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Models show gene flow from crops threatens wild plants
(Friday, July 25, 2003 -- CropChoice news) -- Emily Carlson, Univ. of Wisconsin, 07/23/03: 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."
http://www.news.wisc.edu/view.html?get=8776
---
Wednesday, July 23
Stegemann, et al., "High-frequency gene transfer from the chloroplast
genome to the nucleus," PNAS 100: 8828-33 demonstrates gene transfer from
tobacco chloroplasts to the nuclear genome at a frequency of 1 in 5
million cells. In his commentary (PNAS 100: 8612-4), William Martin writes:
"On an evolutionary or environmental scale, 1 in 5,000,000 cells is a
whopping number; to some it will be unbelievable and by no means will it
make everybody happy. Some biotechnologists are adamant that foreign
genes introduced into the chloroplast can be sequestered there and thus
will not escape via pollen (introgress) from cultivated fields into
wild species like nuclear genes can."
That speculative sequestering (because chloroplast DNA is not expressed
in pollen) is essential to arguments about the future environmental
safety of transgenic crops. | |
