How Mitochondrial DNA Offers Clues to Giant Hornets’ Invasion
By Paige Embry
Oh, the power of a name. Late last year, reports came out that a new invasive species, Vespa mandarinia (sometimes known as the Asian giant hornet), had landed in North America. A nest had been found and destroyed on Vancouver Island, BC, which is just off the coast of northern Washington, where another specimen had been found. The potential introduction of V. mandarinia is particularly worrisome for beekeepers, whose bees are already beleaguered by Varroa mites and an array of other problems, because the hornets can destroy entire colonies. Although those first reports made the news, it was nothing like the splash the hornets made this past May. Gifted with the villainous nickname of “murder hornet,” V. mandarinia took over the news cycle for a few days.
Since those first specimens were found, scientists in Washington State and British Columbia have set up a network to monitor for more V. mandarinia, and have found a few, but they’ve also done a different type of investigation, using genetics to figure out whether the hornets in BC and Washington were related. Their results were published in September in the Annals of the Entomological Society of America.
The scientists specifically looked at the DNA found in the hornets’ mitochondria—the “power plants” that turn nutrients into energy in the cells of eukaryotes (plants, animals, fungi, and a host of other organisms that aren’t bacteria or archaea). Mitochondria are the remains of a long-ago event: a bacterium found its way into another organism and, through a series of evolutionary changes, became part of that organism. Consequently, mitochondrial DNA differs from that found in the rest of the cell. It is also matrilineal, passed down from mother to offspring without input from the father, and therefore shows much less change from one generation to the next than the DNA in the rest of an organism. This relative constancy makes mitochondrial DNA a useful tool for tracking lineages.
For the Vespa mandarinia study, the scientists sequenced the mitochondrial DNA of the British Columbia and Washington specimens, along with specimens from Japan and South Korea, and found that the North American imports do not have the same mother. While the Washington state specimen shared 99.5 percent of its mitochondrial DNA with the one from South Korea, the Canadian and Japanese specimens shared a bit more than 60 percent of their DNA. Lead author Telissa Wilson, with the Washington State Department of Agriculture, writes in an email, “We were all surprised that the U.S. and Canada samples were from different lineages when the locations were in such close proximity.”
Despite the similarity in DNA between the U.S. and the South Korea specimens, we can’t assume the U.S. hornet came from South Korea. “While our study sequenced the [mitochondrial] genome of one individual from South Korea,” Wilson writes, “we cannot say that a second individual from the same region would result in the same genetic structure.” Nor, as the authors note in the paper, can they determine if the Canadian and U.S. “specimens arrived on the same cargo or from the same populations.”
In the paper, the authors advocate for more genomic research on Vespa mandarinia in its current range. In her email, Wilson explains how that information can be helpful in developing eradication plans when the hornet spreads to new areas: “Once the genetic diversity of this species is known, having genetic information such as the mitochondrial genome will give us clues to how big our problem is and where it came from. For instance, if we had a single introduction (over a large geographic area) we would cast a larger survey grid. If there were two separate introductions, then perhaps two smaller surveys would be appropriate.”
Even knowing that the first two findings were from different queens is helpful, because it strengthens the argument for a broad monitoring and eradication effort since all the insects aren’t fanning out from one location. Plus, if two queens made the trip, there may have been more. We may have a better idea when the specimens found this year get sequenced.
Annals of the Entomological Society of America
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.