A new genetic study of wild honey bees living in forests near Ithaca, New York, sheds light on how they rapidly evolved resistance in response to the deadly parasitic mite Varroa destructor. The mite, originally from Asia, has been implicated in causing the deaths of millions of bee colonies across North America and Europe, and yet the population in Ithaca is still going strong, despite being infected with the parasite in the mid-1990s.
“They took a hit, but they recovered,” explains Alexander Mikheyev, a professor at the Okinawa Institute of Science and Technology Graduate University in Japan and lead author of the paper published in Nature Communications. “The population appears to have developed genetic resistance.”
The researchers were able to track genetic changes that occurred in the population because samples of bees from the same forest were collected in 1977. This allowed the team to compare the DNA of the 1977 bees to some collected in 2010, covering the period during which the population became infected with the Varroa mites. An opportunity to do such an analysis – especially with bees – is incredibly rare, because few people collect the insects and fewer still preserve them in ways in which DNA can then be extracted. In fact, the scientists had to develop a new DNA analysis tool that can work using degraded DNA from museum specimens.
By comparing the bees from the same colonies, only 33 years apart, they were able to observe natural selection in action through the bees' genomes. Firstly, they found that the diversity of mitochondrial DNA in the insects, which is only passed on through the female line, was massively reduced. This implies that at some point in the bee population’s history, most of the queen bees were wiped out, with maybe only four or five surviving to repopulate the forest.
Despite this huge reduction in the diversity of their mitochondrial genes, the diversity in the rest of the bees' DNA remained high. It is this genetic diversity that increases the chance that an organism will be able to adapt when faced with pressures such as disease. The researchers were also able to observe a change in a gene called AmDOP3, which is related to a dopamine receptor gene that has been shown to play a role in aversive memory formation in bees. They also noted that the bees evolved to be smaller, perhaps to reduce the developing time of their young and thus beat the mites to adulthood.
The researchers, however, stress that because these changes have occurred over a 33-year period, they cannot say for certain if such changes were in direct response to the mite infestation, though due to the high degree of change, it is unlikely to have happened by chance. They now plan on now looking at which genes might offer resistance to Varroa mites, a process that could help commercial bee keepers and breeders to develop bees that are naturally resistant to the parasite and thus reduce their reliance on yet more pesticides.