Inbreeding Helps Gorillas Survive

Nearly everything we head about inbreeding leads us to believe that it is harmful to genetics. And we'd be right. It's when offspring are produced from the mating of individuals that are closely related, and can lead to genetic weaknesses and defects. But when a population shrinks to the brink of extinction, there isn't much choice in a mate, and so inbreeding occurs. One example of this is the mountain gorillas, which has become one of the most intensively studied creatures in the wild. This has made them one of the most iconic endangered species in the world, with less than 900 individuals left. Half of these live in one of three national parks on the Virunga mountain range in East Africa. Although 880 is a drastically low number, the population of mountain gorillas has increased a fair bit since 1981, when the population hit a low of only 235 left in the parks. Thankfully, due to conservation efforts and support from the public, the amazing creatures are pulling themselves back - but they still have a long way to come. As you might expect, the drop in population size led to an increase of inbreeding, which raised concerns that such a small gene pool would harm the long-term future of the mountain gorilla. A low level of genetic diversity exposes the animals to environmental changes and diseases by reducing their genetic ability to adapt, and also potentially causing harmful mutations. To study this further, an international team of scientists launched what would become the first in-depth, whole-genome analysis of the mountain gorilla. What they discovered was quite shocking, and their findings have been published in Science. Inbreeding has actually benefitted mountain gorillas, removing many harmful genetic variations instead of adding them. It means that they have genetically adapted to living in small populations. However, the researchers found out that this isn't a recent change for the animals. "We worried that the dramatic decline in the 1980s would be catastrophic for mountain gorillas in the long term," said Dr Yali Xue, one of the study's authors from the Sanger Institute near Cambridge. She explained that, on the contrary, the study results suggest that gorillas have been living in this manner for thousands of years, which is much longer than previously thought. Over the course of several years, blood samples were collected from seven mountain gorillas and six eastern lowland gorillas, which is the other subspecies of eastern gorilla. In mountain gorillas, it was found that inbreeding had led to a significant loss of genetic diversity, with them inheriting identical segments of genes from both mother and father in nearly a third of their genome. In fact, both of the eastern subspecies were found to be between two and three times less genetically diverse than western lowland gorillas, which have a larger population. Dr Mike Cranfield, another author of the study, is the co-director of the Mountain Gorilla Veterinary Project, which collected many of the blood samples in the study. Dr Cranfield explained that further collaborations with small, isolated populations of gorillas would help to lead to a consistent and viable approach to conserving them. “The fact we think the genetic health of the population hasn’t been damaged by the decline it went through, is just amazing,” he added. “And [it] eliminates the need for possibly introducing outside genes to recuperate the population.” Dr Chris Tyler-Smith is the Sanger Institute senior author and said the team will now be able to compare all of the gorilla population genomes. The study will allow the team to "begin to understand similarities and differences [of gorilla subspecies], and the genetic impact of inbreeding". And according to the senior author from Cambridge University's Department of Genetics, Dr Aylwyn Scally, despite the mountain gorilla being critically endangered and low in genetic diversity, there is hope for them yet. “They can continue to survive,” he said, “and will return to larger numbers if we help them.”