Mountain gorillas are among the world’s most endangered animals. But while they’ve captured both the public eye and the attention of primatologists, Dr. Chris Tyler-Smith of the Welcome Trust Sanger Institute says we’re just now figuring out how their unique position has shaped their genes. Tyler-Smith and a large team of other researchers have just published the results of a genome sequencing study, which sheds light on how gorillas developed, how to improve conservation, and how inbreeding works in some of humankind’s closest relatives.
On a broad scale, humans are genetic outliers among the great apes — we’re a single species that lives in huge numbers all over the world. But mountain gorillas are at the very opposite end of the spectrum. This subspecies of gorilla was once reduced to a few hundred members, and even now, only about 800 are estimated to exist. In the 1980s, Dian Fossey’s book Gorillas in the Mist (and the Sigourney Weaver film adaptation) helped draw attention to their rapidly dwindling numbers, and conservation efforts pulled them back from the brink of extinction.
Mountain gorilla populations have probably been small for thousands of years
For a study published in Science today, the team sequenced the genomes of seven mountain gorillas and six eastern lowland gorillas — two divisions of the same eastern gorilla species. They compared them against genomes from the far more numerous western gorilla species. Unsurprisingly, there was less genetic diversity and more inbreeding. Less predictably, though, this wasn’t because humans had pushed them towards extinction. "I had thought beforehand that it was probably modern human influence in the last centuries that had really decimated the mountain gorillas," says Tyler-Smith. "But we see from the genetics that they've been very small in number for much longer than that."
Specifically, the team believes mountain gorilla populations have been in the hundreds for "many thousands of years." By comparing the mother and father’s contributions to a gorilla’s genome, they could roughly predict a common ancestor for specific sequences, then use that to estimate the population size over time. They could also look for genetic similarities with other gorilla communities, which would indicate cross-breeding. The genomes actually suggest that western and eastern gorillas completely separated 20,000 years ago, after drifting apart for 150,000 years. It’s possible to take these results and speculate about what finally drove them into separate species — the researchers point to the last Ice Age, which radically changed their environment.
Eastern gorillas’ tiny numbers also produced some unusual results — in short, a lot of inbreeding. In a western lowland gorilla, around 14 percent of the genome length will be homozygous, meaning that the mother and father passed down identical genes. In mountain and eastern lowland gorillas, though, the number is closer to 35 percent. "That is the most extreme inbreeding reported in any of the great apes," says Tyler-Smith, "far more extreme than in any human population. So that very large amount of inbreeding was a surprise." Inbreeding can increase the chance of getting rare genetic diseases, since it’s more likely that both parents will have a copy of a harmful gene. In the case of the mountain gorillas, though, the population was so small that the process appeared to purge the most harmful variations altogether. Geneticists have predicted this effect, Tyler-Smith says, but until now, it’s never been found in great apes.
"You can't just go up to a gorilla and take a blood sample from it."
Genome sequencing is a well-tested technology, but this study could suffer from the fact that a fairly small number of gorillas were involved. Mountain gorillas live in two separate groups, and all seven were from one place, says Tyler-Smith. "So we'd definitely like to sequence some mountain gorillas from the other area where they live to compare them." One major barrier is that it’s not exactly easy to recruit participants. For samples, the team relied on Gorilla Doctors, a veterinary program inspired by Fossey’s fight against the poaching, diseases, and injuries that were killing gorillas. "You can't just go up to a gorilla and take a blood sample from it," says Tyler-Smith. "You can only do that if a gorilla is anesthetized, and you don't anesthetize a gorilla just for research or for interest. You only do that when the gorilla's life is in danger."
Ultimately, Tyler-Smith says there are firm practical reasons to sequence as many gorillas as possible. "We have the technology to do it, it's not that expensive, and that will give us the best genetic basis for understanding their populations," he says. "It's only a small part of conservation, but it's still a useful part." Populations are distinct enough that it might be possible, for example, to trace a captured gorilla’s lineage and return it to the right place, or to figure out where a poacher killed one and bring charges. Or genome sequences could be used by zoos for optimal breeding.
This study doesn’t change the fact that mountain gorillas’ survival is still precarious. But Tyler-Smith thinks that what the team has learned about their genetics should make conservationists hopeful. "The [population] size has been small for thousands of years," he says. "So there's no reason it shouldn't continue this way for thousands more."