March 24, 2005

Tiny weed defies gene laws

LONDON (Reuters) - A tiny weed has defied accepted laws by receiving traits from its grandparents that were not carried by its parents, scientists say.

According to the scientific laws of inheritance described by Gregor Mendel in the mid-1800s, characteristics are determined by unique units of inheritance that are passed on intact from one generation to the next.

But scientists at Purdue University in Indiana have discovered the classic rules don't apply to a plant called Arabidopsis thaliana, which has bypassed genetic abnormalities carried by both parents and reverted to normal traits from the grandparents.

"This means inheritance can happen more flexibly than we thought in the past," said Robert Pruitt, a molecular geneticist at the university.

"While Mendel's laws that we learned in high school are still fundamentally correct, they're not absolute."

If the mechanism he and his colleagues discovered in the plant exists in animals, they believe it could pave a path for gene therapy to treat diseases in plants and humans.

In research reported in the science journal Nature, the scientists said they found the anomaly when they noticed normal flowers on plants that were the offspring of deformed plants.

The parents had a mutated gene that prevented its flowers from opening. But the grandparents and 10 percent of the grandchildren had normal flowers.

"If you take this mutant Arabidopsis, which has two copies of the altered gene, let it seed and then plant the seeds, 90 percent of the offspring will look like the plant, but 10 percent will look like the normal grandparents," Pruitt said in a statement.

"Our genetic training tells us that's just not possible. This challenges everything we believe," he added.

Nature 434, 505 - 509


A fundamental tenet of classical mendelian genetics is that allelic information is stably inherited from one generation to the next, resulting in predictable segregation patterns of differing alleles1. Although several exceptions to this principle are known, all represent specialized cases that are mechanistically restricted to either a limited set of specific genes (for example mating type conversion in yeast2) or specific types of alleles (for example alleles containing transposons3 or repeated sequences4). Here we show that Arabidopsis plants homozygous for recessive mutant alleles of the organ fusion gene HOTHEAD5 (HTH) can inherit allele-specific DNA sequence information that was not present in the chromosomal genome of their parents but was present in previous generations. This previously undescribed process is shown to occur at all DNA sequence polymorphisms examined and therefore seems to be a general mechanism for extra-genomic inheritance of DNA sequence information. We postulate that these genetic restoration events are the result of a template-directed process that makes use of an ancestral RNA-sequence cache.


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