in naturally infected squash plants” (USDA 1996). Synergy was not considered an environmental risk but rather an agronomic problem. “Asgrow inoculated CZW-3 with several common squash-infecting viruses. No synergistic symptoms were seen in infected plants” (USDA 1996).
In both response documents, APHIS concluded that the transgenic squashes should pose no greater risk of evolution of new viruses than naturally infected plants. In the second document, that conclusion was extended to cover the case of wild relatives that pick up the transgenic traits through introgression.
Increased Weediness in the Transgenic Squash Relative to Convention ally Bred Squash. For both ZW-20 and CZW-3, APHIS addressed the risk that the virus resistance genes would increase the weediness of yellow crookneck squash. Yellow crookneck squash is not listed as a common or troublesome weed anywhere in the United States; for example, it is not on the Weed Science Society of America’s “Composite List of Weeds” (available online at http://ext.agn.uiuc.edu/wssa/). Squash volunteers occur adjacent to squash production fields and, if necessary, are controlled mechanically or with herbicides. They do not readily establish as feral or free-living populations (USDA 1994b, 1996).
For ZW-20, Upjohn/Asgrow supplied APHIS with data comparing the transgenic squashes with their nontransgenic counterparts, showing “no major changes in seed germination, cucurbitin levels, seed set viability, susceptibility or resistance to pathogens or insects (except ZYMV and WMV2), and there are no differences in overwintering survivability” (USDA 1994b). For CZW-3 the APHIS response document stated that “Asgrow has reported that there are no major changes in CZW-3 performance characteristics (except for resistance to CMV, ZYMV, and WMV2)” (USDA 1996). Given that the transgenic squashes would be expected to be grown in the same regions as squash is typically grown, APHIS concluded that “there is no evidence to support the conclusion that introduction of virus resistance genes into squash could increase its weediness potential. Many pathogen and insect resistance genes have been introduced into commercial varieties of squash by conventional means in the past without any reports of increased weediness” and noted that conventionally improved cultivars having resistance genes to viruses had already been developed. In both response documents, APHIS concluded that the virus resistance transgenes are unlikely to increase the weediness of yellow crookneck squash (USDA 1994b, 1996).
Impact on Non-target Organisms Other Than Wild Relatives. APHIS pointed out that both ZW-20 and CZW-3 transgenic squash plants have