During the 40,000-year period between their deaths, the dwindling population underwent a reduction in genetic diversity of about 20 percent, the Swedish team reported, suggesting that the reduced fitness of the Wrangel mammoths might have contributed to their extinction.
In fact, the Wrangel mammoth's genome carried so many detrimental mutations that the population had suffered a “genomic meltdown,” according to Rebekah Rogers and Montgomery Slatkin of the University of California, Berkeley. Analyzing the Swedish team's mammoth data at the gene level, they found that many genes had accumulated mutations that would have halted synthesis of proteins, making the incomplete proteins useless, they reported in PLOS Genetics.
The mammoth had lost many of the olfactory genes that underlie the sense of smell, as well as receptors in the vomeronasal gland, which detects pheromones, hormonally active scents that influence the behavior of other individuals. Loss of such genes, to judge by the situation with elephants, could disrupt mate choice and social status.
Another damaged gene is called FOXQ1, which affects the structure and translucency of hair. Mammoths once had thick, rough hair that provided essential insulation in ice-age climates. Damage to FOXQ1, at least in some mice and rabbits that carry the same damaged form of the gene, causes hairs to become less stiff and shiny and to take on a satin appearance. A herd of Wrangel mammoths in moonlight might have shimmered like ghosts, but any compromise to their insulation accompanying the hair change would have jeopardized survival.
Biologists have long debated whether or not species are driven to extinction before genetic factors can play a role. The snapshots of the mammoth genome, one while it flourished and the other near extinction, support the idea that genomic meltdown contributes extinction. “This is probably the best evidence I can think of for the rapid genomic decay of island populations,” said Hendrik Poinar, an evolutionary geneticist at McMaster University.
The discovery that individual genes were deleted in the Wrangel mammoth's genome is a “very novel result,” and if confirmed “will have very important implications for conservation biology,” Dr. Dalen said.
These implications do not seem particularly hopeful, because they imply that once genomic decline has begun in a threatened species, it is irreversible. The upside, Dr. Rogers said, “is that it took hundreds of generations on this island to get a signal as strong as we saw.”
Several mammoth specialists and the biologist George Church of Harvard Medical School have proposed resurrecting the mammoth by making genetic changes in the very similar elephant genome and then bringing the altered genome to life in the womb of an elephant surrogate mother. Though little more than a charming fantasy in 2008, when Dr. Church proposed the idea, some of the many technical obstacles have come to seem less daunting.
But it's now evident that some mammoth genomes would produce hardier animals than others. “I wouldn't recommend using a Wrangel Island mammoth as a template,” said Beth Shapiro, a biologist at the University of California, Santa Cruz, and author of “How to Clone a Mammoth.”
