Comparison of past and present honeybee genomes reveals rapid evolution in wild honeybees populations exposed to a novel parasite.
The common honeybee Apis mellifera has been battling a mite named Varroa destructor in North America and Europe in recent decades. These invasive Asian mites are parasitic to honeybee larvae and also act as a reservoir for viruses and disease, often spelling disaster for an infected hive.
Because honeybees are an essential resource to human food supply, the massive die-offs of colonies have attracted widespread attention and research. Along with epidemiology, entomology, and parasite research, evolutionary biology could also help understand and curb the spread of this invasive parasite.
In a study published last week in Nature Communications researchers compared the DNA sequence of a past population of pre-mite bees to the sequence of a modern mite-resistant population in New York.
Their goal was to study whether or not the genome of modern day bees, after being exposed to parasitic mites in the early 1990s, changed relative to their pre-mite counterparts (specimens from 1977). Any changes could be studied to evaluate whether evolution acted upon the bee genome to confer mite resistance.
The group studied two separate pools of DNA in the bee cells: DNA from the mitochondria, which are cellular organelles used to produce energy, and DNA from the nucleus, a specialized organelle for storage of cellular DNA. Importantly, the mitochondrial DNA is passed only from mother to offspring, while the nuclear DNA is inherited approximately 50/50 from mother and father. The researchers found many differences between the mitochondrial DNA of the past and present bee populations, but very few differences were observed in nuclear DNA.
These large mitochondrial genetic differences indicate that at some point between 1977 and the current day a “bottleneck”, or large die-off, event occurred that probably killed many queen bees in the area, lowering the overall genetic diversity of this mother-to-offspring gene pool. Despite losses in mitochondrial DNA diversity, the similarity between past and present nuclear DNA shows that this pool of diversity was largely maintained.
This retention of diversity probably helped give the bees an advantage over the parasite, giving natural selection more overall sequence to act upon.
Indeed, when the researchers focused in on the few differences in past and present honeybee nuclear DNA, they found that modern day mite-resistant bees carried a change in their dopamine receptor. Since dopamine has previously been linked to grooming, the change could increase a bee’s ability to rid itself of parasites by removing them from their bodies. No experiments were conducted in this study to observe that potential phenomena since the phenotype (in this case, behavior) of the museum specimens could not be observed.
Genes responsible for larval development were also found to contain differences between the two generations of bees, which could be important because the mites are most harmful to developing bees.
Though these changes cannot be ascribed solely to the pressure of the mite-bee arms race (since other evolutionary forces could have acted in the 30+ years between the two populations), the researchers are hopeful that these insights could lead to future research on natural selection’s role in honeybee resistance to parasites.
The study also shows, importantly, that retaining nuclear DNA genetic diversity can provide natural selection with enough material to select for disease resistance over time. Therefore, maintaining genetic diversity in natural populations, especially in organisms we rely so heavily upon, will be key in future conservation efforts.