May 4, 2004
Researchers string together players in pesticide resistance orchestra
WEST LAFAYETTE, Ind. - A Purdue University research team has found a set of genes that may orchestrate insects' ability to fight the effects of pesticides.
"Our study suggests that more than one gene may be involved in making insects resistant to certain pesticides," said Barry Pittendrigh, associate professor of entomology. "Using a music analogy, metabolic resistance may not be a single individual playing a single instrument. It's more likely a symphony with numerous instruments playing a role in producing the music."
The ultimate aim of the research is to develop methods to prevent insect damage to plants, he said. Results of the initial study are published in the Tuesday (May 4) issue of Proceedings of the National Academy of Sciences.
The scientists looked at approximately 14,000 genes from both metabolically resistant and non-resistant wild-type fruit flies. They identified dozens of genes that were different in resistant fly lines compared to non-resistant wild-type flies, Pittendrigh said. This indicates that a number of genes may be part of the metabolic resistance-causing orchestra, he said.
In metabolic resistance, an organism, in this case an insect, breaks down a toxin that normally might be fatal. Organisms metabolize the toxin or turn it into something that disables the harmful molecules, and then dispose of it.
"We have identified a series of genes that are interesting because the high abundance, or expression, of their genetic traits in resistant flies signifies they may be part of the orchestra that leads to resistance," Pittendrigh said. "But more research must be conducted before we claim whether any of these genes actually cause resistance.
"Another interesting finding that emerged from our study is that a series of genes are common to both resistant insects found in the field and those used in the laboratory. Hypothetically, this could lead to common genes that consistently have the same resistance traits across fly lines or even potentially across insect species."
If further research proves this to be true, these genes might be tools for controlling many different insects, he said.
Joao Pedra, an entomology doctoral student and lead author of the paper, said data from the study suggest that more than one detoxification gene is over-expressed in resistant insects.
"Different resistant fly lines also may have different levels of expression of these genes," Pedra said. "This may affect how resistant they are to a pesticide."
Knowing genes involved in resistance and their relationship to each other would provide scientists with information needed to develop ways to halt insects' detoxification of chemicals designed to kill them.
"It would be great if we would ultimately identify a 'conductor' gene that is critical for directing the biochemical processes that allow insects to detoxify pesticides," Pittendrigh said. "A gene or genes that may be critical for resistance, in turn, may become targets, enabling us to develop compounds to control pesticide-resistant insects."
The scientists already have found that some of the genes they're studying are involved in the process of metabolizing some pesticides, rendering them ineffective.
"We have a relatively firm grasp of target insensitivity - when a toxin will no longer bind with a molecule in an insect so the chemical no longer kills the insect," he said. "But to date, we still don't understand many aspects of metabolic pesticide resistance.
"Finding genes involved in the fundamental resistance process that also are found across insect species may provide for better resistance monitoring or even resistance management strategies."
One type of bug, the tarnished plant bug, includes two species native to the United States that cause moderate to severe damage to fruits, vegetables, tree seedlings, cotton and alfalfa. The total annual losses and control costs attributed to this one insect are $2.1 billion to $3.5 billion, according to the U.S. Department of Agriculture's Agricultural Research Service.
Pittendrigh's team used a recently developed technology to simultaneously look at all the genes in a common research animal, the fruit fly (Drosophila). The technology, high-density micro-array analysis, makes it possible to scan the insect genome and record differences between resistant and susceptible insects.
"Understanding the gene or genes that conduct the metabolic resistance orchestra would give us a way to soften the crescendo of insect damage," Pittendrigh said.
The other researchers involved with this study are: Lauren McIntyre, associate professor in the Department of Agronomy and a member of the Purdue Genomics Center Micro-Array Core Facility, and Michael Scharf, an entomology research specialist, director of the Industrial Affiliates Program and a member of the Purdue Center for Urban and Industrial Pest Management. Pittendrigh and Pedra also are members of the Purdue Molecular Plant Resistance and Nematode Team.
Writer: Susan A. Steeves, (765) 496-7481, firstname.lastname@example.org
Sources: Barry Pittendrigh, (765) 494-7730, email@example.com
Joao Pedra, (765) 494-6313, firstname.lastname@example.org
Related Web sites:
A publication-quality photo is available at http://ftp.purdue.edu/pub/uns/+2004/pittendrigh-orchestra.jpg
Genome-wide transcription profile of field and laboratory selected DDT resistant Drosophila
and Barry R. Pittendrigh
Genome-wide micro-array analysis (Affymetrix array) was used to determine if (i) only one gene, the cytochrome P450 enzyme Cyp6g1, is differentially transcribed in DDT resistant vs. susceptible Drosophila and (ii) to profile common genes differentially transcribed across a DDT-resistant field isolate (Rst(2)DDTWisconsin) and a laboratory DDT-selected population (Rst(2)DDT91-R). Statistical analysis (ANOVA model) identified 158 features that were differentially transcribed among Rst(2)DDT91-R, Rst(2)DDTWisconsin and the DDT susceptible genotype Canton-S (P( 0.01). The cytochrome P450 Cyp6a2 and the diazepam binding inhibitor gene (Dbi) were over transcribed in the two DDT resistant genotypes when compared to the wild-type Drosophila and this difference was significant at the most stringent statistical level, a Bonferroni correction. The list of potential candidates differentially transcribed also includes 63 features for which molecular function ontology annotation of the features did not exist. A total of five genes (Cyp6a2, Dbi, Uhg1, CG11176 and CG11893) were significantly different (P-value ( 5.6 e-06) between Rst(2)DDT 91-R and Canton-S. Additionally, two probe sets encoding Cyp12d1 and Dbi were significantly different between Rst(2)DDT Wisconsin and Canton-S after a Bonferroni correction. Fifty-two features, including those associated with pesticide detoxification, ion transport, signal transduction, RNA transcription and lipid metabolism were commonly expressed in both resistant lines but were differentially transcribed in Canton-S. Our results suggest that more than Cyp6g1 is over-transcribed in field and laboratory DDT resistant genotypes and the number of commonalities suggest that similar resistance mechanisms may exist between laboratory and field selected DDT resistant fly lines.