Consider Susan Lolle’s current study of over 200 000 members of a cabbage family plant ...

If it backs up her controversial 2005 findings, scientists may have to rethink their understanding of inheritance. 

The question is, was Mendel wrong?  Arabidopsis thaliana might suggest it.

According to Mendel’s theory of genetic inheritance, offspring inherit one gene (allele) from each parent. If both parents pass on identical mutant alleles, the offspring are said to be homozygous and will be sure to resemble their parents in that trait. And, barring contamination from wild plants, offspring should not be able to revert to the non-mutant (“wild”) state.

Why might Mendel be wrong?

Lolle’s 2005 paper with Robert Pruitt of Purdue University, “Genome-wide non-mendelian inheritance of extra-genomic information in Arabidopsis”, suggested that a mutant variety of this species overrides its genetic code and does indeed revert back to its wild state.

Starting in the 1990s, the researchers began using specimens of A. thaliana to study plant cuticles.

Lolle and Pruitt bred plants with a mutant gene called Hothead (HTH2). The plants used in their research received the HTH2 gene from both parents.

Hothead mutants have fused reproductive organs, making breeding with wild A. thaliana plants from outside their study unlikely. Lolle and Pruitt should have had only HTH2 mutants to conduct their future research with. That’s not what happened.

By 2000, Lolle and Pruitt noticed some descendants of their original hothead mutants did not have fused reproductive organs. After running experiments to rule out outcrossing or pollen contamination, they theorized that something else must be happening.

One possibility was that an RNA copy of ancestral DNA was allowing plants to retrieve information they needed to return back to their wild state. And while that state makes reproduction easier, the homozygous H2H2 mutants should still not have been able to return to their wild state.

Findings challenged

The paper has started a flurry of controversy. It has been challenged by scientists who explain away Lolle’s findings as the result of contamination or outcrossing.

The findings sparked intense debate as researchers set about looking for explanations that did not contradict Mendelian inheritance. Molecular geneticist, Steve Mount , Associate Professor, Department of Cell Biology and Molecular Genertics and participant in the Arabidopsis thaliana Research Initiative at the University of Maryland (AtRIUM),believes outcrossing can explain much of Lolle and Pruitt's 2005 findings. But their discovery of a double wild-type embryo and a female reverted mutant in their Hothead sample needs investigation.

Perhaps the most serious challenge to Lolle’s work has been a study conducted by Steve Jacobson, Professor of Molecular Cell and Developmental Biology at the University of California, Los Angeles.

That study found that plants grown in isolation did not revert back to the wild state, and Jacobson chalks up Lolle and Pruitt’s findings to outcrossing.

But Lolle points out that Jacobson’s sample was too small to demonstrate the 10% reversion of her 2005 study. And Jacobson did not verify homozygous HTH2 in his sample by dissecting embryos or genetically typing plants.

And it appears there may be other research that puts Mendel's theories in doubt.

Lolle has joined forces with, Reid Palmer ,a Professor in the College of Agriculture and Life Sciences at Iowa State University to investigate possible reversion mechanisms in soybeans. Palmer's research has already uncovered allele frequencies that contradict Mendelian genetics.

There is evidence that flax also violates Mendel's theories. In 2005, biologist Christopher A. Cullis at Case Western Reserve University reported that a single copy 5.7 kilobase DNA fragment inserts itself into stable lines of flax as a result of environmental conditions. And that genetic change is heritable.

New study said to support Lolle’s findings

We may soon know who is right. Since 2005, Lolle has been pinning down the percentage of outcrossing in her plants and replicating her findings. She is set to release a new study of over 200,000 plants. So far, she’s found a rate of 40% reversion in samples isolated to protect them from outcrossing.

If Lolle’s new study holds up to scrutiny, theories of Mendelian inheritance will have to be revised.

Background

Susan Lolle, BSc Honors (Queen's), PhD (McGill), is an Associate Professor at the University of Waterloo in Biology.

She has been challenging our knowledge of Mendelian inheritance since she began researching plant cuticles' role in helping plants protect themselves, produce wax and regulate water supply while at the Department of Molecular and Cellular Biology at Harvard University in the 1990s.

In the 1990s Lolle and Robert Pruitt of Purdue University began to work with A. thaliana. It was a good choice for a few reasons: Plants reach maturity in only five weeks, after which they produce about 10,000 seeds. The plant’s small genome also simplifies research, and the tendency to self-fertilize creates a uniform gene pool that is also resistant to outcrossing.