Wild Sheep Descended From Single Pair Show Surprising Genetic Diversity (6/23/2007)

Researchers demonstrated that the mouflons genetic diversity increased over time, contrary to what the usual models predict. - Photo Courtesy: Université du Québec à Montréal

Scientists at Université du Québec à Montréal have reconstructed the genetic history of a population of mouflons (wild sheep) descended from a single pair. The researchers demonstrated that the animals’ genetic diversity increased over time, contrary to what the usual models predict. These results contradict the belief that a population descended from a small number of individuals will exhibit numerous deficiencies and reduced genetic diversity.

The mouflon population of the Kerguelen archipelago

The Kerguelen Islands in the Indian Ocean are one of four districts in the French Austral and Antarctic Territories. These islands, one of the most isolated places on Earth, house a military base and a science station. In 1957, the local authorities decided to offer residents the opportunity to hunt mouflons (a type of wild sheep). A pair of Corsican mouflons was imported from the Vincennes Zoo in Paris. Initially, the mouflon population grew exponentially, and then, from the early 1980s, it fluctuated between 300 and 700 individuals.

The history of the research project

Denis Réale discovered this mouflon population while doing his French civilian service in 1991. For 16 months, he participated in an ecological research program under the supervision of Jean-Louis Chapuis of the Muséum national d'histoire naturelle in Paris. He studied the ecology and behaviour of mammals (mouflons, sheep and reindeer) introduced into the Islands.

Ten years later, Renaud Kaeuffer, a doctoral student in biology at UQAM, supervised jointly by Denis Réale and Dominique Pontier of the Université Claude Bernard Lyon I in France, went to the Islands to study the impact of introduced cats on the bird populations. Noticing that the mouflon population appeared to be thriving, he suggested to Réale that this population could be studied from a genetic viewpoint. Dave Coltman, a professor at the University of Alberta and a specialist in ungulate genetics, agreed to perform the analyses of the genetic material.

Combining their efforts, the researchers used hair, horns and tissue to reconstitute the evolution of the genetic diversity of the mouflon population from 1958 to 2003. Through the contribution of Jean-Louis Chapuis, Denis Réale had access to samples from populations living on the Islands between 1988 and 1996. For the missing years, the researchers appealed to the hunters who wintered there. "We got the missing DNA samples from hunting trophies and managed to go all the way back to the son of the founders," said Denis Réale with a smile. "We were even able to obtain genetic material from the population of origin from the Vincennes Zoo. "We took the DNA from these samples, and looked at specific genetic sites," explained Renaud Kaeuffer. "We expected that the genetic diversity of this population of mouflons would be very homogeneous, and that this genetic diversity would decline over time. Instead, we observed the opposite."

The researchers attribute this increase in genetic variety to natural selection, as the timeframe was too short for this diversity to be attributable to genetic mutation, and the Islands are much too isolated to have undergone migrations. "This variety can be explained by elimination, over the generations, of individuals with low genetic diversity. In small isolated populations, related individuals are likely to reproduce amongst themselves, resulting in inbreeding and homozygotes.

The genetic variety of the population becomes impoverished and its evolutionary potential decreases. Furthermore, consanguinity is known to produce genetic diseases. The most heterozygous individuals are better able to resist these diseases," explains Renaud Kaeuffer. The researchers stress the point that the genetic variety of the mouflons on the Kerguelen Islands is still less than what could be observed in a larger population.

Very few researchers have carried out longitudinal studies on the evolution of genetic variety in a population. The environment of many animal and plant populations has been modified by human activity. In many cases, we are witnessing a loss of biodiversity. While scientists ask themselves about our impact on the genetic diversity of populations, this study by Denis Réale and his co-workers sheds new light on mechanisms that can regulate this genetic diversity.

This research project received financial support from the Natural Science and Engineering Research Council of Canada (NSERC).

Note: This story has been adapted from a news release issued by the Université du Québec à Montréal

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