A quick whiff of insect spray easily knocks down most house flies. Butsometimes it doesn't, depending on whether the fly has evolvedresistance to the insecticidal chemical in the spray.In 1945, DDT (dichloro-diphenyl-trichloroethane) emerged as thelong-sought, all-purpose, all-powerful means of exterminating insectpests, from body lice to malarial mosquitoes, to house flies. The rudeawakening came as early as 1947, when target insects acquiredresistance and began to shrug off the lethal compound.DDT is long out of favor, not only because of its threat to the planet'secology, as witnessed by Rachel Carson's Silent Spring in 1962, butfor the crasser reason that DDT no longer packed its initial insecticidalwallop. The bugs had simply wised up and selected to withstand itslethality.Worse was to come. Insect resistance has cropped up not only in otherDDT-like chemical bug killers, but in the natural compoundpyrethrum, distilled from the flower petals of Dalmatianchrysanthemums. Pyrethrum was hailed as the organic answer toDDT. Modern synthetic pyrethroid analogs are today among thewidest used insect control compounds - and the bugs are on to themtoo."Worldwide use of DDT may have preselected resistant flies and bitinginsects," suggested insect molecular geneticist Douglas Knipple, "andextended their cross-resistance to other agents, including pyrethrum,by transferring the kdr gene."Kdr, which stands for "knockdown resistance" factor, is the genetictrait that saves flies from falling to the floor and dying. It drives thedanger of a worldwide insect comeback, wherever the use ofinsecticides fuels the march of kdr.Kdr-equipped flies are a household nuisance, but a dairy farmers baneand a cotton grower's menace.Knipple, a researcher in Cornell University's entomology department,pointed out that DDT and pyrethrum (floral or chemical) both act onthe insect nervous system. But pyrethrum (widely used, for example, inflea powders), "is different from DDT and more desirable because ofits low toxicity in the environment," he said.Knipple is working on ways to understand how the kdr gene works tofrustrate its insecticidal action.Last week's issue of Proceedings of the National Academy of Sciencescarried a report by Knipple, et al., titled "Tight genetic linkagebetween the kdr insecticide resistance trait and a voltage-sensitivesodium channel in the house fly, Musca domestica."The molecular mechanism by which kdr's gene product shields aninsect's nervous system from insecticidal paralysis, followed by death,remains unknown. So far, kdr, and an even stronger resistance factor,super-kdr, have been mapped onto chromosome 3.What is known is how, or rather, where, DDT and the modernpyrethroids work: They modify the inactivation kinetics of voltage-sensitive sodium channels, which open and close in nerve-cellmembranes. Reasoning backward, Knipple hypothesized that kdrarises from a mutation in the sodium channel gene.To test this idea, he cloned a segment of M. domestica's genome thatmatched the sodium channel gene of the well-mapped fruit fly(Drosophila melanogaster) genome. Then, using PCR, he developed adiagnostic probe to determine the sodium channel genotype ofindividual flies. Genetic linkage analysis confirmed that the kdr generesides very close to the apparently mutant allele of the channel genesegment.Armed with this genetic insight into how insects develop resistance totheir chemical killers, Knipple and his team are currently sequencingthe entire transcription unit and constructing a heterologousexpression system in Xenopus laevis, the African clawed frog, to mapthe resistance trait.Meanwhile, insecticide biochemist James Ottea of Louisiana StateUniversity is "trying to get some kind of field kit together to helpcotton growers adjust their chemical crop-protection strategies to thechanging resistance patterns of the tobacco budworm (Heliothisvirescens), which ravages cotton bolls."Right now in the insecticide world," he told BioWorld, "we havethree major target sites in the insects' nervous system that we canattack with insecticides: One is that sodium channel, which right nowis the most important. It's the paradigm under which Knipple andSoderland at Cornell are working."Then there's a second target, chloride channels, some of which areopened and shut by the neurotransmitter GABA - gammaaminobutyric acid. And thirdly, acetylcholinesterase, the enzyme thatbreaks down a different neurotransmitter, acetylcholine."Ottea explains that growers need to know whether they are facing awave of pests resistant at a specific target site so they can switchchemical classes of insecticide."What's nice about Cornell's kdr in insects, " Ottea said, " is that it'sinherited as a recessive trait. So they can look at the genetics of thechannel expression, its inheritance, and the toxicity should matchwhat they're seeing in their molecular assay."He calls it "terribly important work from both a fundamental and anapplied standpoint."
-- David N. Leff Science Editor
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