Black Cottonwood Trees Have Leaves that are Genetically Different from its Roots

Black Cottonwood treeA team of scientists just discovered that a certain species of Black Cottonwood trees or Populus trichocarpa are actually capable of cloning themselves to produce their offspring. As a result, the offspring are all related to the parent in terms of the same root system. Most scientists originally supposed that the clones would have the same genetic codes as the parents, however, recent research shows that there is actually a vast genetic difference between the offspring and the parent plant.  In the case of some trees, like the Black Cottonwood Trees, the leaves have a different genetic make up from the roots.

Ken Paige, an evolutionary biologist at the University of Illinois at Urbana-Champaign led a team that conducted the very first genome wide analysis of this tree species. He exclaims that “The variation within a tree is as great as the variation across unrelated trees.”

Brett Olds, a colleague of Ken Paige and the biologist at his laboratory adds: “This could change the classic paradigm that evolution only happens in a population rather than at an individual level.” Olds was the one to present the results of the study to the 2012 Ecological Society of America Annual Meeting in  Portland, Oregon.

But this is not new news to some people of the scientific community — Horticulturists have long been aware of these mutations in genetics. They are called somatic mutations and they occur in cells that are not sperm or egg cells. Horticulturists have used the information to graft certain types of mutant branches onto other types of plants that were considered to have “normal” stalks.

However, Paige and his team’s genome study is the very first of its kind. No one has ever catalogued the total number of somatic mutations in a plant before. Olds remarks that all the previous studies have focused solely on a certain region or a certain set of genes, while genome mapping has never been done before.

Black Cottonwood LeavesFor the genome wide study, Paige chose the Black Cottonwood because of its age and growth speed. The plant is ideal for this project because it grows to 30- 50 metres tall and ages up to 200 years old. This growth pattern means that its tissues are separated by long spans of growth. In addition, its offspring forms clones that are connected by the roots.

Thanks to new technology, Olds managed to take a samples of full genomes from the tissue of the highest buds, the lowest branches, and the roots. After sequencing them and comparing the outcomes, Olds discovered over 188,000 mutations that were unique to just one particular tissue sample, proving that the leaves of the tree have a very different genetic make up from the roots of the tree.

They also discovered that the tissue specific mutations particularly affect certain cell functions including DNA binding, cell communications, the immune function, metabolism, and cell death. The study used to be too expensive to fund, but thanks to new technology, genome mapping is much more affordable.

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