Transgenic resistance

Some ninety years after the Mendelian-biometrician conflict started, the molecular biologists give every appearance of following a similar route. With great scientific excitement on their side, but with rather few opportunities for practical application, the molecular biologists are pursuing the idea of transgenic resistance in crops with great vigour. There is a very real danger that most of these single-gene resistances will prove as ephemeral as vertical resistance, when they are employed in farmers' crops on a basis of uniformity. Many irresponsible claims have been made for newly discovered resistance genes, and it is of the utmost importance that this matter be properly assessed.

There can be no doubt that a large area of crop, uniformly protected by a single-gene resistance, will exert very strong selection pressure on the parasite. Clearly, the crucial question is whether these transgenic resistances are within, or beyond, the capacity for micro-evolutionary change of the parasite. And this is not an easy question to answer with any degree of confidence. However, there can be no doubt that great uncertainty exists, and that the molecular biologists should be much more cautious at claiming to have found the answer to crop parasites.

One possible argument is to examine the nature of immunity. The maximum level of horizontal resistance may confer an apparent immunity but it is not true immunity. This is because the horizontal resistance can be host-eroded (see 6.6.1), and the apparent immunity will then be revealed as mere resistance. But it is polygenically inherited resistance, which is beyond the capacity for micro-evolutionary change of the parasite, and it is stable. Similarly, vertical resistance confers an apparent immunity, but only to non-matching vertical pathotypes. This is not true immunity because it is within the capacity for micro-evolutionary change of the parasite, and it is unstable.

Immunity means that wheat is immune to coffee rust, and coffee is immune to wheat rust. We must enquire whether this true immunity can be genetically controlled by a single gene. And we are compelled to conclude that this is most unlikely. It follows that a transgenic resistance conferred by a single gene will not be true immunity. And, because it is conferred by a single gene, this resistance is likely to result from a simple resistance mechanism, such as hypersensitivity. And a simple resistance mechanism is likely to be within the capacity for micro-evolutionary change of the parasite. Such a mechanism will thus confer a temporary resistance. It will be unstable.

An alternative argument is that a transgenic resistance could involve many genes, and be stable. Unfortunately, the transfer of a sufficient number of genes to confer such stability is beyond the capacity of genetic engineering, which is concerned with single genes. Such a transfer of many genes is also likely to alter the target host, beyond acceptable limits.

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