Researchers at the University of Chicago have gained new insight into the co-evolution of plants and the microbes that attack them. In the August 12 issue of Nature, the researchers report findings that go against the widely held 'arms race' theory in which plant resistance genes fight brief battles with microbes before both plant and pathogen mutate to higher and higher levels of resistance and virulence.

Joy Bergelson, assistant professor of ecology & evolution at the University of Chicago, favors an alternative hypothesis she calls 'trench warfare', in which cycles of disease epidemics maintain relatively stable forms of resistance and susceptibility genes over long periods of time.

"The arms race theory has been a generally accepted model for the evolution of disease resistance genes because it is intuitive, but its never been scientifically tested" says Bergelson. "Our results were surprising in demonstrating that an arms race is not occurring for the resistance gene we studied."

The researchers focused on the Rpm1 gene in Arabidopsis, the common mustard plant. Rpm1 is either present or absent in a given plant. These two alternative states are called the gene's alleles, which are located at a specific spot on the DNA called the gene locus. The Rpm1 allele confers resistance to certain pathogens. When it is absent at the Rpm1 locus (the null allele) the plant is susceptible to pathogenic attack.

To understand the evolutionary dynamics of the Rpm1 resistance gene, Bergelson and colleagues looked at the number of base pair differences in the DNA surrounding the Rpm1 gene locus. Base pair differences, also known as polymorphisms, accumulate over long periods of evolutionary time, and serve as a kind of molecular clock that gives researchers clues as to the age of alleles. The more polymorphisms, the older the alleles.

Bergelson's new analysis found that the number of polymorphisms was much higher than would be predicted by the arm
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Contact: Sharon Parmet
sparmet@mcis.bsd.uchicago.edu
773-702-6241
University of Chicago Medical Center
12-Aug-1999