All clones are not alike.
What's the News: Foresters have long noticed that trees with the exact same genes, when raised in separate nurseries, have very different responses to drought. While one shoots up through lean times, the other droops. Why the divergence? Scientists have now found that twin trees raised separately are, just like human twins
, expressing different genes. In other words, nurture is affecting nature. What's the Context:
Commercially grown trees are often genetically identical: they are clones of some long-ago tree that had just the right traits for sandy soil or the shiniest apple. Some clone lines are quite old--that young maple on your street could be the genetic twin of a tree that lived in the 1850s.
While clones have the same genes as each other, whether those genes are expressed the same way is a function of epigenetics. The addition of methyl tags to DNA bases, called methylation, is an epigenetic change that allows some genes to be transcribed while others are not. How cells get their cues to methylate or demethylate DNA, though, is still fuzzy.
How the Heck:
The team compared poplar tree clones grown in separate nurseries. There was nothing unusual about how the trees were raised---they were all grown indoors under standard conditions. But sure enough, when deprived of water, two of the three varieties showed diverging reactions depending on where trees were grown.
Looking closer, the team found that the methylation patterns of the trees' DNA were quite different.
Additionally, a survey of all the transcribed genes showed that the trees from one location were deploying one set to deal with drought while their twins from the other location were using another.
They also noticed that the longer a particular variety had been grown as a clone---one poplar variety, for example, dated back to the early 1900s---and the longer it had been since a pair of trees shared a common clone ancestor, the more divergent the clones' response to drought.
They checked the trees' genomes to see if they'd picked up a random mutation or two over the years, but within each variety, the DNA sequences were identical. This suggests the intriguing possibility that epigenetic changes accumulate over time, so trees from older lines of clones are more apt to show variability in their responses to stress. Scientists, in fact, have observed similar effects with identical twins as they age.
The Future Holds: In addition to this new insight into epigenetics, the finding has practical applications: managed forests often have large swaths of trees that have the same genes, which means that a whole grove can be destroyed when a pathogen or environmental stressor attacks, the researchers point out. But if foresters can classify clones by these epigenetic characteristics, they can plant a more robust forest, one that has some trees that will stand up to drought and others that will withstand flooding. Reference: Sherosha Raj, Katharina Bräutigam, Erin T. Hamanishi, Olivia Wilkins, Barb R. Thomas, William Schroeder, Shawn D. Mansfield, Aine L. Plant, and Malcolm M. Campbell. Clone history shapes Populus drought responses. Proceedings of the National Academy of Sciences, July 11, 2011 DOI: 10.1073/pnas.1103341108
Image credit: James Jordan / flickr