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Environmental Effects of Transgenic Plants: The Scope and Adequacy of Regulation
no direct pathogenic properties. The protein products of the transgenes are already present in high concentration in naturally infected plants, and the levels of cucurbitin—a naturally occurring plant defensive compound—are likely to be unchanged. Therefore, APHIS concluded that “there is no reason to believe deleterious effects on beneficial organisms could result specifically from the cultivation of” the new transgenic squashes. APHIS noted Upjohn/Asgrow taste tests for cucurbitin levels, but otherwise its conclusions were based on the fact that the coat proteins present in the transgenic squashes are already present in the environment in virus-infected plants. In the second determination, APHIS briefly examined the issue of whether insecticide usage might be reduced by the introduction of the CZW-3 but did not reach a conclusion.
Impacts on Free-Living Relatives of Squash Arising from Interbreeding. Most of the APHIS discussion in the response documents, particularly the 1994 one focuses on whether wild relatives could benefit from virus-resistance alleles, leading to the evolution of increased weediness. The effort was, in part, in response to several negative comments received after the three APHIS Federal Register announcements associated with ZW-20. Many of the comments questioning the decision did so because it marked an important APHIS precedent. As noted in Chapter 2, the sexual transfer of beneficial alleles from a transgenic crop to a wild relative might result in the evolution of a more difficult weed. This issue is perhaps the most widely discussed risk associated with transgenic crops (e.g., Colwell et al. 1985, Goodman and Newell 1985, Snow and Moran-Palma 1997, Hails 2000).
This case study has all three elements that could create such a risk— transgenes of a type that could confer a fitness boost in the wild, a sexually compatible wild relative, and the fact that the wild relative has been classified as a weed. If the crop mates with the wild relatives introducing virus resistance into wild populations and if the primary factor limiting the aggressiveness of wild populations is disease caused by the same viruses, introgression of the transgenes could result in increased weediness of the wild relatives. To obtain more information on the relevant biology of the wild relative, APHIS commissioned a report on the risks that might be posed by crop to wild gene flow by Hugh Wilson, an expert on cucurbit taxonomy and ecology. Wilson (1993) concluded that free-living Cucurbita pepo (FLCP) is a significant weed that might benefit from protection from ZYMV and WMV2. Key information on squash and its weedy North American relatives is summarized below with APHIS’s conclusions and the committee’s evaluation of those conclusions.