Forensic geneticists are tracking down an elusive killer wantedin nine or more states, from Florida to California. In 1991, thissmall, sap-sucking insect, the whitefly, cost vegetable andcotton growers in California and Texas alone more than $200million in crop damage, and the loss of some 12,000 farm jobs.

"Whiteflies are not flies," said entomologist Thomas M. Perringof the University of California, Riverside. True flies, he toldBioWorld, belong to the taxonomic order, Diptera; so-calledwhiteflies to the Homoptera, which includes aphids, hoppersand cicadas.

In the case of whiteflies, the traditionally dull discipline oftaxonomy has suddenly become a crisis of plant life or deathfor farmers the world over. This novel, ferocious strain nowdecimating food, fiber and flower farms has driven out thebetter-known and less-damaging pest, "sweet potato whitefly."

The founder whitefly, Bemisia tabaci, was identified on tobaccoplants in Greece more than a century ago, in 1889. By 1894, itturned up in Florida and started west, reaching California in1928. Then suddenly in 1986, the new and deadlier strain ofwhitefly appeared in Florida, attacking greenhouse poinsettias.By the end of the decade, it too had crossed the Sun Belt toCalifornia, wreaking crop havoc in its path.

Farmers dubbed the newcomer "superbug." Perring proposescalling it "silverleaf" whitefly because it afflicts squash withsilverleaf disease.

To the naked eye and the highest-power microscope, there isno telling the sweet potato whitefly apart from its fiercersilverleaf lookalike.

Are they two truly different species, or merely mutants?Riding on the answer to this taxonomic question is how to copewith the interloping insect in the years ahead. Researchersworldwide are fanning out to locate silverleaf's originalancestral habitat, because that is where they hope to find itsnatural biological enemies, especially a species of microscopicwasp that lays its eggs in whitefly larvae.

These parasitoid wasps are highly fastidious host-specificwhitefly foes. Only if "silverleaf" is indeed a wholly separatespecies from "sweet potato" is there hope that whatever mini-wasps inhabit its native haunts can and will attack it. That iswhy the entomologists have enlisted molecular biology todetermine if superbug is a case of mistaken identity or guilt bydisassociation.

Perring is first author of a paper in Science for Jan. 1, titled"Identification of a Whitefly Species by Genomic and BehavioralStudies."

Microbiologist Russell J. Rodriguez of UC Riverside's plantpathology department used a variant of polymerase chainreaction (PCR) analysis to determine that silverleaf is indeed aseparate species.

Instead of the two different primers usually used in PCR toamplify a specific region of DNA, Rodriguez employed singleprimers, based on microsatellite (highly-repetitive) DNA, whicharbitrarily amplify throughout the genome.

"Historically," Rodriguez told BioWorld, "the basic problem withtaxonomy is that methodologies have always been dependenton gene expression. With the whitefly, we don't have to dealwith gene expression at all, but rather scattering our viewing ofthe whole organism across the whole genome."

Rodriguez electrophoresed the PCR-amplified runs on agarosegels and based his estimate of species-similarity on comigratingbands. "Within a species," he said, "we found 80 percent to 100percent similarity; between species, less than 20 percent."

He added: "It's an idiot-proof method, and fitted neatly into therest of Perring's Science paper," which also came down on theside of two separate species, based on behavioral andreproductive incompatibilities.

The single-primer method Rodriguez deployed is a variant ofstandard forensic DNA-fingerprinting practice, he explained,"which employs two PCR primers to amplify a known region ofthe genome, then uses different restriction enzymes to cut thatamplified band and look for polymorphisms."

"For our purposes, the single-primer technology gives ussensitivity so high that we can take two insects off a leaf anddetermine whether they are of the same, or different, species,"Rodriguez said.

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.

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