Worldwide, zoos spend millions of dollars each year transporting rare animals thousands of miles in order to breed them with their most distantly related relatives. Some have questioned the need for such programs, which can stress rare animals, even to the point of death.
Rice's results, which are available online, are scheduled to appear in an upcoming issue of the journal Zoo Biology. The results are based on a yearlong study of 11 generations of houseflies. The study is the first to compare the so-called "maximum avoidance inbreeding," or MAI strategy, with regimens that allow limited inbreeding.
"In previous studies, a number of groups identified short-term benefits for breeding schemes that used limited inbreeding in order to produce stronger individuals," said Lisa Meffert, assistant professor of ecology and evolutionary biology and the lead researcher on the project. "Ours are the first tests of the long-term consequences of these strategies. In particular, we wanted to simulate several generations of captive breeding followed by several generations of breeding in the wild."
Meffert, post-doctoral researcher Stacey Day, graduate student Sara Hicks and pre-med student Nsuela Mukana found that populations in both breeding groups exhibited similar levels of fitness and fertility as long as they remained in a controlled, "captive" setting. However, after the simulated release into the wild, the MAI populations were less likely to go extinct or to suffer population crashes than were the populations that had undergone limited inbreeding.
"The benefits of maximum-avoidance inbreeding were difficult to detect as long as the populations remained captive," said Meffert. "It's not clear why this is the case, but it could be that genes lost through minimal inbreeding allowed the MAI populations to better adapt to the harsh conditions in the wild."
Meffert chose to conduct her experiments on common houseflies because they breed every few weeks. This model system allowed her to simulate several years of breeding for endangered species that breed only once or twice per year.
In order to simulate small populations of rare animals in zoos, breeding lines were founded by just five pairs of males and female flies. Moreover, because flies lay many eggs, and most higher-order animals have small broods, each fly breeding line was allowed to grow by no more than 50 percent per generation.
Each time a pair of flies mated, researchers measured the fitness of the pair by counting the number of eggs the female laid, and by measuring the number of eggs that hatched.
In the MAI breeding scheme, brothers and sisters were never allowed to mate and were instead paired only with offspring from other breeding pairs. In the alternate breeding scheme, the females were allowed to contribute a disproportionate share of offspring to subsequent generations, based on their fitness scores.
The release into the wild was simulated by adding environmental stress. In the "post-release" breeding scenario, temperatures in incubators were altered on a 12-hour cycle based upon the high and low temperatures recorded in Houston that day.
In both strategies, all breeding lines survived in captivity. In the "wild" setting, four of the inbred populations went extinct, compared to just one extinction for the MAI lines.
"Apparently, the stressful environment served to select only the most fit lines while the more benign environment allowed low-fitness lines to persist in captivity," said Meffert.
Though the findings clearly support the case for expensive MAI breeding schemes for some endangered animals, Meffert suspects that might not be the case for species that reproduce faster or have larger broods. For those animals, alternate breeding schemes might indeed prove more effective, and her group is conducting research to see if that's the case.