Thursday, 15 November 2012

Is Resistance Futile? How useful can GM really be in the battle to protect our crops?

Anyone who has studied military history will probably be familiar with the concept of an 'arms race'. One army develops some new type of weaponry which gives them the edge over their rival. The rival responds by developing a slightly better type of weaponry so that they now have a slight edge. This prompts a new development from the first army, which then prompts another new development from the second, and so on. As the process continues, both armies make a great deal of progress in terms of military technology, but very little progress in terms of their ability to overpower the other rival. They are both always either slightly stronger than, slightly weaker than or roughly the same strength as the other army.

Arms races are not just a human phenomenon, but are also a key part of evolution, including plant evolution. All plants are constantly locked into evolutionary arms races with two groups of living things: pathogens (organisms such as microbes and viruses that cause disease) and pests (organisms such as insects and grazing mammals that eat plants). As soon as a plant species evolves a new line of defence (a new poison perhaps), these pathogens and pests begin evolving some way of overcoming that defence (an antidote, for example). This arms race occurs not because the organisms want to outdo each other, but because of the inevitable pull of natural selection.

A cabbage plant which happens (not through design, but through sheer genetic luck) to be born with a new type of anti-slug poison will be more successful than the other cabbages around it and will produce more seed. Its offspring (which inherit the new line of defence) will similarly out-compete the cabbages around them. Within a few generations, the entire field will be full of slug-proof cabbages. It is only a matter of time, however, before a slug is born which (not through design, but through sheer genetic luck) is resistant to the poison. This fortunate creature will have access to all of the cabbage that the other slugs cannot eat and will therefore grow big and strong and produce lots of babies. Before long, all of the slugs in the field will be resistant to the poison. And so the arms race goes on. It is important to note the this slug only had an advantage because the poison existed. If the the poison did not exist then this slug would have had an antidote to a non-existent poison, which is no advantage. The existence of a defence actually causes the evolution of a counter-defence. It is a battle that has been raging for hundreds of millions of years and is not going to stop any time soon.

At some point, just a few thousand years ago, we got ourselves involved in this fight, and ever since we have been doing everything we can to protect our crops from pests and disease. Our most recent weapon is the use of genetic modification to give crops defence mechanisms that they might never have evolved naturally. For example, there are a group of genetically modified (GM) crops called the 'Bt crops' which have been given genes from a bacterium in order to protect them from insects that like to eat them.

The bacterium which donated the genes is a soil-dweller called Bacillus thuringiensis (Bt for short). Bt has a set of genes that allow it to produce a special group of anti-insect proteins. If an insect eat the bacterium, these proteins damage the inside of that insect's stomach, causing it to die. For decades, farmers (including organic farmers) have sprayed the bacterium onto their crops as a form of natural pesticide. Each of the anti-insect proteins affects only one type of insect, meaning that there is no harm to other organisms in the field, or to humans who eat the food.

There are two drawbacks to spraying crops with the bacterium. Firstly, it can be washed off by rain. Secondly it does not get inside plant stems, which is where some insects (such as the corn borer) like to lay their young. Transferring the Bt genes to crops solves these problems because it means that the plants themselves produce the proteins.

In the late 90s Bt cotton and Bt corn (which each contain one Bt gene for one Bt protein) were invented. They are now grown on hundreds of millions of acres around the world (mostly in the USA, Australia, China and India, but they are becoming more widespread in other countries; other Bt crops such as Bt rice are also in the pipeline). Not only has this raised yields, it has also reduced the use of pesticides, which can be harmful to the environment due to, among other things, their effects on non-target insects. In many countries, such as India and China, farmers previously sprayed powerful pesticides from containers on their backs, resulting in many farmers being poisoned. Since the introduction of Bt cotton the number of poisonings has gone down significantly.

But there is a problem with Bt crops, which you might be able to guess by now. At some point an insect might be born which is resistant to one of the Bt proteins. This would give it an advantage and before long there would be many resistant insects. Unfortunately, this has already happened, multiple times in multiple countries (including the USA, Pakistan and China). At the moment, resistant insects account for only few percent of the insect pests in those areas, but if nothing is done, it is only a matter of time before Bt-resistance spreads and Bt-crops become completely useless. But what can be done?

Well, ever since Bt crops were first grown, scientists have tried to prevent insects from becoming resistant by urging farmers to plant 'refuges' of non-Bt crops next to their Bt crops. This reduces the advantage that a Bt-resistant insect would have, since the other insects can still find some food. However, the advantage is only reduced, not completely eliminated, so while this strategy can delay the problem it cannot completely prevent it.

So is it hopeless to expect GM to protect our crops in a sustainable way? Well, actually, it might not be.There is a new technique called 'engineering durable resistance' that might just put an end to the arms race. It is a very simple idea and it works like this: rather than giving a GM crop one way to kill a pest or pathogen, you give give it multiple ways to kill that one pest or pathogen. For example, a new version of Bt cotton has been released in the US and Australia, which has two different Bt proteins that both kill the cotton bollworm. Now, a cotton bollworm which is born with resistance to one of the proteins has absolutely no advantage, because the cotton plant will still kill it. In order to have an advantage a bollworm would have to be born with resistance to both proteins, which is very unlikely. If the number of proteins was increased to say, 4 or 5, then the chances of a bollworm being born with resistance to all of them are vanishingly small.

Of course, it is impossible to know for certain whether this represents a long-term victory. In theory, durable resistance should provide lasting protection, but theory doesn't always turn out to be right. The appropriate response at this stage is probably something along the lines of quiet optimism.

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