These include a possible limit on the length of genetic sequences introduced into crop plants and the banning of genes that make functional proteins. The department is also worried about particularly high-risk sequences, such as those that trigger the process of viral replication. But critics believe that not enough is yet known to say what the risks of recombination are. And some of those at the meeting suggested that it might be a smoke screen. There is, however, evidence that existing viruses may pick up a trait from a transgenic plant.
Some viruses seem to be recent products of recombination that arose naturally.
And in laboratory experiments, viruses from which a genetic sequence has been removed have reacquired it from transgenic plants carrying the missing genes. Traits such as the ability to move more efficiently from cell to cell or to infect other kinds of plants could prove dangerous if transferred to another virus. The technology's proponents claim that if recombination were likely, new hybrid viruses would be turning up all the time. You can go to a potato field or a tomato field, or corn or wheat, where all these viruses are living together. But Allen Miller, a plant virologist at Iowa State University, points out that transgenic plants will contain the viral genes in all their cells all the time, increasing the risk of recombination.