Beth Zaiken
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 animal populations lead to inbreeding and genetic defects, and it has long been thought that the Wrangel Island mammoths eventually succumbed to this problem around 4,000 years ago.
However, a paper in Cell on Thursday compared 50,000-year-old genomes from mainland and isolated Wrangel Island mammoths and found that 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, but also has important implications for conservation efforts today.
A serious 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; the others were listed 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 it was isolated. The oldest genome was from a female Siberian mammoth that died about 52,300 years ago. The youngest were from the male mammoths of Wrangel Island, which were destroyed at the exact time when the last of these mammoths died (one of them died only 4,333 years ago).
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It’s a remarkable and revealing time frame: the sample included mammoths from a population that started out large and genetically healthy, went into isolation, and eventually went extinct.
The mammoths, the team noted in their paper, experienced a “turbulent climatic period,” particularly during an episode of rapid warming called the Bølling-Allerød interstadial (roughly 14,700 to 12,900 years ago)—a time when others have suggested that it may have led to local extinctions of woolly mammoths. However, mammoth genomes studied during this time period do not show that warming had any negative effects.
The negative effects appeared – and drastically so – after the population became isolated on that island.
The team’s simulations show that, in the smallest case, the total mammoth population of Wrangel Island was less than 10 individuals. This represents a serious population barrier. This was seen genetically through increased levels of homozygosity within the genome, caused when both parents contribute nearly identical chromosomes, both descended from a recent ancestor. Homozygosity flows within isolated Wrangel Island mammoths were four times greater than before sea level rise.
Despite that 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 seem to indicate that inbreeding eventually shifted to pairs of more distant relatives, suggesting either a larger mammoth population or a change in behavior.
Within 20 generations, according to their simulations, the population would have grown to about 200-300 mammoths. This is consistent with the slower decline in heterozygosity they found across the genome.
Long-term negative effects
The mammoths of Wrangel Island may have survived against the odds, and harmful genetic defects may not have been the reason for their extinction, but research suggests their history is complicated.
At about 7,608 square kilometers today, slightly larger than the island of Crete, Wrangel Island would have offered a fair amount of space and resources, although these were large animals. For 6,000 years after their isolation, for example, they suffered from inbreeding depression, which refers to increased mortality as a result of inbreeding and the resulting defects.
This kinship also promoted the clearance of deleterious mutations. This may seem like a good thing – and it can be – but it usually happens because individuals who carry two copies of the harmful mutation die or fail to reproduce. So it is only good if the population survives.
The team’s results show that clearing genetic mutations can be a long evolutionary process. Lead author Marianne Dehasque is a paleogeneticist who completed her PhD at the Center for Paleogenetics. She explained to Ars that, “Cleaning up deleterious mutations for over 6,000 years essentially shows long-term negative effects caused by these extremely deleterious mutations. Since the purge in the Wrangel Island population continued for such a long time, this indicates that the population was experiencing negative effects from these mutations until its extinction.
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