Sequencing one of the world's oldest trees to learn how mutations occur in clonally reproducing organisms

04 Nov 2024, 14:40 by Bob Yirka

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Conceptual model of somatic mutation inheritance between ramets within an aspen clone. When a mutation arises, we expect it to propagate down to the new tissues as the clone continues to grow. New mutations are symbolized with the lightning bolt. The mutation identity is marked as a colored star and the dark marks corresponds to where samples could be collected from the clone. Credit: Evolutionary Biology (2024). DOI: 10.1101/2024.10.19.619233

A team of biologists, environmental scientists and geneticists affiliated with multiple institutions across the U.S. has learned more about how mutations occur in clonally reproducing organisms by sequencing one of the oldest trees in the world. The group has posted their paper on the bioRxiv preprint server.

Most research on evolution involves the study of diploids—creatures with two sets of chromosomes. Such creatures generally reproduce sexually. In this new study, the research team focused their efforts on a triploid—a species that has three sets of chromosomes. In this case, it was a type of tree called the Pando aspen, which is not able to reproduce sexually. Instead, it clones itself.

Prior research has shown that Pando aspen trees grow to an extraordinary size—but not in a way that is readily apparent. Instead of growing tall or circumferentially large, the tree sends out roots from which other tree-like plants grow. In this way, a single Panda aspen can be easily mistaken for a small forest of nearly identical looking trees.

Each of the trees (known officially as stems) are clones of the original tree—all connected by a single root. One such Pando aspen, residing in Utah's Fishlake National Forest, occupies approximately 42.6 hectares of land. Such trees are also known to have long lifespans, much longer than diploid trees.

For this new study, members of the research team ventured to Utah to collect samples of the massive, ancient tree and several of its neighbors.

The main part of the research consisted of genetically sequencing different parts of the trees to learn more about the role mutations play in their development. Then, after deleting variants found in both the main tree under study and its neighbors, and mutations found in single samples, the researchers found approximately 4,000 variants that have arisen over the main tree's long life.

The team also found evidence that the tree was between 16,000 and 80,000 years old—making it one of the oldest in the world. And finally, they found evidence that because the tree was a triploid, it had bigger cells in addition to being a big organism, and because of that, better overall fitness—traits that have allowed it to live for so long.

More information: Rozenn M. Pineau et al, Mosaic of Somatic Mutations in Earth's Oldest Living Organism, Pando, biorxiv (2024). DOI: 10.1101/2024.10.19.619233

Journal information: Evolutionary Biology , bioRxiv