New clues to evolution of mammals found in woolly-mammoth genome

University Park, Pa. -- Woolly mammoths died out several thousand years ago, but the genetic material they left behind is yielding new clues about the evolution of mammals, according to a study by Penn State scientists published last week in the early online edition of the journal Genome Research (http://www.genome.org).

The analysis of mobile DNA elements in the mammoth genome reveals new insights into how some of these elements arose in mammals and how they shaped the genome of a species headed for extinction. The authors of the study are postdoctoral researchers Fangqing Zhao and Ji Qi, and Professor of Biochemistry and Molecular Biology Stephan C. Schuster.

Interspersed repeats, also known as transposable elements, are DNA sequences that can "jump" around the genome, causing mutations in the host and contributing to expansion of the genome. Interspersed repeats account for a significant fraction of mammalian genomes, and some of these elements remain actively mobile. In humans, interspersed repeats account for approximately 44 percent of the entire genome sequence. Even more extreme is the opossum genome, where more than half of the sequence is composed of repetitive elements.

Scientists recently sequenced the genome of the woolly mammoth, using DNA samples obtained from preserved specimens. Schuster and his research group at Penn State, who were involved in the sequencing and analysis of the mammoth genome, now are looking deeper into the sequence for interspersed repeats. The mammoth genome is an excellent candidate for comparative analysis of interspersed repeats in mammals because it had a remarkably large genome of approximately 4.7 billion bases -- 1.5-times larger than the human genome. Using the sequence of the mammoth genome and sequences of other mammals for comparison, Schuster's group found that the mammoth genome contained the largest proportion of interspersed repeats of any other mammal studied. In fact, a single class of elements, known as the BovB long-interspersed repeat, alone accounted for nearly 12 percent of the mammoth genome.

Fangqing Zhao, a researcher in Schuster's group, emphasized that the BovB family of repeats is particularly interesting because, while this family has been identified in other mammalian genomes such as ruminants, opossum, and now the mammoth, its distribution in the mammalian lineage is inconsistent.  Zhao explained that this finding in the mammoth further supports the hypothesis that BovB may have been acquired "horizontally," meaning that vertebrate genomes attained the element from another organism, rather than inheriting it from ancestors.

Many species within the Afrotheria group of mammals, which includes the woolly mammoth, are at high risk for extinction or are already extinct. Underscoring the need to study genomes of species on the brink of extinction, Zhao said, "Further analyses examining if the genomes of extinct and endangered Afrotherians contain more repetitive elements than do the genomes of nonendangered mammals may elucidate whether there is an interplay between repetitive elements and extinction."

This work in Schuster's lab is supported by the Gordon and Betty Moore Foundation.

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Last Updated November 18, 2010