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Eastern lowland gorillas on their way to "genetic breakdown"



Gray Gorillas.
Image: Dear Dalén / Tom van der Valk

The eastern lowland gorillas in the Democratic Republic of the Congo have experienced a dramatic decline in population in recent decades, leading to a sensational lack of genetic diversity and a threatened outbreak. Numerous harmful mutations according to new research results.

Comparing the genomes of living gorillas of the Eastern lowlands, also known as Gray's gorillas, with genomes from museum specimens, researchers from the Swedish Museum of Natural History (SMNH) and the University of Uppsala have shown that genetic diversity decreases in species while the inbreeding increases. This double whammy syndrome is due to declining populations and causes an effect called "temporal genomic erosion" where the lack of genetic diversity introduces deleterious mutations into gray gorillas, including fertility problems and reduced ability to fend off infectious diseases. The new research was published today in Current Biology.

It is needless to say that this makes an already dire situation worse for the Gray Gorilla. But while the genetic diversity of these gorillas is sinking sharply, it has not yet reached the point of no return ̵

1; a condition called "genetic breakdown". Thereafter, there is not enough diversity in the gene pool to be able to reproduce. There is still time to bring these monkeys back from the abyss, all we have to do is act.

Museum samples of Grauer's gorilla skulls.
Image: Love Dalén / Tom van der Valk

Over the past two decades, the population of Gray Gorillas in the Democratic Republic of the Congo has declined by 80 percent. Due to poaching and habitat loss, less than 4,000 people now live and the species is considered critically endangered. To see whether this decline in population was reflected in a loss of genetic diversity, the researchers at SMNH and Uppsala investigated past eastern gorilla genomes in the past century by examining samples collected in museums. It is the first study to use historical genomes of a vulnerable species to study changes in genomic variation over time.

"We believe that this approach can be implemented to a much greater extent in endangered species in general," said Dear Dalén, researcher at SMNH and co-author of the new study, told Earther.

Dalén and his colleagues sequenced 11 genomes from eastern gorillas that were 100 years old. The DNA of the archived samples obtained from teeth and dry tissue samples has been severely degraded. By using the same methods that are normally used to restore older samples such as mammoths and Neanderthals, scientists were able to recover good and reliable genetic data.

"Of course, it would have been great if we could have sequenced more museum pieces, as this would have enabled a higher resolution," Dalén told Earther. "Overall, finding significant changes in Gray's gorillas is robust, in my opinion, because statistical significance accounts for the small sample size."

A juvenile Gray Gorilla.
Image: Dear Dalén / Tom van der Valk [19659003] The results confirmed the loss of genetic diversity in live gray gorillas from which DNA samples from seven individuals were taken. In addition, an alarming series of genetic mutations have crept in over the last four to five generations – mutations that make these gorillas more vulnerable to transmissible and genetic diseases and less able to adapt to environmental changes. Many of these mutations are called "loss of function" (LoF). This happens, as the name suggests, when a gene loses its developed function. In the last 100 years, Gray's gorillas have acquired multiple LoF mutations related to changes in immune response and male fertility in humans and other related species. The researchers also discovered LoF in genes associated with the development of fingers and toes, which may explain why many living gray gorillas have fused digits.

Interestingly, the researchers also carried out a similar study of mountain gorillas using the same techniques. No significant genetic changes were detected. These gorillas, a closely related subspecies, seem to maintain their genetic diversity and reproductive capability, although the population has recently been lost due to similar factors.

The reason for the observed increase in harmful mutations in Gray's gorillas compared to mountain gorillas, said Dalén, may have something to do with the differences in their long-term history. Gray gorillas experienced a strong population increase a few thousand years ago, while the population of mountain gorillas remained low for several thousand years. Somewhat contrapuntally, the small population size of the mountain gorillas could have enabled natural selection to eliminate acquired harmful mutations in a process known as genetic cleansing.

In contrast, the population increase in Grauer's gorillas resulted in a large number of mutations – including harmful ones -.

"When the Gray Gorilla began to decline in recent decades, we believe that these low-frequency damaging mutations have increased frequency due to stochastic allelic changes – so-called genetic drift," said Dalén.

As the new study suggests, it is crucial to stop and ideally reverse the demographic decline in Gray's gorilla.

"The smaller the population, the more damaging mutations are likely to occur more often," said Dalén. "This could lead to a negative feedback loop called a mutational melt. The descent must therefore really be stopped. "

Action will not be easy. The ongoing conflict in the Democratic Republic of the Congo and the threat of Ebola have made conservation efforts difficult, if not impossible. We hope that reason will prevail, and we will be able to make significant efforts to preserve this remarkable group of animals.

[Current Biology]


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