherein crop genes replace genes in wild
populations, and demographic swamping, wherein wild populations shrink in
size because crop-wild hybrids are less fertile.

"Genetic assimilation and demographic swamping could change a wild plant
in some way that might be important for its survival in some habitats or
for other organisms that depend on them for their survival," says
Haygood. "The potential ramifications are huge and diverse."

The research team starts with a simple model, where a wild population of
large and constant size receives pollen from a crop that differs
genetically by only one gene. They then add complexity, or, as Ives says,
"more realism." That is, they consider a crop that is more different
genetically and a wild population that is small or varies in size.

The researchers are quick to point out that the models do not distinguish
between crops developed through traditional breeding and genetic
engineering. "How the genes get in the crops doesn't matter," explains
Haygood. "What's important is what they do once they're there."

In both the basic and expanded models, the researchers find that crop
genes rapidly can take over wild populations and, sometimes, just a small
increase in the rate of pollen flow can make a big difference in the
spread of a crop gene. When this happens, says, Ives, "There's no going
back. It's irreversible."

The findings, explains Haygood, show that few conditions are needed to
enable genetic assimilation and demographic swamping. "You don't need
high rates of pollen flow or strongly favored traits," he says. "Crop
genes, even fairly deleterious ones, can easily become common in wild
populations within 10 to 20 generations."

At the same time, the combined forces of these two processes on the wild
populations can change their genetic make-up in unfavorable ways and
drastically shrink their population size, leading to what evolutionary
biologists call a "migrational meltdown."

Although the models look at gene flow from a crop plant to a wild
relative, the researchers say that the models probably also could apply
to gene flow from a commercial to a landrace crop raised each season from
the previous year's seed. But they add that more investigation is needed.

The goal of the gene flow models, explain the researchers, is to provide
qualitative insight that they hope will enhance the public dialogue on
gene flow from crop to wild plants.

"Gene flow from crops to wild relatives is one of a host of environmental
issues that humans must deal with," says Haygood. "These models are a
resource that can contribute to the discussion."### 

- Emily Carlson (608) 262-9772,

Ralph Haygood
(+1-608) 262-9226
Tony Ives
(+1-608) 262-1519


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