NEW YORK (GenomeWeb News ) – New research is revealing the genetics underpinning honey bees’ response to Varroa destructor parasitism — and providing new clues about what makes some bees tolerant to these destructive mites.
In a paper appearing online in BMC Genomics last week, an international team of researchers compared gene expression in honey bee pupae from two Varroa-tolerant and sensitive colonies. Using this approach, the researchers not only pinpointed gene expression changes associated with Varroa mite parasitism, they also started to uncover the potential basis of Varroa tolerance — findings that may have biological and economic implications.
“These genes are potential markers of this tolerance and thus could be used to screen natural populations for tolerant colonies,” University of Illinois post-doctoral researcher Cedric Alaux, an author on the paper, said in an e-mail message to GenomeWeb Daily News. “This Varroa diagnosis will allow the selective breeding of mite-tolerant honey bees for apiculture.”
Varroa destructor is an external mite that parasitizes honey bees, feeding on bee hemolymph, a substance akin to bee blood. The mites parasitize larvae and then move on to adults, where they pass viruses from one bee to another. For instance, the mites are a vector for the deformed wing virus, a virus that affects bee morphology and reduces their lifespan.
While there’s good evidence that Varroa move diseases around in honey bees, it’s unclear whether the mites have a role in the colony collapse disorder that has plagued North American honey bees since 2006. But since it distributes viruses and weakens colonies, it’s possible that Varroa contributes to or exacerbates CCD, author Jay Evans, a research entomologist with the United States Department of Agriculture-Agricultural Research Service Bee Research Lab, told GenomeWeb Daily News.
Not every bee is equally susceptible to Varroa, though how some bees survive the parasitism while others perish is unknown.
Alaux, Evans, and their colleagues reasoned that understanding the genetics of Varroa parasitism and tolerance could ultimately help curb Varroa damage by facilitating the development of diagnostic tools or by augmenting mite-resistant bee breeding efforts.
They used microarrays created from honey bee expressed sequence tags to compare the expression of roughly a third of the honey bee genes — about 5,000 — in pupae from four different colonies representing tolerant and sensitive bees that were or were not parasitized. They verified their microarray results by using real-time quantitative PCR to test the expression of four of the differentially expressed genes detected by microarrays.
The researchers detected 116 cDNAs that were expressed differently in Varroa-tolerant and sensitive bees. Meanwhile, the expression of 32 genes differed in parasitized and non-parasitized bees. Of these, nearly half were up-regulated and just over half were down-regulated when the pupae were exposed to Varroa. Just two genes had expression profiles that depended on both tolerance/sensitivity and Varroa parasitism.
“We’re still puzzling over some of them,” Evans said. When they embarked on the project, he added, the team suspected they might find see expression differences related to immune response.
Instead, differentially expressed genes tended to be those related to nervous system function and processes such as neuronal development, locomotion, neuron excitability, learning and memory, and smell. That led them to speculate that behavioral differences — namely distinct grooming habits — may augment resistance in Varroa-tolerant bees.
“Previous studies have shown tolerant bees show more efficient hygienic behavior, probably because of a higher olfactory sensitivity and responsiveness than sensitive bees,” Alaux explained. “These genes are potential candidates for the behavioral tolerance, but this remains to be tested.”
More work will also be necessary to tease apart the consequences of Varroa parasitism from those of concurrent viral infections, Evans noted. Even so, the results provide new insights into the hallmarks of bees’ response and tolerance to Varroa and open the door for more research in this area.
For their part, Alaux and his co-workers are looking at a broader set of genes and comparing tissue-specific gene expression, focusing on gene expression in the brains of Varroa-sensitive and tolerant bees.
“We are now using a new honey bee oligonucleotide microarray, based on the recently sequenced honey bee genome which contains all the genes of the bee genome,” Alaux said.