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Monday, August 14, 2006

Mussels Quickly Evolved To Foil a New Predator Crab

Mussels living along the U.S. Atlantic coast have acquired a novel genetic defense against a newly introduced species of predator crab, providing an example of how quickly evolution can work when the pressure to adapt is intense.

The story begins back in 1817, when the European shore crab, Carcinus maenas, began to settle in U.S. waters. New England mussels were easy prey for those crabs. But over time, evolution favored mussels that, by chance, had developed a new capacity: They could detect a chemical released by the crabs and use that waterborne signal to spur the growth of an extra-thick protective shell. Thus, mussels living in "tough" neighborhoods had thicker shells, and all was well.

That balance changed in 1988, when a new invasive species of crab -- the Asian shore crab, Hemigrapsus sanguineus -- arrived in ships. Asian crabs do not exude the same chemical that their European predecessors do, so mussels did not thicken their shells in neighborhoods where the Asian crabs settled, and they became a favorite snack for the new invaders.

Now University of New Hampshire zoologists Aaren S. Freeman and James E. Byers have found that mussels living along stretches of the East Coast inhabited by Asian crabs have developed the ability to grow thicker shells in response to the chemical released by those Asian invaders.

By contrast, they found, mussels living off the coast of northern Maine -- where the Asian crab has not yet spread -- lack that capacity. That is exactly what evolutionary theory would predict, the team notes: A mutation will become common in populations where it is useful, reflecting the enhanced survival of individuals lucky enough to have it, while it will generally remain rare in other populations where it offers no particular advantage.

The work is described in the Aug. 11 issue of the journal Science.

-- Rick Weiss

A 5th-Century Apartheid May Explain Britons' Genes

Long before Adolf Hitler dreamed of doing it, Germans colonized Britain and probably instituted a South Africa style apartheid regime there, scientists have concluded.

Archaeologists and historians have long believed that no more than 200,000 Anglo-Saxons from Germany invaded Britain during a 300-year period starting in the 5th century. But given that there were 2 million native Britons at the time, biologists have puzzled over how -- in less than 15 generations -- 50 percent of the English gene pool has acquired Anglo-Saxon Y chromosomes.

Now researchers at University College London and other institutions have concluded that the only way to square those facts is by visualizing an apartheid regime, in which the invaders restricted intermarriage and instituted a brutal system of exploitation such that Anglo-Saxon offspring were far more likely to survive into adulthood than the children of local Britons, who were then called Welshmen. Legal texts from the time support the idea: Anglo-Saxon lives were valued two to five times as much as the lives of Welshmen.


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