Genetic differences seen in the same tree

Published in The Hind on August 16, 2012

Can clones be genetically different, even by a small extent, from the parent body? For that matter, can there be genetic differences even within the same tree, such that tissues from the top and bottom of a single tree are genetically different?

Ken Paige, an evolutionary biologist at the University of Illinois at Urbana-Champaign and his team made this startling discovery. Their finding was based on a genome-wide analysis of black cottonwood trees (Populus trichocarpa).

“This could change the classic paradigm that evolution only happens in a population rather than at an individual level,” Brett Olds was quoted as saying in Nature. Dr. Olds, a biologist in Paige’s laboratory, presented the research at the 2012 Ecological Society of America Annual Meeting in Portland, Oregon.

Black cottonwood trees have the power to clone themselves. And by sharing the same root system, the resulting offspring continue to remain connected to their parents.

According to the journal, the reasons why the scientists chose to study black cottonwood trees are simple. The full genome sequence is available. Most importantly, the trees live up to 200 years, and they grow to 30-50 metres height and thus the spacing between individual tissues is great both in terms of distance and years. Moreover, the trees exhibit connected clones, thereby providing an ideal opportunity to study both the parent and the offspring.

Samples were collected from 11 parent-clone sets and the genomes sequenced from the root to the highest buds. The genome results were compared with the reference sequence.

The results were stunning — 188,406 mutations were unique to just one tissue sample and these mutations were different from other parts of the same tree! Some of the mutations were seen within protein-encoding DNA sequences, and some in the amino-acid changes in the encoded proteins.

According to Nature, Dr. Olds found the top buds of the parent and offspring in one tree were genetically closer to other tissues in the same plant. In another instance, the differences showed up in a peculiar manner – “the top bud was closer to the reference cottonwood genome than to any of the other tissues from the same individual.”

“When people study plants, they’ll often take a cutting from a leaf and assume that it is representative of the plant’s genome,” Dr.Olds was quoted as saying in Nature. “That may not be the case. You may need to take multiple tissues.”