The woolly mammoths of Wrangel island were survivors. Trapped on a hunk of rock in the Arctic Ocean after rising sea levels cut them off from present-day Siberia, they were the last of their species to go extinct. Palaeontology textbooks have explained their eventual demise around 4,000 years ago as a classic case of extinction through inbreeding, in which severely damaging genetic mutations spread through an isolated population and kill it off. New work published in Cell , a journal, on June 27th reveals that the textbooks are wrong.
Genetic diversity can be thought of as an insurance policy for a species. If there are enough unrelated individuals in a population, there is a good chance that one will possess a heritable trait capable of protecting it from a novel threat. So long as that individual is capable of passing that trait on to its offspring, the species can avoid extinction. When small isolated populations are forced to breed with one another over multiple generations, however, harmful mutations become inescapably concentrated. Known as mutational meltdown, this phenomenon has the power to wipe out entire populations.
Love Dalén and Marianne Dehasqueat at the Centre for Palaeogenetics in Sweden were curious to know if the Wrangel island mammoths experienced mutational meltdown. Since Wrangel island is dry and bitterly cold for most of the year, the remains of woolly mammoths excavated there have often been very well preserved. Even DNA, which degrades quickly in warm climates, can be collected from these specimens. This made it possible for Dr Dalén’s team to study the genetics of 14 mammoths that had become trapped on the island. They then compared the genetics of these animals with those of seven individuals that were alive in the region before sea levels rose and cut the population off.
Some mammoths from the isolated population died as recently as 4,333 years ago. Others perished 9,219 years ago, about 800 years after Wrangel island became isolated. Similarities in their DNA led the researchers to conclude that approximately eight individual mammoths founded the population. This probably corresponds to a single herd. Moreover, just as textbooks suggest, the mammoths did face severe genetic challenges early on. But, remarkably, the most harmful mutations did not overwhelm the population. Instead, individuals with severely damaging mutations either died or failed to reproduce, leaving the rest of the population healthy. (Whether this was simply natural selection at work, or the result of mammoths choosing not to mate with sick individuals, remains to be seen.) For 6,000 years, a population of 200 to 300 mammoths lived in this relative stability. They did not have much genetic diversity. Very harmful mutations were still making some individuals ill and mildly harmful ones were building up, but the mammoths were not wasting away from mutational meltdown.
Then, quite suddenly, they went extinct. Archaeologists have found no evidence of people being present on the island at the time, so human hunting seems an unlikely explanation (even though humans are thought to have hunted mammoths in places like North America). A tundra fire or a few seasons of poor weather could have done them in too, but disease seems more probable. One part of the Wrangel island mammoth genome that was hit hard by isolation was the major histocompatibility complex (MHC). This is a key part of the immune system that gathers materials from pathogens and trains attack cells to recognise them. The researchers calculate that the MHC of island mammoths lost 49.2% of its genetic diversity compared with that of the mammoths that were not isolated. Paired with the build-up of mildly harmful mutations, that dramatic decline in genetic diversity probably made the mammoths vulnerable to infection.
This work has practical implications for conservation efforts. A great many species, like the California condor and the scimitar-horned oryx, have come perilously close to extinction. The efforts made to save their remaining populations from mutational meltdown through selective breeding programmes are certainly to be lauded, but Dr Dalén’s work suggests that this is only the first challenge. Maintaining a species’ genetic insurance policy over the longer term is equally important. But it is not clear that conservationists can do that for species that are threatened by a dramatic loss of genetic diversity.
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