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Transcriptome Sequencing Study Offers Clues to Bed Bug Genetics, Pesticide Resistance

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – In a study appearing online last night in PLoS ONE, researchers reported that they have sequenced the transcriptome of the bed bug, Cimex lectularius, creating a large database of expressed sequence tags for the pest insect.

An Ohio State University research team used Roche 454 sequencing to characterize the transcriptional profiles of insecticide-sensitive bed bugs raised in the lab as well as resistant bugs collected from a residence. In the process, they identified tens of thousands of transcript sequences in the insect, including transcripts for two detoxification-related enzymes that appear to have expression differences in pesticide-sensitive and –resistant bugs.

"This is the first study to elucidate the genetic makeup of the insect and to obtain fundamental molecular knowledge regarding potential defense pathways and genes that may be involved in metabolic resistance to commonly used pesticides," corresponding author Omprakash Mittapalli, an entomology researcher at Ohio Stat University, said in a statement.

Bed bugs are nocturnal insects that feed on human blood and live in bedding, clothing furniture, and other household items. Their numbers have exploded in North America, Europe, Australia, Eastern Asia, and elsewhere over the past decade, the researchers noted, due to factors ranging from global travel to insecticide resistance and changes in insecticide use.

But though the rise in bed bug numbers comes with economic, social, medical, and mental health consequences, they explained, not much is known about the insect's genetic repertoire and vulnerabilities.

"While bedbugs are poised to become one of the major household pests across the United States in the coming years, we know very little about their genetic makeup and their mechanisms of resistance to insecticides," Mittapalli said.

In an effort to gain a more complete understanding of bed bug genetics — and get clues about the genes that might contribute to pesticide resistance — Mittapalli and his team used the Roche 454 GS FLX Titanium platform to sequence cDNA libraries created using RNA isolated from two groups of bed bugs: a pesticide-sensitive laboratory strain and a pesticide-resistant strain derived from bugs collected in an apartment in Columbus, Ohio in 2009 and 2010.

They then assembled the sequences into 35,646 potential bed bug ESTs. Along with apparent bed bug sequences, the researchers also detected dozens of sequences representing the common insect endosymbiont Wolbachia.

When they compared bed bug sequences with those reported for other insects, the researchers found that almost half of the predicted bed bug proteins — 44.8 percent — were distinct from those housed in the GenBank database, while nearly a third resembled other GenBank sequences. Of these, almost 86 percent of predicted proteins were most similar to those in the other insects, particularly sequences from the body louse, pea aphid, and fruit fly.

Along with their protein domain, gene ontology, and pathway analyses, the team looked for genetic variants in bed bug sequences, tracking down 296 SNPs and 370 microsatellite loci that, if verified in future studies, could help in classifying and characterizing bed bug populations.

Because studies of other insects have linked chemical metabolism to some forms of insecticide resistance, the researchers also explored the potential roles of two enzymes — the cytochrome p450 gene CYP9 and the glutathione S-transferase gene Delta-epsilon — in pesticide susceptibility and resistance.

"[T]he role of detoxification and antioxidant enzymes in pesticide resistance of bedbugs is poorly understood," Mittapalli explained in a statement. "Enzymes such as cytochrome P450s and glutathione S-transferases (GSTs) have been shown in other insects to act as detoxification agents, allowing the insects to get rid of toxic compounds such as insecticides and not be killed by them."

Indeed, the team found that, at every development stage tested, bed bugs that had been exposed to pesticides had elevated levels of transcripts representing the cytochrome p450 gene CYP9.

On the other hand, they found that the GST gene Delta-epsilon was more highly expressed in resistant bugs from the so-called "late-instar" stage of development than in the sensitive bugs at this stage, though adult bed bugs from sensitive and resistant groups showed comparable levels of the gene.

By exploiting such knowledge of bed bug gene expression, researchers explained, it may eventually be possible to come up with newer and more targeted strategies for controlling the pests.

"The insecticides being used right now are based on the idea that resistance in bedbugs is caused by point mutations in genes," Mittapalli said. "But we are finding out that the mode of resistance could be attributed to a combination of changes in the bug's genetic makeup (such as mutations) as well as transcriptomic adjustments leading to differential gene expression."

Data from the study has been submitted to NCBI's Sequence Read Archive.

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