Fly bites plant, but plants can bite back, Purdue scientists find

The Hessian fly changes wheat growth by injecting poisons into the plants, but a newly discovered resistance gene that can kill the insect may add a new defensive weapon for the grain crop.

Using the new gene in combination with other genes is expected to extend resistance time to the most economically damaging insect of wheat by as much as six times. Scientists from Purdue University and the U.S. Department of Agriculture—Agricultural Research Service (USDA-ARS) mapped the new gene and two closely linked markers, or bits of DNA, that indicate its presence in soft red winter wheat.

Results of the study are published in this month’s issue of the journal Theoretical and Applied Genetics.

“Although 30 other genes resistant to the Hessian fly are known, this is the first resistance gene found on this particular chromosome,” said Christie Williams, Purdue entomology assistant professor and USDA-ARS scientist. “The unique chromosomal location is important because it will allow us to easily pyramid the gene with other resistance genes to extend the durability of resistance against this pest.”

When several genes are combined in one plant to create the desired effect, in this case better resistance to the Hessian fly, it is called pyramiding. In order to pyramid genes successfully, they must be in different locations in the genome.

Now that Purdue researchers have discovered the gene, called H31, and know that it’s on a different chromosome than previously known Hessian fly resistance genes, they will intentionally breed wheat plants with three different Hessian fly resistance genes, Williams said. This should be especially effective because all of the genes to be used are strong genes – in other words, 100 percent of the plants containing them would be resistant under almost any stress, such as drought.

Conventional agricultural crossbreeding and selection is used to transfer the Hessian fly resistance genes into a single plant. It doesn’t involve any genetic engineering.

The soft red winter wheat studied in this research is used mainly for pastries, although the H31 resistance gene has its origin in pasta wheat. The researchers used an insect that is a widespread and highly virulent strain, the L biotype of the Hessian fly.

Hessian fly infestations have been controlled for about 60 years in the United States by wheat varieties naturally resistant to the fly. Hessian flies can overcome a single newly released resistance gene in about eight years, Williams said. However, by combining several different genes that afford protection from the pest, scientists believe resistance can be extended for 50 years.

“Computer modeling predicts that if three Hessian fly resistance genes are combined in one cultivar – or line of wheat – and planted along with a few susceptible plants that serve as a refuge for weaker strains of the fly, we can extend the durability of resistance,” she said. “We want to pyramid the resistance genes in wheat plants because it’s much harder for the Hessian fly to overcome three different resistance genes simultaneously.”

For the flies and the plants, it’s the old axiom: survival of the fittest.

The flies conquer the plants’ resistance because a few of the insects are genetically strong enough to survive on resistant plants that kill the majority of the larvae. When two surviving Hessian flies mate, their offspring are capable of overcoming the plant’s resistance. This continues until all the flies in the area are able to withstand the plants’ genetic protection.

At that point, a new line of plants with different resistance genes must be found.

The method of using natural genes in the plants to protect against a pest is called host plant resistance.

“Host plant resistance is really the preferred way of dealing with many insect problems because it lessens the need to apply chemicals that can degrade the environment,” Williams said.

The Hessian fly, which German mercenaries apparently introduced to North America during the Revolutionary War, causes catastrophic losses if not controlled by resistant plants. In Morocco, which didn’t have resistant plants until recently, the Hessian fly destroyed 36 percent of the country’s wheat crop annually. During the 1980s the state of Georgia suffered $28 million in lost wheat in one year after the fly overcame the plants’ resistance gene used in the area at the time.

The Hessian fly is particularly insidious because it actually can control the wheat plant’s development.

The adult fly lays eggs on the plant leaves. After the eggs hatch, the resulting tiny, red larvae crawl down to the base of the wheat where they feed on the plant. If the plant isn’t resistant to the insect, the larvae inject chemicals from their saliva into the plant that completely alter the wheat’s physiology and growth.

The plant stops growing and actually begins producing more sugar and protein in order to feed the larvae. Specialized cells develop in the wheat plant so that the insect has the perfect environment to grow, Williams said.

“If the plant is resistant, there is no visible sign that the flies have been on the plant,” she said. “Resistant plants will kill the larvae in about four days.”

Williams and her research team hope to determine the biochemical processes that allow the Hessian fly to control the plants and also the ones that enable the plants to kill the insect.

Other scientists involved with this study are: Chad Collier, Department of Entomology and USDA-ARS laboratory technician; Nagesh Sardesai, Department of Entomology postdoctoral fellow; Herb Ohm, Department of Agronomy professor; Sue Cambron, USDA-ARS research associate.