Nature examines a tree that was alive in the time of Napoleon, yet has DNA that’s remarkably free of the usual damage of aging:
Each time a cell divides, mutations can arise because of errors made while copying the genome. Animals shield their reproductive cells from these mutations by isolating them early in development. These cells, called the germline, then follow a different developmental path, and typically have a low rate of cell division.
But plants do not have a dedicated germline: the cluster of stem cells that gives rise to the reproductive parts of flowers also generates plant stems and leaves. Because of this, scientists thought that the stem cells would accumulate many mutations, and that newer branches at the top of a long-lived tree would be remarkably different from the lower branches.
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Plant biologist Philippe Reymond and his team at the University of Lausanne decided to test this hypothesis using the university’s prized oak tree. They sequenced the genome from leaves on lower, older branches and upper, younger ones, and tallied the number of single-letter changes they found in the tree’s DNA.
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The team found that the number of mutations was much lower than would be expected based on calculations of the number of cell divisions that occurred between the lower branch and the higher one.
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For [University of Bern biologist Cris] Kuhlemeier, the results provide an answer to a question that has troubled him ever since a trip to Oregon 20 years ago. As he looked up at a soaring, 400-year-old Douglas fir, Kuhlemeier wondered how the branches towards the top of the tree would differ from those at the bottom. “I had always thought of a tree not as an organism, but as a collection of organisms with different genomes — more like a colony,” he says. Many ecologists shared his view, but now he has begun to question his earlier idea.