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Ancient Swarm of Locusts Made Trans-Atlantic Trip

By Pippa Wysong, Access Excellence

Toronto (01/25/06) - Genetic detective work has revealed that various grasshopper species in North America are descended from the desert locust from Africa that migrated to the Americas between three to five million years ago. The research solves a number of mysteries about grasshoppers and locusts on both continents, according to Nathan Lovejoy, PhD, professor of life sciences at the University of Toronto. He and Sean Mullen, PhD, of the University of Maryland, led a US-Canada study that was published in the Proceedings of the Royal Society.

Scientists have wondered whether grasshoppers in the genus Schistocerca originated in the Americas or Africa, and also how the insect managed to travel from one continent to another. A further mystery was the fact that there are more than 50 species of Schistocerca in the Americas, but only one in Africa - an unusual sort of biogeographic distribution.

The key grasshopper studied was the desert locust, or Schistocerca gregaria. "Locust is a term that applies to grasshoppers that sometimes form large swarms," Dr. Lovejoy told Access Excellence in an interview. The desert locust lives in arid habitats of Africa and Asia, but its closest relatives are all found in the Americas. Schistocerca probably evolved after the Cretaceous period when continental drift separated Africa and South America. Had they appeared prior to the separation of the continents, their appearance on both continents, and the distribution of species would not have been as much a puzzle.

The desert locust tends to be solitary in dry environments with little foliage for food. However, years with increased rainfall lead to more plant growth along with an increased number of locusts. "The locusts are more successful, they're laying more eggs and they take on a radically different biology. They actually develop swarming behaviors," Dr. Lovejoy said. As well, locusts that live a more solitary life are green, but when they go into swarming mode take on a black and yellow coloration.

Swarms are massive and can consist of billions of the insects. "A single swarm with a billion locusts can cover 1,000 square kilometers. Within each square kilometer there can be 40 to 80 million locusts," he said. And the swarms migrate, moving about 100 kilometers a day eating every bit of greenery in their path. "You can be in a country with no locusts, then the next day you can have a billion locusts sitting on top of you, eating all your crops." It was not believed that swarms of these small insects could make a trans-Atlantic journey, but that changed in 1988 when a swarm traveled from Africa to the Caribbean, a distance of more than 5,000 kilometers.

Researchers took mitochondrial DNA samples from Schistocerca from both the Americas and Africa. "We then used that DNA to make an evolutionary family tree to understand how these different species are related to each other," Dr. Lovejoy said. Analysis of the Schistocerca family tree showed that the desert locust was at the base of the evolutionary tree.

"It was a very early locust to diverge. The family tree showed that all the Schistocerca species in the Americas are closely related to each other and that they had a single ancestor. This all suggests there was a single movement from Africa to the New World to give rise to all the species in the Americas," he said. Patterns of mutations in the different species of Schistocerca indicate where on the family tree each species sits.

"The bottom line of our study is that not only did locusts cross the Atlantic in 1988, we think they also crossed the Atlantic about three to five million years ago. And that there was a single invasion, from Africa to the Americas which gave rise to all the different species in the Americas. Our study shows there was a prehistoric trans-Atlantic flight," Dr. Lovejoy said.

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