Tuesday, April 14, 2015

Mountain gorilla genome sequenced

Mountain gorillas (Gorilla beringei) are arguably the cutest species of gorilla. They're especially fuzzy but they're also especially scarce. There are fewer than 800 individuals left on the planet and they are found only in the Virunga Volcanoes in Uganda, Rwanda, and The Democratic Republic of Congo (IUCN). That's it. There are zero individuals in zoos right now.

Juvenile mountain gorillas, Photo credit: Philip Milne
Because there are so few individuals left, mountain gorillas are at risk of genetic inbreeding.  This is when there are so few individuals that mating opportunities are limited. One of the greatest benefits of sexual reproduction is genetic diversity, creating new gene combinations in your offspring that you don't have. With inbreeding, individuals mate with those closely related genetically. This can be damaging to a population because diversity protects us: one disease can't wipe out the entire population if we're all different. (Check out UC Berkley's excellent explanation of inbreeding depression here for a refresher on the topic or to learn it for the first time.) With less than 800 individuals remaining, the risk of inbreeding is much higher in mountain gorillas than in other great ape populations.

A paper by Xue and colleagues (2015) published in Science last week details the genetic consequences of prolonged population decline in eastern lowland and mountain gorillas. Completing a whole-genome sequence on this species has led to some fascinating and some surprisingly optimistic findings.

Xue and colleagues report homozygosity, or having two forms of the same gene that are identical, is higher in mountain and eastern lowland gorillas than in western lowland gorillas and even the most inbred populations of humans. Within the study populations, chromosomes were found to be homozygous on over one third of their total length, indicating that the parents of these individuals are related. The degree to which this homozygosity extends suggests that mountain gorillas have experienced several recent doses of inbreeding.

Mountain gorilla adults and infant. Photo credit: Derek Keats
The effective population size of mountain gorillas is currently 273 individuals (plus or minus 54). A term used in genetics, effective population size refers to the number of breeding individuals in an ideal population that would display the same gene frequencies as would occur randomly due to genetic drift. This is a good measure of genetic diversity.

The study also found that mountain gorillas have a lower rate of the harmful loss-of-function variants than do their more populous relatives, western lowland gorillas. Loss-of-function variants can be fatal. Mountain gorillas also appear to have survived at low population numbers for a long time, thousands of years. This is excellent news! Although it's certainly doesn't mean we should relinquish our conservation efforts.  The authors of this study find further evidence to suggest that the last great environmental change (tropical forests to savannah) caused a collapse in mountain gorillas and western lowland gorillas.

While inbreeding usually makes for a risky situation as environmental changes or disease, for example, could wipe out substantial parts of a population, it seems mountain gorillas are doing better than expected. They are inbred but they're doing well.

Food for thought: How does this new study relate to gorilla conservation issues? How might a better understanding of population genetics help combat poaching and the illegal wildlife trade?

Links of interest:
IUCN Redlist overview of species
Gorilla Doctors Website
Paper, "Great ape genetic diversity and population history"

Xue, Y., Prado-Martinez, J., Sudmant, P. H., Narasimhan, V., Ayub, Q., Szpak, M., ... & Scally, A. (2015). Mountain gorilla genomes reveal the impact of long-term population decline and inbreeding. Science, 348(6231), 242-245.

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