Finally, some good news for mountain gorillas. The highly endangered apes have been so rare for so long that they have already weathered the initial genetic storms of inbreeding and could face clearer sailing ahead, if conservationists can protect their habitat.
Mountain gorillas, the "gorillas in the mist" made famous by primatologist Dian Fossey, are among the most threatened primates on the planet, numbering barely 800 individuals in two enclaves in the mountains of central Africa.
The gorillas were even more inbred than they expected, roughly the equivalent of the result of a mating between great-uncle and great-niece, they found. "It's more inbreeding than we've seen so far in any other great ape," says Scally.
High levels of inbreeding increase the odds that an individual will get copies of a harmful mutation from both parents, and thus the risk of genetic disease.
But the team show that both mountain and eastern lowland gorillas actually carry fewer of the most harmful mutations, those that knocked out gene function completely, than their more common cousins, western lowland gorillas.
To understand why, the researchers scanned the gorillas' genomes for genetic markers of low populations in the past. This enabled them to estimate how gorilla population sizes have changed over the past few million years.
Populations of mountain gorillas plummeted about 100,000 years ago and have remained low ever since, they found.
In the club
This prolonged small population may have allowed evolution to purge the gorillas' genomes of the most harmful mutations tens of thousands of years ago, thus reducing the present-day genetic cost of inbreeding. If so,conservation biologists may have one less problem to worry about as they try to preserve mountain gorillas. "There's no reason to suspect that they've gone past some genetic point of no return," says Scally.
That conjecture is plausible, but not yet proven, says Katy Gonder, a geneticist at Drexel University in Philadelphia, Pennsylvania. Further study will be needed to show whether the lack of #genetic diversity could still leave mountain gorillas more vulnerable to disease and other challenges, she says.
Scally's study makes a big contribution to conservation, says Gonder. The genome sequences show that mountain gorillas are indeed genetically distinct from the eastern lowland gorilla. This settles a long-running debate about the question, and clarifies the importance of conserving mountain gorillas despite their inbreeding. "They're still an important reservoir of genetic diversity within the species," she says.