Field trials will be required to study further the recombination events as they occur in nature. Is there a message from the containment experiments for people who are going to use these kinds of transgenic plants? Firstly, the fact that recombination happens means that the transgenic lines are not going to stay as they were made for long. This has economic implications. Secondly, it is important to know what can be done to minimise recombinations. Various small parts of the viral genome could be introduced into the potato so that the need for several recombination events to occur would reduce the overall frequency of successful recombination. A mixed population of transgenic plants could be used each with a different type of resistance. Crop rotation could be employed. The same transgenic should not be grown in the same field two years running. The rotation would need to be organised regionally so that farmers would know what is going on in their locality. Such regional control would be easy in a small country like Switzerland. Thirdly, ideally the smallest effective transgene would be used. If these were all of the same sequence when multiple copies of the gene are introduced, the requirement to minimise insertion of the minimum necessary genetic information would be met.
Multiple copies are needed only for resistance based on gene silencing. Other types of poty virus resistance have been obtained with one transgene copy insertion. These require synthesis of a functional protein (e.g. tobacco mosaic virus coat protein) and thus using gene fragments in these cases is ruled out. The resistance mechanism in such cases is not well understood but probably works by disturbing the viral coating/uncoating equilibrium so that viral replication is inhibited.
If resistance selection and use in conventional monocultures is pushed too far, not only will resistance be lost but also there will be a selective pressure in favour of the emergence of new viral strains, thus giving rise to a new problem hitherto not encountered.
There are three economically important potato viruses in the area (PVX, PVY & PVA) which are routinely tested for. The early transgenics aimed at dealing with these had the ampicillin gene but this was dropped in favour of having only the kanamycin and coat protein genes. They were put into a local variety named 'Charlotte'. A potato breeder visiting the research establishment was obviously unimpressed by the resulting virus resistant plants. He can assure virus free crops by selling tested, certified tubers as seed. Even if they succumb to primary infection there is no reduction in crop yield in the first year. It only means that the farmers cannot use their own seed for the second year because the results of secondary infection are far more devastating. So in France and Switzerland it is not in the seed potato producer's interest to have virus resistant plants. In Germany, however, regulations allow for royalties to be claimed on reuse of seed tubers from resistant strains.