Did you know that the female European roe deer (Capreolus capreolus) uses embryonic diapause to control when her fawn will be born? From the late-summer mating through the end of December, the embryo remains at about the 30-cell stage in a state of diapause. This delays implantation of the fertilized egg until a more ideal time for birth.
Diapause is a genetic switch that results in suspended animation in which embryonic cell growth and development are reversibly stopped or slowed. Diapause occurs in a diverse range of vertebrates and invertebrates that includes insects, copepods (crustaceans), fish, birds, rodents, and marsupials (animals with pouches, such as the kangaroo) and other mammals, including deer.
In chicken eggs, development stops whenever the hen gets off the egg and the eggs cool down and starts back again when the hen returns to her nest. This is considered a temperature-dependent diapause.
Diapause differs dramatically from hibernation and aestivation in that during diapause, every cell ceases growth, achieving complete dormancy, with the only energy expended to maintain cellular integrity. In hibernation and aestivation, metabolism only slows as a survival reaction to harsh conditions. Diapause, on the other hand, begins before these severe conditions occur and is brought about by early triggers that presage a change in the environment.
During diapause, cell growth and development are reversibly stopped or slowed. Annual killifish eggs in diapause, for example, can survive for months or even years! Recent findings suggest that the female killifish, in response to environmental cues, relays a message to the egg to stop growing for a fixed amount of time. Essentially, the biological clock stops. These “sleeping” embryos can remain alive for periods two times longer than their normal life expectancy! When diapause is broken and the embryo “wakes up,” it continues to develop and hatch – as if time had stood still until then!
Here are examples of diapause, hibernation and aestivation:
Did you know the yucca moth (Prodoxus y-inversus) has the longest reported diapause among insects? Adults emerged 19 years after pods of the plant Yucca baccata containing the pre-pupa stage of the moths were collected in Nevada and stored.
Source: Webster's New International Dictionary of the English Language, published in 1911.
Diapause lasting more than a year has been observed in many insects, including pests, and the times of hatching can be variable because of this delayed hatching mechanism. This delay is critical in agriculture, where the application of insecticide must be applied intermittently because the casings surrounding the unhatched young as well as the dormant young themselves are often resistant to insecticides. If diapause researchers found a way to force hatching to occur all at once among these prehatched insects, then farmers could eradicate all of them in one application and greatly reduce the pesticide load on the environment.
Diapause could also assist in curtailing disease. In the case of malaria, for example, diapaused eggs of Triops, killifish or other temporary pond animals that thrive on mosquito larvae, could be easily transported to countries where malaria is rampant and air-dropped into larvae-infested ponds. These biological control agents would quickly hatch and eliminate the larvae and thereby slow the spread of the disease. This would be particularly beneficial in Africa, where the spread of malaria has reduced the immune systems of its victims and left them more vulnerable to AIDS infection.
Understanding the factors that control diapause could be applied to human medicine in other ways. For example, the rapid growth of embryonic cells are quite similar to the out-of-control growth of cancer cells. If diapause researchers can isolate the chemical factors that induce diapause in embryonic cells, this might be applied to slow or even stop cancer cells.
Diapause research could even assist research on aging and give a boost to the cosmetic industry.
It is even within the realm of possibility that one day, diapause research could even be applied to planetary travel, which requires long periods of time and could be eased by periods of animal dormancy.
Students from Glenbrook High School in Illinois
selected our killifish for two NASA-sponsored studies aboard
the Space Shuttle in a program was underwritten by Instrumentation
Technology Associates, Inc. (ITA Student Space Shuttle Experiments on CMIX-5 STS-80.
Media coverage of these incredible killifish has included articles in Discover, First, Woman's World, Family Circle and Forbes' Egg magazine; coverage on Fox Network News, Prime Time Pet, New York's Eyewitness News and MTV; plus features in thousands of local newspapers.
PART 3: WHY USE THE ANNUAL KILLIFISH
TO STUDY DIAPAUSE?
Killifish are a group of fish that live in temporary ponds in the tropical and subtropical regions of Africa and South America. They are called “annual” because the seasonal drying up of these temporary ponds kills off the entire adult population of killifish. The killifish life cycle starts all over again, however, when the rainy season returns and the killifish eggs that were deposited in the mud of these ponds begin to hatch daily over a period of weeks. “Killi” means “small pond.”
Because of their unusual ability to survive long periods of drought, annual killifish are a remarkable example of species survival through diapause. For research, they are especially important as the only vertebrate in which diapause can be predictably controlled for scientific study. This has led to a rich historical record of research regarding these fish.
Diapause occurs naturally in killifish embryos when the temporary ponds that are the killifish's habitat disappear during the annual dry seasons. While all the adult killifish face extinction during these dry periods, the female assures species survival by laying eggs that partially develop and then go into diapause. When the ponds dry up, the adult killifish die off, but the eggs survive for days, months - or even years! - until the rains return and the ponds fill once again. Under normal conditions, newly hatched fry can reach adult size and reproduce in less than six weeks, making them great candidates for scientific study.
The stages of development and diapause of the annual killifish have been fully described by Wourms (1972) and summarized here.
Using the techniques published on this Web Site to simulate the temporary pond habitat of the colorful, exotic African and South American annual killifish(Teleostei: Cyprinodontidae), scientists at the Diapause Research Foundation are discovering the secrets of suspended animation that have allowed the embryonic eggs of these fascinating jumping fish to withstand months — and even years! — out of water.
Because of DRF’s ability to mass produce these embryos, the Foundation is in a unique position to pioneer research that will for the first time illustrate how environmental stimuli induce the adult killifish to either produce diapausing or non-diapausing eggs.
By carefully establishing same-age killifish embryos that either remain in diapause or that bypass diapause, a biological assay can be developed to extract and isolate chemicals that affect diapause. These chemicals, then, can be used to test their value as drugs of biomedical importance.