Beth Zaiken
A small population of mammoths became trapped on Wrangel Island when rising sea levels cut it off from the Siberian mainland about 10,000 years ago. It has long been thought that small, isolated populations of animals led to inbreeding and genetic defects, and that the Wrangel Island mammoths finally succumbed to the problem about 4,000 years ago.
But a paper published Thursday in the journal Cell, comparing 50,000 years of genomes from mainland and isolated Wrangel Island mammoths, finds that this isn’t the case. What the authors found not only calls into question our understanding of this isolated group of mammoths and the evolution of small populations, but also has important implications for conservation efforts today.
Serious bottleneck
This is the culmination of years of gene sequencing by members of the international team behind the new paper. They studied the genomes of 21 mammoths, 13 of which were newly sequenced by lead author Marianne Dehusque. The others were sequenced years ago by co-authors Patricia Pechnerová, Foteini Kanelidou and Héloïse Muller. The genome was found to belong to the Siberian woolly mammoth (Mammutus primigeniusThe oldest genome was from a female Siberian mammoth that died about 52,300 years ago. The most recent genome was from a male mammoth from Wrangel Island that died around the same time as the last mammoths went extinct (one of them died just 4,333 years ago).
Love Darren
This is a remarkable and thought-provoking period: the sample included mammoths from a population that was initially large and genetically healthy, but became isolated and eventually went extinct.
The researchers note in their paper that mammoths experienced “periods of climatic instability,” specifically: Bolling-Allerod Interglacial (approximately 14,700 to 12,900 years ago) — It has been suggested that this may have been the localized extinction of mammoths, but studies of mammoth genomes from this period have found no evidence that warming had any adverse effects.
It was only once the population was isolated on the island that adverse, and dramatic, effects began to appear.
According to the team’s simulations, the Wrangel Island mammoth population was at its lowest point fewer than 10 individuals, indicating a severe population bottleneck. Genetically, this was confirmed by parents donating nearly identical chromosomes inherited from their most recent ancestors, causing an increase in homozygosity in the genome. The increase in homozygosity among the isolated Wrangel Island mammoths was four times higher than it was before sea-level rise.
Despite dangerously low numbers, mammoths recovered; population sizes, inbreeding levels and genetic diversity remained stable for 6,000 years until their extinction. Unlike an earlier population bottleneck, genomic signatures seem to indicate that over time, inbreeding eventually shifted to pairing with more distant relatives, suggesting an increase in mammoth population size or a change in behavior.
Their simulations suggest that within 20 generations the mammoth population would have increased to around 200–300 individuals, which is consistent with the gradual decline in heterozygosity they found in the genome.
Long-term adverse effects
While the Wrangel Island mammoths may have survived despite the hardships and a harmful genetic defect may not have been the cause of their extinction, research suggests their story is complex.
about 7,608 Wrangel Island has a modern area of 100 square kilometers, a little larger than the island of Crete. The island would have had considerable space and resources to house even the larger animals. For example, the island suffered from inbreeding depression during its 6,000 years of isolation, which means high mortality rates due to inbreeding and the resulting defects.
This inbreeding also helped eliminate harmful mutations. This may sound like a good thing, and it may be, but this usually happens because individuals with two copies of the harmful mutation die or fail to reproduce, so it’s only a good thing if the population survives.
The team’s results show that the elimination of genetic mutations can be a long-term evolutionary process. Lead author Marianne DeHask is a paleogeneticist who earned her PhD at the Center for Paleogenetics. She explained to Ars: “The elimination of deleterious mutations over more than 6,000 years basically shows the long-term negative effects caused by these highly deleterious mutations. The fact that the elimination of the Wrangel Island population lasted for such a long period of time shows that the population was experiencing the negative effects of these mutations right up until the point of extinction.”
