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Locust Upset: DNA puts swarmer's origin in Africa

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12:40pm, January 4, 2006
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The desert locust, often blamed for modern crop ruin and biblical plagues, was not an ancient export from the Americas, say DNA analysts.

Some biologists had recently argued that Africa's storied locust arose from ancestors of today's New World Schistocerca species that crossed the Atlantic Ocean. That's backwards, Nathan R. Lovejoy of the University of Toronto at Scarborough now says.

Lovejoy and his colleagues used DNA sequences as the basis for a new family tree of the Schistocerca genus. Their analysis of that tree suggests that ancient locusts from Africa gave rise to the 50-or-so modern New World species, Lovejoy and his colleagues say in an upcoming Proceedings of the Royal Society B. They propose that the African locusts crossed the Atlantic several million years ago.

"We were surprised," says Lovejoy.

The term locust refers to grasshoppers that gather in swarms to feed. One of the most famous, the desert locust (Schistocerca gregaria) of Africa, can take either of two forms depending on environmental conditions. The mild-mannered green grasshoppers forage individually, but as food and water become abundant, the next generation turns black and yellow. Its members congregate into groups that can include more than a billion. These swarms can fly 100 kilometers in a day.

In 1988, biologists observed a swarm of desert locusts from Africa arrive at the coast of South America. This confirmation of extremely long-distance travel ignited interest in which direction the insects' ancestors had moved.

In 2004, Hojun Song of Ohio State University in Columbus argued that New World locusts had traveled to Africa. He based his claim on a family tree constructed with body characteristics of Schistocerca locusts. According to that tree, Song says, the desert locust seems the closest relative only to some recent New World species. Thus, he argued that species were diversifying in the New World when some of them crossed the Atlantic and gave rise to today's African desert locust.

For the new family tree, Lovejoy and his colleagues analyzed a DNA stretch that covered several genes from various species' mitochondria, or cell powerhouses. On the resulting family tree, the African desert locust forms the lowest branch and so represents the most-ancient lineage, says Lovejoy. That pattern argues that ancestors of the desert locust crossed the Atlantic to give rise to a lineage that branched out in the New World, he says.

Song, so far, is sticking to the view that desert locusts migrated out of the Americas. The new family tree, he says, doesn't include as many locust species as the old ones did. Only a family tree based on both morphological data and sequences from more genes will settle the debate, he says.

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