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.
end
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