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Don't Jump to Wrong Conclusions on Jumping Genes

Don't Jump to Wrong Conclusions on Jumping Genes

There has been confusion in the media about the relevance to GM crops of a complex paper by Dr Jeremy Timmis and colleagues, published today by the journal Nature.

Timmis himself has stated (as well-reported by the Australian and ABC) that the work does not suggest that GM plants are less safe.

Timmis specifically rejected suggestions that his work means that the risk of gene spread to neighbouring plants is high.

As Professor Timmis noted in the Australian: "It's easy for the public to misinterpret such complex science."

The plants studied by Timmis and his colleagues took advantage of a type of GM that is currently used only in experiments, and is found in no crop currently registered or proposed for release anywhere in the world. Contrary to some media reports, it has not been considered a stand-alone strategy for avoiding superweeds.

Further, even if the kind of genes studied do jump within the cell, they can't make superweeds because the genes become effectively silent once they have left their home base in the cell.

Here are the key points to understand about this fascinating study into the evolution of plant cells.

In addition to the DNA on chromosomes within the cell nucleus, plants have DNA in the light-gathering photosynthetic compartments of the cell, the chloroplasts. DNA and other evidence proves that chloroplasts originated as bacteria that became symbiotically included in plant cells hundreds of millions of years ago.

As noted in the first line of the paper in Nature, "Gene transfer from the chloroplast to the nucleus has occurred over evolutionary time".

Some evidence for this is that bacterial DNA is coded somewhat differently to the plant nuclear DNA. In fact, when bacterial DNA is injected into the nucleus to make GM plants, it is very poorly expressed; it sits there without doing much (doesn't make protein or otherwise affect the plant's resistance).

The Nature paper also showed that genes expressed in the chloroplast were poorly expressed in the nucleus and vice versa. That is, because the genetic code is slightly different, the plant nucleus can no longer act on any message.

It's a bit like the chloroplasts read only French and the cell nucleus only English. They use the same letters and some of the words are similar, but complex French messages would be virtually meaningless to the English nucleus.

The chloroplasts stay with the mother plant in the developing seeds, and are not transferred through pollen, but are only "maternally" inherited.

Thus, to actually measure the rare event of gene transfer from chloroplast to nucleus, Timmis and his colleagues Chun Huang and Michael Ayliffe had to put DNA into the chloroplast that would provide a kind of chemical resistance when it moved to the cell nucleus and was then transferred via pollen to offspring. This allowed the researchers to screen 250,000 plants to find the 16 that showed the gene transfer.

In sum, the paper shows that genes can move from the chloroplasts to the cell nucleus, but at low frequencies, and then be transferred to other plants via pollen. However, the genes can't make "superweeds" because they could no longer cause resistance or other traits to other plants because the DNA is no longer read.

Because chloroplast DNA is not often transferred in pollen (in fact, less than 0.01% of the time in the Nature paper), it has been proposed that genes inserted into chloroplasts would be less likely transferred from crop pollen to weeds.

However, because this method does nothing to prevent crop-weed hybrids from occurring via weed pollen moving to the crop, chloroplast-based GM plants have never been accepted as a stand-alone method of restricting gene transfer.

For the great majority of crops, gene flow is naturally very limited anyway.

Further, before the registration of any GM crop any where in the world, the regulatory systems must be satisfied that there is no significant risk to human health or the environment, no matter how the crops could be dispersed.

In sum, (1) insertion of novel genes into chloroplasts remains experimental, (2) it is not relevant to any current biotech product and (3) it's thus not relied upon to reduce gene flow issues related to current crops, including canola in Australia.

The Nature paper is a fascinating insight into cellular evolution, but not relevant to the current GM debate.

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