Researchers from the Hebrew University of Jerusalem last month reported on the discovery that double-stranded RNAs could be used to fight the honey bee parasite Varroa destructor when fed to the host, opening the door to a new strategy for fighting one of apiculture's biggest threats.
The discoveries are licensed exclusively to Beeologics, which develops RNAi-based treatments for bee diseases such as Israeli acute paralysis virus and was acquired by Monsanto in 2011.
According to the publication, which appeared in PLoS Pathogens, Varroa feed on the hemolymph of developing and mature bees. Hives infected with the mites, which carry a number of bee viruses, typically collapse within a few years.
Ilan Sela, the senior author of the paper and former CSO of Beeologics, noted that chemicals have been used successfully to kill the mites, but they quickly develop resistance, creating the need for alternative control strategies.
Noting that dsRNAs ingested by bees remain functional and that the approach had been successfully used by Beeologics, he and his colleagues hypothesized that the RNAi-mediating molecules could be horizontally transferred from honey bees to Varroa and then to parasitized bees, he told Gene Silencing News.
In initial testing, bees were fed a sugar solution containing dsRNAs against GFP, and the researchers found a direct transfer of the dsRNA from the bees to phoretic mites feeding on their hemolymph, according to the PLoS Pathogens paper.
“To test for direct horizontal transfer, we placed 30 worker bees in plastic containers, and fed them with dsRNA-GFP in a 50 percent sucrose solution for 8 days,” the investigators wrote. “Varroa mites were introduced to the containers on the fifth day of feeding.” After three days, the mites that were attached to bees were removed. An analysis revealed the presence of the GFP sequence in the parasites, indicating bee-to-mite transfer.
Additional testing demonstrated that the dsRNAs were delivered by food ingestion, in light of previous findings that showed mites can absorb dsRNA and siRNAs through physical contact.
Notably, the researchers also found that Varroa that had acquired dsRNAs from a bee host could transfer them to untreated bees, indicating direct reciprocal transfer of the RNAi triggers from bee to mite and on to another bee.
With the ability to transfer dsRNA from bee to mite established, the scientists examined whether an RNAi approach could be used to control Varroa. Sela noted that the mite's genome was not published at the time, so the team focused on key genes that were conserved across various insect and mite species.
Specifically, they looked to “fundamental housekeeping genes involved in cytoskeleton assembly, energy transfer and transcription,” according to the paper. “In addition, we chose genes involved in apoptosis inhibition, assuming that their silencing would enhance apoptosis.”
“From these conserved sequences, we went into the Varroa genome to fish out the actual Varroa genes,” Sela said. “Once we had the sequence of the Varroa genes, we targeted them.”
The researchers found that when these dsRNAs were fed to bees, mites infecting the insects displayed suppressed levels of the targeted genes by between 35 percent and 60 percent.
“Once we had demonstrated silencing of several Varroa genes, we proceeded to monitor mite survival,” they wrote in their paper. After confirming that the dsRNA treatment had no deleterious effect on bees, they measured the effect of the RNAi approach on mite populations in infested hives.
They found that treatment could cut the mite populations by up to 61 percent compared to untreated controls. This, Sela said, is on par with standard chemical-based interventions.
Despite the positive outcome of the study, it is unclear whether it will be advanced further.
Sela said that he is no longer working on the project since he left Beeologics, and officials from Monsanto did not return a request for comment.