‘Born-again’ eggs offer peek into past

CORNELL (US)—Researchers in a field known loosely as “resurrection ecology” are breathing new life into microscopic creatures, finding it possible for them to remain viable for decades—and in some cases—even centuries.

Nelson Hairston Jr., professor and chair of Cornell University’s ecology and evolutionary biology department, studies the eggs of zooplankton—tiny, free-floating water animals—buried in lake sediments. The deeper the eggs lie, the older they are.

The researchers take sediment cores from lake floors to extract the eggs and then place them in optimal hatching conditions, such as those found in spring in a temperate lake, and let nature take its course.

“We can resurrect them and discover what life was like in the past,” explains  Hairston. “Paleo-ecologists study microfossils, but you can’t understand much physiologically or behaviorally” with that approach.

By hatching these eggs, Hairston and others can compare time-suspended hatchlings with their more contemporary counterparts to better understand how a species may have evolved in the meantime.

Hairston first became interested in the possibilities of studying dormant eggs in the late 1970s. He noticed little red crustaceans—known as copepods—in the pristine lake behind his Rhode Island home disappeared in the summer and  returned as larvae in the fall.

Further research revealed the copepods stay active under the ice in the winter, but die out as their eggs lie dormant on the lake floor through the summer when the lake’s fish are most active. When the fish become less active in the fall, larvae hatch from the eggs, and the copepods continue their life cycle.

This time suspension, where zooplankton pause their life cycles to avoid heavy predation or harsh seasonal and environmental conditions, also increases a species’ local gene pool, with as much as a century’s worth of genetic material stored in a lake bed, Hairston says.

When insects, nesting fish, and boat anchors stir the mud, they can release old eggs that hatch, offering a wider variety of genetic material to the contemporary population.

Hairston and colleagues published a paper in Nature in 1999 that described how 40-year-old resurrected eggs could answer whether tiny crustaceans called Daphnia in central Europe’s Lake Constance had evolved to survive rising levels of toxic cyanobacteria, known as blue-green algae.

In the 1970s, phosphorus levels from pollution rose in the lake, increasing the numbers of cyanobacteria. The researchers hatched eggs from the 1960s and found they could not survive the toxic lake conditions, but Daphnia from the 1970s had adapted and survived.

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