In a startling discovery, geneticists at Purdue University say they have found plants that possess a corrected version of a defective gene inherited from both their parents, as if some handy backup copy with the right version had been made in the grandparents’ generation or earlier.
The finding implies that some organisms may contain cryptic backup copies of their genomes that bypass the usual mechanisms of heredity.
If confirmed, it would represent an unprecedented exception to the laws of inheritance discovered by Gregor Mendel in the 19th century. Equally surprising, the cryptic genome appears not to be made of DNA, the standard hereditary material.
The discovery also raises interesting biological questions – including whether it gets in the way of evolution, which depends on mutations changing an organism, not being put right by a backup system.
“It looks like a marvelous discovery,” said Elliott Meyerowitz, a plant geneticist at the California Institute of Technology. David Haig, an evolutionary biologist at Harvard, described the finding as “a really strange and unexpected result” that will be important if the observation holds up and applies widely in nature.
The new result – reported online Tuesday in the journal Nature by Robert Pruitt, Susan Lolle and colleagues at Purdue in Indiana – has been found in a single species, the mustardlike plant called arabidopsis that is the standard laboratory organism of plant geneticists.
But there are hints that the same mechanism may occur in people, according to a commentary by Detlef Weigel of the Max Planck Institute for Developmental Biology in Tübingen, Germany. Weigel describes the Purdue work as “a spectacular discovery.”
The finding grew out of a research project started three years ago in which Pruitt and Lolle were trying to understand the genes that control the plant’s outer skin, or cuticle. As part of the project, they were studying plants with a mutated gene that made the plant’s petals and other floral organs clump together. Because each of the plant’s two copies of the gene were in mutated form, they had virtually no chance of having normal offspring.
But as many as 10 percent of the plants’ offspring reverted to normal. Various rare events can make this happen, but none involve altering the actual sequence of DNA units in the gene. Yet when the researchers analyzed the mutated gene, known as hothead, they found it had changed, with the mutated DNA units returned to normal form.
A mutated gene can be put right by various mechanisms that are already known, but all require a correct copy of the gene to be available to serve as the template. The Purdue team scanned the DNA of the entire arabidopsis genome for a second, cryptic copy of the hothead gene but could find none.
Pruitt and his colleagues argue that a correct template must exist, but because it is not in the form of DNA, it probably exists as RNA, DNA’s close chemical cousin. RNA performs many important roles in the cell, and is the hereditary material of some viruses. But it is less stable than DNA and has been regarded as unsuitable for preserving the genetic information of higher organisms.
Pruitt said he favored the idea that there is an RNA backup copy for the entire genome, not just the hothead gene, and that it might be set in motion when the plant was under stress. International Herald Tribune