Response

Norman C. Ellstrand's disagreement with aspects of our review1 reflects genuine differences of opinion among scientists in the field of biosafety. The disparity in these views is exemplified in a recent review2 of Ellstrand's book3, in which Ellstrand's “state of the world” is summarized as one in which “gene flow and genetic pollution are everywhere, varying only by degree and crop alleles are ubiquitous, infiltrating native ecosystems with unknown consequences”, whereas other researchers view potential introgression problems as being more limited in extent, and geographically restricted to a smaller set of crops and locales. With experts in the field at loggerheads, it is not difficult to understand why the general populace and politicians are confused about agricultural biotechnology and the cultivation of transgenic plants. A book designed to illustrate this dilemma and aimed squarely at these confused parties is currently in production (a gratuitous advertisement)4.

We stand by all our statements in the Nature Reviews Genetics Review on transgene introgression1. The fact remains that all domesticated plants come from wild plants, not vice versa. Throughout our paper, we used the commonly agreed definition of introgression as “the permanent incorporation of genes from one set of differentiated populations (species, subspecies, races and so on) into another.” Therefore, we used stringent molecular evidence in assessing whether crop-to-weed introgression has indeed taken place — apparently, a more stringent screen than used by Ellstrand and some others when discussing the cases of transgene or crop gene introgression3,5,6.

Most of our knowledge from known introgression events is the result of work performed by plant breeders who purposefully incorporate DNA from wild germplasm into largely domesticated plants for crop improvement — not a trivial task. There are, in fact, more documented examples of weed-to-crop introgression than crop-to-weed introgression7. Proving crop-to-weed introgression is inherently more difficult. Nature must be surveyed and a high degree of confidence must be assigned about genetic point-of-origin — that sequences in putative introgressed plants indeed came from the crops in question. Nonetheless, in our review, we noted that there is convincing molecular evidence of introgression that involves certain crops and weeds, and furthermore, with regards to regulating transgenic plants, that our energy should be focused on these crops and wild relatives. Nor were we dogmatic to suggest that introgression in crops, such as soybeans to wild soybeans, will never happen. It is simply that the peer-reviewed publications so far do not provide convincing evidence that introgression has occurred in all the cases that Ellstrand would lead us to believe.

Rather than classifying our review as optimistic, we strove to present an even-handed and realistic view of the world in the hope that the information would be helpful to regulators and to those interested in transgene introgression. We see potential introgression risks as being pinpointed to certain crops, weeds and locales, but not as a global problem.