Jumping genes make genetic leaps

Agricultural Research, Jan, 1994 by Doris Stanley

Impetus for Finding the New Gene

McClintock's discovery of transposable elements was based on her work with the genetics that controlled the purple and bronze color in Indian corn.

She found that when there was a change in corn kernel color, there was a corresponding change in the place where chromosomes tended to break. She hypothesized that this breakage was caused by a "jumping" gene.

A particular gene caused the kernels to be purple. But when the chromosomes broke apart, a different genetic element showed up next to the gene for purple color. So the offspring of that particular kernel produced golden-brown or bronze-colored kernels.

This color change also involved a third gene, which apparently activated the jumping gene. The different-colored kernels were produced by the color gene that did the work; a controlling element, or jumping gene, that told the color gene what to do; and an activator that started the work.

"This is the way we proved that a jumping gene was in glossy-15," Sisco says. "When the transposable element is present in the gene, it turns off the gene function, producing sections on the corn plant that have no wax. When the element hops out during leaf development, the gene turns back on, producing waxy sections on the plant."

Paul H. Sisco is in the USDA-ARS Plant Science Research Unit, North Carolina State University, Box 7620, Raleigh, NC 27695-7620; phone (919) 515-2705, fax (919) 515-7959.

An Ancient Crop, Updated

A staple in the diets of the Aztecs, Incas, and other native American peoples, corn as a crop is about 8,000 years old. Today, it is important as a raw material in producing industrial products such as ethanol. A major feed and food crop, both domestic and foreign, corn can only grow as a cultivated crop. It can't survive in the wild. The many superior corn hybrids that we have today may be attributed largely to one man: George F. Sprague.

"In the late 1920's when I started my research, farmers depended on open pollination, saving their own corn seed for next year's crop. This means that yields were relatively low--I'd say about 30 bushels per acre," muses Sprague. "Nowadays, U.S. average yields exceed 100 bushels."

Before retiring in 1973 as head of ARS Corn and Sorghum Investigations at Beltsville, Maryland, Sprague conducted research on corn genetics that is considered to be among the greatest plant breeding achievements of the 20th century.

He helped develop the scientific principles that provide the foundation for corn breeding and genetics research. Famous for using basic science to increase corn's productivity, Sprague applied his knowledge of corn's genetics to develop varieties that helped the farmer produce more.

"In the early days of corn research, we developed cooperative agreements with Corn Belt states to work on hybridization. Federal researchers were set up in universities in Iowa, Illinois, Indiana, Kansas, Missouri, and Ohio," he says.

The first hybrid corns were introduced in the early 1930's, and by 1940, most of the major corn-producing states were using only hybrids. Sprague says, "By the 1960's, 100 percent of U.S. corn acreage was planted with hybrids."