According to a new study published in the journal PLoS Genetics, dwindling populations created a ‘mutational meltdown’ in the genomes of the last wooly mammoths, which had survived on Wrangel Island in the Arctic Ocean until about 3,700 years ago.
This is an artist’s rendition of a woolly mammoth. Image credit: Flying Puffin / CC BY-SA 2.0.
Woolly mammoths (Mammuthus primigenius) were among the most common large herbivores in North America, Siberia, and Beringia during the Pleistocene and early Holocene. However warming climates and human predation led to extinction on the mainland roughly 10,000 years ago.
Lone isolated island populations persisted out of human reach until roughly 3,700 years ago when the species finally went extinct.
In the new study, University of California-Berkeley researchers Dr. Rebekah Rogers and Dr. Montgomery Slatkin compared existing genomes from a mainland mammoth that dates back to 45,000 years ago, when the animal was plentiful, to one that lived about 4,300 years ago on Wrangel Island, an isolated island in the Arctic Ocean that was separated from the mainland by rising sea levels about 12,000 years ago.
“One specimen is derived from the Siberian mainland at Oimyakon, dated to 45,000 years ago. This sample comes from a time when mammoth populations were plentiful, with estimated effective population size of 13,000 individuals,” the scientists explained.
“The second specimen is from Wrangel Island off the north Siberian coast. This sample from 4,300 years ago represents one of the last known mammoth specimens. This individual comes from a small population estimated to contain roughly 300 individuals.”
“These two specimens offer the rare chance to explore the ways the genome responds to pre-extinction population dynamics.”
The analysis showed that the Wrangel Island mammoth had accumulated multiple harmful mutations in its genome, which interfered with gene functions.
The animals had lost many olfactory receptors, which detect odors, as well as urinary proteins, which can impact social status and mate choice.
The genome also revealed that the Wrangel Island mammoth had specific mutations that likely created an unusual translucent satin coat.
“We observe an excess of detrimental mutations, consistent with genomic meltdown in woolly mammoths on Wrangel Island just prior to extinction,” Dr. Rogers and Dr. Slatkin said.
“We observe an excess of deletions, an increase in the proportion of deletions affecting gene sequences, and an excess of premature stop codons in response to evolution under low effective population sizes. Large numbers of olfactory receptors appear to have loss of function mutations in the island mammoth.”
“We also observe two independent loss of function mutations at the FOXQ1 locus, likely conferring a satin coat in this unusual woolly mammoth.”
For those interested in wooly mammoth ‘de-extinction,’ this study demonstrates that some mammoth genomes carry an overabundance of negative mutations.
The work also offers a warning to conservationists: preserving a small group of isolated animals is not sufficient to stop negative effects of inbreeding and genomic meltdown.
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R.L. Rogers & M. Slatkin. 2017. Excess of genomic defects in a woolly mammoth on Wrangel island. PLoS Genet 13 (3): e1006601; doi: 10.1371/journal.pgen.1006601
