Research into flea tanning boosts complex evolution hypothesis

For the water flea Daphnia melanica, catching a tan may be a death sentence. A darker appearance makes the micro-crustacean more visible to predatory fish. But how fast can the water flea lose its tan when necessary? The answer to this question in fact could change humankind's grasp of the relationship between genetics, developmental biology, evolution, ecology and conservation.

Now, University of Kansas researchers have shown that genetic traits that react to the environment - in this case tanning in response to sunlight - can evolve quickly in response to other needs.

"We provide evidence for rapid evolution via genetic accommodation, which had been widely hypothesized but not yet demonstrated," said Alison Scoville, a postdoctoral researcher at KU's Higuchi Biosciences Center.

Scoville and her colleagues studied water fleas in California lakes. The lakes are exposed to more sunlight in summer than in winter.

As with many human beings, the fleas' color pales in winter.

Scoville compared water fleas in lakes without predatory fish to water fleas in lakes that had become home to fish during the last few decades. The normally dark color of the water flea makes it highly visible to fish and, therefore, easy prey.

Scoville discovered that Daphnia melanica in lakes with fish were lighter in color - even though the level of ultraviolet radiation had not decreased. Her research shows that over this short period of time the water fleas had evolved a permanently high expression of genes that previously were turned on only in low ultraviolet conditions.

In other words, her research showed that the water flea could use its preexisting ability to alter skin tone in response to sunlight to hide from predators instead.

"Phenotypic plasticity refers to the ability of plants and animals to adjust their growth and development in response to environmental signals," said John Kelly, associate professor of ecology and evolutionary biology and Scoville's mentor. "For example, plants will lengthen their stems under low light conditions. Animals routinely increase muscle mass with sustained exercise. Ali's research indicates that such responses to the environment are often complex."

Scoville sought to discover if such developmental adjustment could occur without the usual environmental signal - sunlight, in this case.

"As an example, muscle mass increases with exercise. Imagine that we could turn that developmental pathway on all the time, so that we would all be very strong and muscular whether or not we exercised," she said.

The Proceedings of the National Academy of Sciences recently published Scoville's results in a paper co-authored with Michael Pfrender, an associate professor of biological sciences at the University of Notre Dame.

The paper, "Phenotypic plasticity facilitates recurrent rapid adaptation to introduced predators," shows the potential for organisms to evolve physical changes without the usual environmental input, Kelly said.

In this case, water fleas were able to turn on a genetic trait, reduced pigmentation, which normally occurs as a result of reduced exposure to light, in order to make themselves less visible to predators. This complex physical change occurred rapidly and may have prevented the extinction of the water fleas in lakes with fish.

An Institutional Research and Academic Career Development Award from the National Institutes of Health and grants from the National Science Foundation supported Scoville's work.

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