By Melissa Mayer
If you sweep a net in just the right spot in grassy fields along the Gulf and Atlantic Coasts—from eastern Texas to central North Carolina—you might encounter something peculiar: the only all-female species of praying mantis.
That’s precisely what three researchers from Washington and Lee University in Lexington, Virginia, did in 2016 and 2017. The results of those research road trips, published in March in the Annals of the Entomological Society of America, may finally solve the puzzle that is the praying mantid Brunneria borealis, which reproduces entirely asexually.
“I had been interested for a long time in doing some basic biology on this particular praying mantis because there’s almost nothing in the refereed literature since 1948 when White said quite a few things about this mantid and apparently closed the door,” says Lawrence Hurd, Ph.D., professor of biology at Washington and Lee and lead author of the paper. “White” is Michael J.D. White, a zoologist whose four-page article about B. borealis 76 years ago has long stood as one of the only close studies of the species. “I mean, he just shut the door, and nobody opened it.”
A Puzzling Mantid
While there are other mantids that reproduce asexually under the right circ*mstances, the fact that B. borealis only reproduces asexually—with no males specimens at all—has intrigued scientists for decades. (Reproducing asexually is known as parthenogenesis; animals that do under some circ*mstances are called facultative parthenogens, while those that do so exclusively are obligate parthenogens.)
But it isn’t just their asexual mode that’s peculiar. These mantids lay unusual egg cases, too. While most mantid egg caes (or oothecae) bear a ridge from which a flush of nymphs emerge, the B. borealis egg case has a single opening on one end from which the nymphs exit one at a time over months.
Hurd says that means the nymphs come out when prey is available, spreading out the inherent risk of starvation. The process of emptying a B. borealis egg case can take as long as four months—so, by the time the last egg within the ootheca hatches, other individuals from the same egg case have already reached adulthood.
So, how did this wingless mantid, with its unique reproductive strategy, expand its presence to a range comprising more than 2,400 kilometers?
Tracking Them Down
To find that out, the research team set off on a pair of field trips, spending long, hot days figuring out the mantid’s preferred habitat—clumped, warm-season grasses near the coast—and the best way to collect it.
“If you try to find them by searching the vegetation, you’ll never do it,” says Hurd. “But if you sweep net over the grasses and then walk back and look where you’ve just swept, what these animals do—which is unique in my experience—is actually come up in the grass to see what’s going on by the net the second time around.”
Once they knew where to look, the team could collect four to six individuals, including different instars, within 15 minutes of sweeping.
The Mystery Deepens
Back in Virginia, the team turned to DNA barcoding, using a 610-base pair region of the mitochondrial cytochrome oxidase I gene. They found no variation in this genetic sequence for 24 individual mantids collected from nine sites across seven states. They say that’s unusual and suggests the mantid’s introduction to North America was fairly recent.
Without any genetic variation at all among this population, it’s impossible to pinpoint the time of that introduction more precisely. But the team does have a remarkable idea about how it may have occurred.
“We frame it as a hypothesis that it was a single individual,” says Hurd. “Because that makes sense, even though the timing and the distribution looks ridiculous for that supposition.”
It’s possible that one single female mantid was bundled up with grasses in transit to North America near the turn of the 20th century. Once the mantid disembarked from the ship—all alone on a vast continent—she simply hit copy and paste, her wingless offspring slowly making their way across their North American range. Hurd has seen other mantids spread when they lay their egg cases on trucks, trains, or even bicycles.
The team isn’t done cracking the case of B. borealis. Five species of Brunneria mantids live in South America, all facultative parthenogens with both sexes present. That includes an interesting population in Uruguay that the team describes as morphologically indistinguishable from the North American mantid. Their next study will look at nuclear DNA in these species to see if they are truly distinct species. Watch this space!
Read More
Annals of the Entomological Society of America
Melissa Mayer is a science writer and the human behind Washington State University’s science cat, Dr. Universe. Email: melissa.j.mayer@gmail.com.
