Mammoth genome approaching completion

Sequencing of the extinct creature's nuclear DNA about 70 percent done

Even though scientists haven’t yet cobbled together the full genome of the woolly mammoth, the information they’ve gathered so far — enough to assemble about 70 percent of the extinct creature’s nuclear DNA — reveals that the tundra-strolling behemoths were very closely related to modern elephants.

HAIRY DATA Using well-preserved specimens of hair from a variety of extinct woolly mammoths, researchers have assembled about 70 percent of the creature’s nuclear genome. Schuster

So far, the estimated overlap between the mammoth genome and that of the African elephant is about 99.4 percent, says study coauthor Stephan Schuster, a genomicist at Pennsylvania State University in University Park. Previous studies have suggested that the two species last had a common ancestor about 7.6 million years ago, Schuster and his colleagues report in the Nov. 20 Nature. Together, these figures suggest that the rate of genetic change in elephants and their kin is about half that seen in humans and their kin during the same period.

Humans and chimps last shared a common ancestor about 7 million years ago, and the genomes of these two species differ by about 1.24 percent, the researchers note.

Until just a few years ago, researchers scrutinizing the DNA of extinct animals took their samples from bone. Then a team of researchers including Schuster developed techniques to extract genetic material from hair and assembled the relatively small mitochondrial genome of the woolly mammoth (SN: 9/29/07, p. 195).

Since then, advances in high-speed DNA sequencing equipment have enabled that team, and others, to compile large portions of the mammoth’s nuclear genome as well. In recent genetic analyses of hair samples from two mammoths — one that lived about 20,000 years ago, and the other one around 59,000 years ago — Schuster and his colleagues assembled about 3.3 billion DNA base pairs, or “letters,” of the creatures’ nuclear genomes.

The team’s analyses suggest that the nuclear genome of the mammoth contains about 4.7 billion base pairs, which means the base-pair sequences now known represent about 70 percent of the creature’s genetic heritage. Previous studies indicate that the mammoth’s mitochondrial genome contains only 16,770 base pairs (SN: 12/24&31/05, p. 403).

Several sequences of base pairs identified in the latest study are seen only in mammoths. However, analogous stretches of DNA found in about 50 other creatures — all of which inhabit warm climes — differ from the mammoth but don’t differ from each other. So, Schuster proposes, the newly identified base-pair sequences may relate to genes that enabled the mammoths to survive the harsh conditions during the last ice age.

“Unique sequences of base pairs are entirely normal,” says Michael Hofreiter, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. After all, he notes, genetic distinction is what helps distinguish one species from another. The only way to determine whether these particular genetic sequences helped survival in cold conditions would be to study their effects in living tissue, he adds.

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