Grey Gustafson discovered that flightless tiger beetles did not cross oceans to reach North America. DNA sequencing showed these insects lived on the supercontinent Gondwana before it split into South America and Africa. "The group is actually older than when the continents were separated," Gustafson said about the ancient lineage.
They can't fly or swim, so why do their closest relatives appear on different continents? Grey Gustafson, an associate professor in the Department of Biological Sciences, discovered through DNA sequencing that the ancestors of some of today's tiger beetles existed in South America and Africa before the two continents separated. What's more, North America's largest tiger beetles are related to these ancestral South American giant tiger beetles.
Gustafson and his colleagues recently published their findings in Proceedings of the Royal Society B.
Gustafson's research began with two groups of tiger beetles found in the western U.S.: the genus Amblycheila, called the giant tiger beetles, and the genus Omus, the night-stalking tiger beetles. The previous hypothesis was that both groups belong to the tribe Manticorini—named for the mythical chimeric monster—which also contains large-bodied, flightless tiger beetles found in the Southern Hemisphere.
If that were true, Gustafson said, it would raise the question: How and why did tiger beetles end up in North America when most species of the tribe live in sub-Saharan Africa, with the only other close relative being found in South America?
"We wanted to know how a large-bodied, flightless beetle could get across the ocean to South America and then, better yet, how our North American beetles fit into this tribe," Gustafson said.
The process
Scientists have done substantial research to better understand the biology, classification, life history and conservation of tiger beetles, but they've done comparatively little to understand the evolutionary history of these insects.
So, Gustafson spent three years generating data that could reveal more about the evolutionary history of North American manticorine tiger beetles. With help from colleagues, Gustafson collected and compared DNA sequences from across their entire genomes.
"We assembled genomic sequences from freshly collected material in the western U.S., ethanol-preserved frozen material from colleagues and specimens held in natural history collections in the U.S. and Europe," Gustafson said.
The research team then used the data to produce a family tree to understand the most recent common ancestors. With this information, researchers were able to trace when the lineages diverged from one another.
"The exciting thing we found was that the group is actually older than when the continents were separated, so they didn't actually get across the oceans," Gustafson said. "They originated before an ocean separated South America from Africa on a supercontinent called Gondwana."
An ancient lineage
Gustafson and his colleagues found that not only are the giant tiger beetles of North America most closely related to the South American tiger beetle, but together these tiger beetles are most closely related to those found in southern Africa.
The family tree produced by DNA sequencing revealed the Manticorini tribe evolved at a time when South America and Africa were part of a landmass known as Gondwana, which later separated into the distinct continents we know today.
"Because of the size of the supercontinent, many biologists hypothesized that lineages with lots of closely related modern diversity in Africa and South America likely originated when the continents were still part of Gondwana," Gustafson said.
"This hypothesis has especially been used to explain how animals that are unable to fly but have close relatives on either side of the Atlantic Ocean could be distributed on two landmasses currently separated by an ocean."
The geographical phenomenon, known as vicariance, occurs when a barrier forms, splitting different populations of the same species between the land on either side. While not limited to tiger beetle research, vicariance continues to be part of a larger ecological narrative regarding evolution.
Like the rheas of South America and the ostriches of Africa, these tiger beetles are large, flightless and separated by an ocean. However, unlike these birds, the ancient manticorine tiger beetles now serve as a well-supported example of Gondwanan vicariance.
"The answer to our question of how these giant beetles got across the ocean was that they actually didn't," Gustafson said. "As the continents separated, they took their giant tiger beetles with them."