DNA from mammoth remains reveals history of last surviving population

A small group of woolly mammoths became stranded on Wrangel Island about 10,000 years ago when rising sea levels separated the island from mainland Siberia. Small, isolated populations of the animals lead to inbreeding and genetic defects, and it was long thought that the mammoths of Wrangel Island finally succumbed to this problem around 4,000 years ago.

But a paper in Cell on Thursday compared 50,000 years of genomes from mainland and isolated mammoths from the Wrangel Islands and found that this was not the case. What the paper’s authors discovered not only challenges our understanding of this isolated group of mammoths and the evolution of small populations, it also has important implications for conservation efforts today.

A difficult obstacle

It is the culmination of years of genetic sequencing by members of the international team behind this new paper. They studied 21 mammoth genomes – 13 of which were newly sequenced by lead author Marianne Dehasque; others were sequenced years ago by co-authors Patrícia Pečnerová, Foteini Kanellidou and Héloïse Muller. Genomes were obtained from Siberian woolly mammoths (Mammuthus primigenius), both from the mainland and from the island before and after he became isolated. The oldest genome came from a female Siberian mammoth that died about 52,300 years ago. The youngest were mammoth males from Wrangel Island, who died exactly at the time when the last of these mammoths became extinct (one of them died just 4,333 years ago).

It’s a remarkable and revealing time span: The sample included mammoths from a population that started out large and genetically healthy, went through isolation, and eventually went extinct.

The mammoths, the team noted in their paper, experienced a “climatically turbulent period,” particularly during an episode of rapid warming called the Bølling-Allerød interstadial (about 14,700 to 12,900 years ago)—a time that others have argued may have led to local extinction of mammoths. However, mammoth genomes studied during this time period do not indicate that warming had any adverse effects.

The side effects did not appear – and drastically so – only after the population on this island was isolated.

The team’s simulations suggest that, at minimum, the total population of mammoths on Wrangel Island was less than 10 individuals. This represents a serious population narrowing. This has been observed genetically through increased homozygosity in the genome, caused by both parents contributing nearly identical chromosomes, both derived from a recent ancestor. Homozygosity trends in isolated mammoths on Wrangel Island were four times greater than before sea level rise.

Despite this dangerously small number of mammoths, they recovered. The population size as well as the level of inbreeding and genetic diversity remained stable for the next 6,000 years until their extinction. In contrast to the initial population bottleneck, genomic signatures over time appear to indicate that inbreeding eventually shifted to more distantly related pairs, indicating either a larger mammoth population or a behavioral change.

Within 20 generations, according to their simulations, the population size would have increased to about 200–300 mammoths. This is consistent with the slower decline in heterozygosity they found across the genome.

Long-term negative effects

Wrangel Island mammoths may have survived against the odds, and harmful genetic defects may not have been the reason for their extinction, but research suggests their story is complicated.

With an area of ​​about 7,608 square kilometers today, slightly larger than the island of Crete, Wrangel Island would have offered a considerable amount of space and resources, even for large animals. For example, for 6,000 years after their isolation, they suffered from inbreeding depression, meaning increased mortality due to inbreeding and resulting defects.

This inbreeding also enhanced the clearance of deleterious mutations. That might sound like a good thing—and it can be—but it usually happens because individuals carrying two copies of the harmful mutations die or fail to reproduce. So it’s only good if the population survives.

The team’s results show that cleaning up genetic mutations can be a lengthy evolutionary process. Lead author Marianne Dehasque is a paleogeneticist who completed her PhD at the Center for Paleogenetics. She explained to Ars that “purging deleterious mutations for over 6,000 years basically suggests long-term negative effects caused by these extremely deleterious mutations. Since the purge of the Wrangel Island population took so long, it suggests that the population was experiencing the negative effects of these mutations until their extinction.