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Research Magazine > ARCHIVE > Summer 01 > Article

Genetic Junk
by Judy Purdy

Strange as it may seem, much of the genome actually has little or nothing to do with genes.

For two decades, Susan Wessler, a UGA Distinguished Research Professor of Botany and Genetics, has studied “genetic junk” in DNA called transposable elements, or TEs for short (Research Reporter, Fall 1992).

These fragments of DNA are found in the genomes of all organisms. In humans, for example, TEs account for about 40 percent of the entire genome.

“Transposable elements make a genome bigger but most TEs apparently don’t cause any harm to the organism,” said Wessler, who studies TEs in rice and corn.
Her research has shown that by moving in and out of DNA, TEs may alter or even create inherited traits. TEs have influenced evolution by determining how and when certain genes are expressed.

“They can shake up an otherwise conservative genome, challenging an organism from the inside,” she said.

Plant populations that have large numbers of active TEs could be more adaptable to changes in the environment, but nobody knows for sure. Such diversity could give a TE-diverse population competitive advantages and a greater ability to survive.

“Over millions of years of evolution, TEs lose their mobility,” Wessler said. “Populations with less TE diversity and/or activity might have diminished ability to adapt to external challenges.”

Wessler’s research is designed to find answers to just how much TE diversity exists within distinct plant populations and what adaptive advantages a population with a rich array of active TEs might possess.

Her research team also looks at whether a specific location of TEs within DNA might beneficially alter gene expression and introduce traits like drought tolerance, or pest or virus resistance in agricultural crops such as rice.

“Rice is the most important direct source of food for humans. About two-thirds of the world depend on rice as an energy source,” said Wessler, who heads a research collaboration that includes UGA, Cornell University and Washington University.

Wessler’s research is funded by a $3.4 million National Science Foundation grant to determine if the position of specific TEs in rice varies among populations, and if so, to what extent.

She is among hundreds of scientists worldwide who are working to better understand rice DNA, which may have as many as 30,000 or more genes spread across a dozen pairs of chromosomes.

Unlike many of its cousins in the grass family — which includes corn, sorghum, millet, wheat and rye — rice has a smaller genome and also much less genetic junk, so the gene-mapping process should be easier, Wessler said.

“For example, rice and corn may have two related genes that are separated by 10,000 base pairs in rice and 100,000 base pairs in corn. The difference is that in corn more TEs have inserted between the genes,” she said. “The rice genome, if written in textbook style, would fill about 150 volumes, while corn would take about 900.”

Wessler’s findings will contribute to the overall knowledge of grass family genetics. Down the road, other scientists will be able to use that knowledge to engineer higher-quality, bigger-yielding varieties of rice, corn and other grass cousins to feed a growing population.

One thing seems certain: As scientists learn more about TEs, they will be less inclined to refer to them as genetic junk.

For more information, e-mail sue@dogwood.botany.uga.edu.

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Rice is the main energy source for two-thirds of the world’s people. Illustration of rice (Oryza sativa cv Nipponbare) by UGA botany graduate student Dorset Trapnell