The Bosmina resting egg pictured above was retrieved from Portage Lake in Michigan’s Upper Peninsula.

In the 1990s, an international team of scientists discovered a method to hatch microscopic animals from eggs more than a century old. The eggs were extracted from the remains of zooplankton collected from lake sediment and hatched in an incubator. The zooplankton subsequently grew to maturity. This feat of perpetual reproduction, which has come to be known as “resurrection ecology,” is revolutionizing the study of evolution.

Researchers at Michigan Technological University, in Houghton, are at the forefront of resurrection ecology. From the sediments of Lake Michigan and Portage Lake, in the Keweenaw Peninsula, eggs dating back nearly a century are being retrieved, hatched and compared with their contemporary cousins to track changes in the species.

A Daphnia retrocurva resting egg retrieved from Portage Lake.

The value of this research process is immeasurable. In the case of zooplankton, as many as 30 successive generations may live and die in a lake in a single year. If the eggs from each generation are viable for 100 years or more, a sediment sequence may contain 3,000 generations in total. In human terms, the hatching of a 100-year-old zooplankton egg is equiva-lent to resurrecting a human being from the dawn of Homo sapiens more than 120,000 years ago.

During the past several years, the Michigan Tech researchers have been hatching the eggs of a small crustacean, Daphnia retrocurva, to test the hypothesis that predators and their prey constantly evolve in response to changes in each — or perish in the process. Their work offers evidence that predators and prey do simultaneously co-evolve as partners in a dance through time. Additionally, there is evidence that individual species undergo rapid changes to increase their resistance to predators, disease and environmental hazards as well as their ability to compete with other species.

The scientists are using D. retrocurva because the eggs are relatively easy to identify visually. Portage Lake and Lake Constance in Germany are ideal sites for this research because both feature sediments that are simple to date using the radioactive isotopes lead 210 and caesium137. These two isotopes are preferable to carbon 14 for dating sediments because their radioactive half-lives (22 years and 30 years, respectively, compared to the 5,700-year half-life of carbon 14) produce more exact measurements. Lead 210 and caesium 137 can accurately determine the age of lake sediments within two years to three years, whereas carbon 14 can only date a sample within a range of 100 years.

In the case of D. retrocurva from Portage Lake, scientists wanted to know what changes, if any, have occurred in the past 80 years, a period when the lake experienced major upheavals due to mining, dredging and stagnation.[1]

It was discovered that D. retrocurva changed significantly during the 80-year period under study. In particular, there were changes in their helmets and spines in direct relation to fluctuations in predator populations — changes that would make D. retrocurva less appetizing. In other words, as the number of predators increased, the D. retrocurva changed in ways that would help to preserve its numbers against greater predation. Such micro-evolutionary adjustments had been observed in D. retrocurva fossils, but resurrection ecology brought the historical record alive.

The pioneers of resurrection ecology[2] assembled in Germany in the 1990s to explore ways to revive the zooplankton populations of eutrophic lakes. By demonstrating the viability of decades-old and century-old eggs, they established a new process to test many hypotheses that had been difficult, if not impossible, to examine because of time scales. By retrieving "resting" or "diapausal"[3] eggs for DNA analysis, enzyme characterization and other testing, scientists can now examine evolution over time and space; document the timing and frequency of local colonization and extinction events; and provide an accurate historical biological assessment of ecosystem perturbations.

The first research employing resurrection ecology involved cladocerans (small freshwater crustaceans) and sediment chemistry that were examined alongside changes that could be documented through conventional study of fossils.[4] The study of D. retrocurva eggs from sediment cores revealed that the species changed genetically over the span of just 80 years. In most instances, the evolutionary change was evident in the spines and helmets of the organisms to adapt to the environment and the threat of predators.

More recent experiments have involved microparasites, pathogens and epibionts.[5] Michigan Tech scientists have hatched eggs from all three categories, although their research has not yet extended to tracking evolutionary changes.

The Red Queen Hypothesis

"Well, in our country," said Alice, still panting a little, "you’d generally get to somewhere else — if you ran very fast for a long time as we’ve been doing."

"A slow sort of country!," said the Queen. "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that."

— Lewis Carroll in Through the Looking-Glass
and What Alice Found There

Researcher Leigh Van Valen, at the University of Chicago, postulated in 1973 that organisms must continually evolve for a species to survive. He dubbed his theory "The Red Queen Hypothesis."

Van Valen was fascinated by changes in the shells of mollusks and their rates of extinction. When he plotted the species’ extinction rates on a logarithmic scale, he obtained a straight line, which suggested a constant rate of extinction. He viewed the changes in the mollusks’ shells as both evolutionary responses to changes in the environment (morphological) and the consequences of natural selection (genetic) based on competitive species and predatory interactions.

Van Valen believed that his hypothesis could not be proved because testing at that time could only be done using fossil records. But using only fossils, which are non-living remains, was very difficult because of the absence of samples from generations uninterrupted across time. When scientists have access to the ancestors of organisms that are still living, it is possible to study behavior and reproduction. In any event, because most fossils are scattered, they do not constitute a continuous time record.

To prove the theoretical co-evolution between host and pathogen, scientists required the genetic feedback provided by resurrection ecology, that is, live specimens from hundreds if not thousands of generations. Prior to the research at Lake Constance and Portage Lake, such study seemed unlikely, if not impossible.

Using resurrection ecology, scientists gathered evidence that predators change along with their prey, proving that the Red Queen Hypothesis holds true for the microorganisms from Portage Lake. Resurrection ecology also has enabled scientists to study evolution prompted by environmental changes. What has been witnessed is the essential ability of species to compete and survive through time. In other words, species must continually evolve in order to survive.

Because a great quantity of aquatic species in temperate lakes produce resting eggs, and these eggs subsequently have been buried in conformable sediments (chronologically layered sediments that form on top of each other), scientists are better able to track lineages of species through time. In terrestrial environments, trees and shrubs produce seeds, but the burial in soil is much more erratic and difficult to date. The viability of eggs from aquatic microorganisms, after more than a century, demonstrates nature’s remarkable resiliency. Within the stores of reclaimed eggs are untold discoveries just waiting to be hatched.

[1] Eutrophication of a lake occurs when an excess of nutrients such as nitrogen and phosphorous promotes inordinate growth of aquatic plants, resulting in the depletion of oxygen.

[2] The author; N.G. Hairston, Jr. and C.E. Caceras of Cornell University; and L.J. Weider of the Max Planck Institute of Limnology.

[3] A form of hibernation marked by reduced metabolism and activity.

[4] Hairston et al. 1999b, 2001; Kerfoot et al. 1999; Kerfoot and Weider 2004; Jankowski and Straile 2003.

[5] Organisms that attach to the exoskeleton of a host organism, but cause no direct harm.