The spotted wing fruit fly is one of the world’s major crop destroyers. Scientifically known as Drosophila suzukii, this peppercorn-size insect uses a serrated organ to lay its eggs inside—rather than on top of—unripe fruit, damaging raspberry, strawberry, and cherry crops across the globe. Now, scientists may have found a way to fight this pest using a strategy called gene drive, which can spread genes rapidly through a population. When coupled with a lethal “cargo gene,” the approach could kill the flies in their tracks when exposed to a specific chemical compound, or just simple summertime heat.
Once seen only in Japan, D. suzukii has now invaded every continent except Antarctica. It made its way to the United States 10 years ago, where it has decimated up to 80% of crops in some places. California’s raspberry industry alone lost nearly $40 million in revenue between 2009 and 2014. Current control strategies depend heavily on an insecticide called malathion, but it’s not always effective and there are concerns the fly will develop resistance.
Scientists have tried a gene drive approach in fruit flies before. In 2007, molecular biologist Bruce Hay of the California Institute of Technology in Pasadena and colleagues created a particular version of it called Medea (short for maternal effect dominant embryonic arrest, but also named after the Greek mythological figure who murdered her own children) in D. melanogaster, another species of fruit fly. The team endowed the flies with an extra gene that kills offspring unless they also have an extra copy of another gene that counteracts it. By supplying both the “toxin” and its “antidote,” Medea can spread quickly through a population, as only offspring with the gene combination will survive.
In the new study, Hay’s former graduate student, genetic engineer Anna Buchman of the University of California, San Diego, set Medea on D. suzukii. Buchman and colleagues collected D. suzukii flies from Corvallis, Oregon, and sequenced part of their genome to create a version of Medea that would work specifically in the pest. Then, the team mated D. suzukii flies injected with Medea to normal flies. Nearly 98% of offspring born to Medea-bearing mothers over six generations also had the genetic element, as opposed to the expected 50%, indicating that Medea was spreading rapidly through the population. The researchers saw similar results in D. suzukii populations around the world, with 88% to 99% of offspring inheriting the Medea gene drive element, they report this month in the Proceedings of the National Academy of Sciences.
In order to combat the pest in the real world, however, the team will need to pair Medea with another gene that would make the flies vulnerable to a predictable trigger. This would cause the flies to die when exposed to a particular chemical, for example, or even to rising summer temperatures, when the flies are at their peak. In a best-case scenario, researchers could introduce Medea-bearing flies into an area overrun with D. suzukii, wait about six generations—or as few as 12 weeks—for the Medea strain to dominate, and then simply spray the fields with a chemical—or wait for summer—and then watch nearly all of the flies die.
Still, experts say Buchman’s team will need to show that its version of Medea can stick around in a population for a long period of time—something that’s still unclear, says Max Scott, a geneticist at North Carolina State University in Raleigh. He points out that Buchman’s version of the gene drive “didn’t work anywhere near as well” as it did in Hay’s original work, because Medea disappeared from the flies in two long-term experiments and some wild D. suzukii flies appeared immune to Medea’s toxin. “To get something that will actually work in the field presents a pretty big challenge.”