The genomes of mountain gorillas (above) are providing new insights into how these endangered apes are surviving.

The genomes of mountain gorillas (above) are providing new insights into how these endangered apes are surviving.

Gorilla Doctors/UC Davis

Mountain gorilla genome provides hope for animal’s future

The sad story of an orphan mountain gorilla has a somewhat happy ending. Rescued as a 4-year-old from poachers in 2007, Kaboko was raised in a gorilla orphanage in the Democratic Republic of the Congo (DRC). He got sick and died at age 9, never having the opportunity to breed and help the mountain gorillas recover from the low numbers that threaten their existence. But with his blood and that of six others, researchers have now delved into the history and health status of these endangered animals and concluded their fate may be less dire than some have feared.

It turns out that mountain gorillas have been scarce for many millennia, over which time they have managed to get rid of potentially species-killing mutations. The findings suggest that the animals “can survive if they are given a chance and protected with ample habitat,” says Stephen O’Brien, a geneticist at the Theodosius Dobzhansky Center for Genome Bioinformatics in St. Petersburg, Russia, who was not involved with the study.

Gorillas divide into several subgroups. Most are western gorillas, which live in the rainforests of Central Africa. Eastern gorillas include mountain gorillas and eastern lowland gorillas in DRC and surrounding areas. The mountain gorillas of the Virunga volcanic mountain range were made famous by the late Dian Fossey, a primatologist who studied and wrote about them 40 years ago and who was killed at her research station in 1985, presumably because of her fight against poaching. In 1981, mountain gorillas there numbered only about 250. These mountains stretch across the borders of Rwanda, DRC, and Uganda. Throughout this range, development, civil strife, and poaching had taken a serious toll.

Fossey inspired a strong conservation movement and much research, but until now geneticists had to rely on hair and stool samples to collect DNA. Early studies suggested the animals suffered from a low genetic diversity and extensive inbreeding, making them highly susceptible to extinction. However, that DNA was not of high enough quality to enable a complete genome analysis.

After sequencing the genome of the more populous western gorilla 3 years ago, the geneticists involved approached conservation organizations for help getting blood samples from the much rarer eastern gorillas. Michael Cranfield, a veterinarian at the University of California, Davis, and co-director of one organization called Gorilla Doctors, and his colleagues provided samples from blood collected while treating injured animals or dissecting dead ones, including Kaboko.

In addition to seven mountain gorillas, Chris Tyler-Smith, a geneticist at the Wellcome Trust Sanger Institute in Hinxton, U.K., and his colleagues sequenced six eastern lowland gorillas. With Aylwyn Scally, a geneticist at the University of Cambridge in the United Kingdom, and others, they compared that data with the western gorilla genome.

Mountain gorilla numbers began declining much earlier than researchers have thought, about 100,000 years ago, perhaps because of climate change, they report online today in Science. Their numbers may have been in the hundreds for much of the past 20,000 years.

That mountain gorilla population size has been declining for a long time is “a surprise,” says Jeffrey Kidd, a geneticist at the University of Michigan, Ann Arbor, who was not involved in the work. The finding highlights “the important role of climate and forestation changes for the evolution of African primates.”

The small population size explains why genetic diversity is so low and inbreeding so extensive. Today, about one-third of each chromosome is exactly the same among mountain gorillas, suggesting that half-siblings are mating, they report. (In eastern lowland gorillas, about 14% of the chromosomes are the same.)

The data also confirmed the current division of gorillas into distinct subgroups and identified almost 26,000 differences that can now be used to tell eastern lowland and mountain gorillas, and even the different mountain gorilla clans, apart. “It’s good to know concrete details about the variation of these gorilla populations,” says Oliver Ryder, a geneticist at the San Diego Zoo Institute for Conservation Research in California who was not involved with the research. “These insights can contribute to long-term risk assessments and conservation management actions.”

Numbering about 480 today, Virunga mountain gorillas have more potentially deleterious mutations than other gorillas, Scally says. Luckily, mountain gorillas have relatively few of the genetic changes that abort protein production all together. Such changes are often lethal in offspring if both parents have them. In small populations, the chances are higher that both parents will carry these deadly mutations, so they are more likely to be purged. With fewer of these mutations in the gene pool, population growth could be slightly faster, Ryder says.

Despite the precarious situation, Scally is hopeful: “If we are able to stop further encroachment of their habitat, maybe they will be able to carry on and recover.”

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