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HOME > PAST ISSUE > July-August 2009 > Article Detail

SCIENCE OBSERVER

Rip Van Winkle Ecology

Biologists awaken dormant eggs to observe life from the past and tease out how it differs from what lives today

Catherine Clabby

Zooplankton that originated as diapausing eggsClick to Enlarge ImageScientists always seek new peepholes into the past. Astronomers chase light, the more dated and distant the better. Geologists hunt for ever-older rock. Paleontologists will work with nearly any trace of what’s gone extinct.

To get deeper into this game, some biologists and ecologists are now embracing life delayed.

Calling themselves resurrection ecologists, many revive so-called resting eggs, some a century old, produced by small aquatic organisms. Once hatched, these organic time capsules produce living examples of what a species used to be like, allowing ready comparisons to what exists today.

“Being able to go back in time and sort out how evolution actually happened, rather than inferring how it might have happened, is really cool stuff,” says Nelson Hairston, Jr., a Cornell University ecologist and pioneer in the field.

With these eggs, Hairston and others capture changes prompted by competition between organisms, exposure to human-made pollution and other environmental pressures. Some researchers also have big hopes of tracking climate change effects.

For the first time, an international meeting devoted to resurrection ecology will convene in September in Switzerland. There, scientists will compare lab results, brainstorm about methodology and, it’s hoped, launch new research partnerships.

This line of research owes much to diapausing eggs, produced by plankton, microparasites and some other tiny aquatic organisms. These specialized eggs are programmed not to hatch immediately. Instead they drop to the bottom of lakes or ponds and wait until local conditions improve—say, a dry spell passes, warmer water temperatures arrive or a predator’s active seasons ends.

Each year some get buried in sediments before they receive the signal—often simply the right light—to stir from metabolic resting states. Year after year, burials repeat until the sediment holds layer upon layer of dormant eggs, stacked older to younger from bottom to top.

Scientists retrieve samples of these natural egg banks from sediment cores, which they date at multiple spots using radioactive isotopes and other methods. Eggs that won’t hatch sometimes yield a different sort of experimental booty. Full genome sequences have been retrieved from eggs more than 200 years old.

“There is the promise of going even further back in time with genomic studies,” says W. Charles Kerfoot, a Michigan Technological University ecologist who coined the phrase resurrection biology about a decade ago.

Because these organisms are short-lived, the time needed to observe their adaptation is relatively brief. As many as 3,000 generations could stand between a normally hatched zooplankton and a diapausing egg of the same species trapped in sediments a century before, Kerfoot says.

Still, the pace of change can be startling. For example, Hairston and collaborators in a 1999 Nature paper demonstrated how tiny crustaceans called Daphnia rapidly adapted to higher levels of toxic cyanobacteria in central Europe’s Lake Constance. Pollution increased phosphorus levels in the lake, which increased the population of cyanobacteria, better known as blue-green algae. Filter-feeding Daphnia hatched from eggs dating from the 1960s could extract very little nutrition from the algae. But Daphnia hatched from eggs dated to the 1970s had adapted to extract more.

In 2007, Ellen Decaestecker of Katholieke Universiteit Leuven in Belgium and colleagues reported, also in Nature, evidence for the so-called Red Queen hypothesis after studying diapausing eggs of both Daphnia and a parasite in European lakes. Proposed by evolutionary biologist Leigh Van Valen in 1973, the theory was named for a scene in Lewis Carroll’s Through the Looking-Glass, where Alice, on the queen’s orders, has to run as fast as she can just to remain in place. Organisms interacting with other living things sometimes have to change simply to hold their ground, Van Valen postulated.

By testing both organisms, Decaestecker found that in as little as two to four years Daphnia species could improve at fending off invasion by a bacterial endoparasite called Pasteuria ramosa. In turn, the parasite would become more virulent, prompting Daphnia to step up its defenses again.

There is also evidence that some relatively quick changes cannot be reversed, potentially raising the stakes when it comes to the topic of human-made pollution. Nora Brede of Goethe University in Frankfurt published findings in March in the Proceedings of the National Academy of Sciences of the U.S.A. detailing changes in three Daphnia species in two European lakes. Both lakes were polluted with phosphorus from human sewage and detergents in the 1960s and 1970s but were cleaned up by the 1990s with pollution controls.

By hatching diapausing eggs dating back 100 years, Brede observed that during periods of peak pollution, two species of Daphnia interbred, creating hybrids. Because of the evolutionary trajectory each started following, she saw little chance for the previous species to re-emerge.

Some plant scientists want to expand the focus of resurrection ecology with what they call Project Baseline. At the Switzerland meeting, they will describe efforts to create plant seed banks superior to what nature produces. The scientists are looking for funding to collect seeds today—so-called “ancestral material”—rather than wait and look later. That would increase the chances of capturing wide samples of precisely dated plant seeds. Comparing plants grown from stored seeds to those that develop in natural environments could show ways climate change influences evolution, supporters say.

“People traditionally thought evolution took a really long time. But evolution can happen really fast, in just a few generations,” says Steven J. Franks of Fordham University, who is active in Project Baseline. “In a few years or so, you can start to see changes.”


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