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Wheat tough enough to take the Hessian fly.

Wheat Tough Enough To Take the Hessian Fly

ARS researchers were part of the team that dealt the Hessian fly a one-two punch when they zapped the chromosomes of a wheat/rye hybrid with x-rays, creating a new source of Hessian fly resistance for wheat.

First identified on Long Island in 1779, the pest was apparently brought to the United States in the straw bedding of Hessian soldiers fighting in the Revolutionary War. Over time, it has spread to all major wheat growing regions of the United States. Larvae of the insect attack young wheat in the fall and again in the spring, stunting plant growth and causing yield losses of 5 to 10 percent each year.

In early years, winter wheat growers depended on the so-called fly-free date to protect the wheat. They planted their crop after the date in the fall when wheat fields in a given region were generally free of Hessian flies. However, this method was not always reliable. More recently, wheat varieties that genetically resist feeding of the larvae have been used to combat the pest.

But some resistance genes used in varieties have been deployed for more than 10 years and are losing their effectiveness.

"The genetic variability in the fly is such that it can overcome resistance," says ARS entomologist J.H. Hatchett. "To continue to protect wheat, we have to be able to come up with new genes. As the deployed genes are lost, we have to have new genes ready to replace them."

Hoping to score a knockout, scientists searched for a suitable species that could lend the much-needed resistance to wheat. They selected the rye plant, a distant relative of wheat, because it was known to be highly resistant to the Hessian fly. The scientists are still uncertain exactly what causes the resistance. The young larvae feed on the resistant plants for 2 to 3 days and then die. The phenomenon known as antibiosis appears to cause an incompatible feeding response. "Because rye is a poor host for the pest, we felt like there was a possibility that in the long run, rye genes may be more durable than those in wheat." Hatchett says. "The trick was figuring out how to transfer the rye gene to wheat chromosomes."

The late Emil E. Sebesta, who was an ARS scientist at Stillwater, Oklahoma, when this study began, had previously used x-rays to move greenbug resistance from rye to wheat.

Based on this research, the scientists opted to use irradiation because of its proven ability to break chromosomes - a necessity to begin the transfer of genes from rye to wheat.

Breaking the chromosomes also eliminates unwanted rye genes, explains Bikram Gill, a Kansas State University geneticist. The research was performed in cooperation with the university's Wheat Genetics Resource Center.

Scientists first crossed resistant Balbo rye and a susceptible common wheat. Chromosomes of the progeny were chemically treated with the compound colchicine, which doubles the number of chromosomes to overcome sterility.

These plants were again crossed with susceptible wheat plants, and the resistant progeny were allowed to self-pollinate. Pollen from the progeny plants was exposed to a low dosage of radiation and then used to fertilize several lines of wheat.

"At this point in the process, it just takes a lot of luck," Hatchett notes. "You hope that the x-rays will break the wheat chromosomes and rye chromosomes and a piece of the rye chromosome carrying the gene for resistance will insert or attach to the wheat chromosome."

Luck was with the researchers. After several generations of testing and selection, pure, resistant sublines were obtained that carried the normal 42 chromosomes of wheat. Using genetic fingerprinting to identify the rye chromatin in the wheat plants, the scientists found three types of translocations: two terminal and one intercalary.

The terminal translocations had a small segment of the rye chromosome attached to the end of a wheat chromosome. The intercalary translocation was formed when a very small piece of rye chromatin was inserted in the middle of a wheat chromosome.

"When a small segment of the rye chromosome is added to the end of a wheat chromosome, a part of the wheat chromosome is usually lost," explains Gill. "But with an intercalary translocation, none of the wheat chromosome is lost, which makes it more valuable than a terminal translocation."

This is the first time an intercalary translocation involving wheat and another species has been found, the geneticist says.

Lines derived from the irradiated pollen were tested for resistance in a greenhouse at Kansas State University. "Resistance was verified by finding dead Hessian fly larvae at the base of the plants," Hatchett says. Germplasm from the three lines will be made available within the coming year to both public and private wheat breeders for use in development of new Hessian-fly-resistant wheat varieties.

PHOTO : About one-eighth-inch long, the female Hessian fly emits a sex pheromone from her ovipositor to attract males.

PHOTO : Plant geneticist Stan Cox and entomologist J.H. Hatchett check for desirable traits in wheat plants carrying wheat-rye chromosomal translocations.

PHOTO : Technician Ken Oppenlander dissects a Hessian fly-resistant wheat seedling to confirm the presence of dead larvae.
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Author:Gerriets, Marcie
Publication:Agricultural Research
Date:Sep 1, 1991
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