The last population of woolly mammoths descended from fewer than ten individuals. Although the tribe recovered, the mammoths suffered from harmful mutations for thousands of years until extinction. This is shown by a new study, published in the journal Cell, carried out at the Center for Paleogenetics in Stockholm. The study provides new insights into the long-term genetic challenges species face when the number of individuals is reduced.
An important question in conservation biology concerns the viability of populations after sudden population declines. Previous studies have investigated the so-called bottleneck effect and the problems that arise from loss of genetic diversity and increased inbreeding rates. But few have investigated how the genetic material is affected in the long term by sudden population declines.
- The woolly mammoth, Mammuthus primigenius, is a perfect species to look at to investigate the long-term genetic consequences of such "bottlenecks". The last mammoth population survived on Wrangel Island for 6,000 years before dying out. Thanks to the cold arctic conditions that prevail on the small island off the coast of Siberia, the DNA of these mammoths is very well preserved, says Marianne Dehasque, lead author of the study published in Cell .
DNA has been extracted from the remains of 21 woolly mammoths
To investigate what happened to the mammoths during their last time on earth, researchers from the Center for Paleogenetics, which is jointly funded by the National Museum of Natural History and Stockholm University, have sequenced the entire genome of 21 woolly mammoths. These individuals lived during the last 50,000 years of the species' existence.
- Until 10,000 years ago, the genetic diversity of mammoths in northeastern Siberia was largely unchanged. It suggests that the mammoth population was surprisingly stable during the last ice age. This despite the arrival of modern humans and several climate changes that took place during the second half of the last ice age, says Marianne Dehasque.
Fewer than ten mammoths the base of the last population
However, the last surviving mammoths met a different fate. The study shows that fewer than ten mammoths eventually colonized Wrangel Island, when the island was formed due to rising sea levels 10,000 years ago. These individuals then formed the entire base of the population that remained on the island for another 6,000 years. Genomic data show that, after the initial bottleneck, the population rapidly recovered in size and that genetic diversity also stabilized. Despite the recovery, the researchers can see that the mammoths suffered from harmful mutations for thousands of years to come.
- Although the population recovered from near extinction to approximately 300 reproducing individuals, our analyzes show that they suffered from inbreeding depression. The results suggest that the mammoths suffered from genetic diseases for hundreds of generations after the recovery, says Love Dalén, professor of evolutionary genomics at the Center for Paleogenetics.
According to Marianne Dehasque, the study has implications for conservation biology in general.
- Our research underlines the importance of long-term genetic monitoring in work with endangered species. Even after a population has recovered, the effect of deleterious mutations can persist for many generations, jeopardizing its viability.
Still unclear why the last mammoths became extinct
What ultimately caused the extinction of the mammoth on Wrangel Island, some 4,000 years ago, remains a mystery. The study shows that, despite inbreeding depression, the population size remained constant at around 300
reproducing individuals until just a few generations before the species became extinct.
- We do not know what it was that ultimately caused the extinction of the woolly mammoth, but we can state that the population must have decreased very quickly at the end, says Love Dalén and continues.
- It's a bit like a murder mystery. Something caused the extinction, now we just have to keep collecting evidence to find the answer.