most common cucurbit viruses in the United States are watermelon mosaic virus 2 (WMV2), cucumber mosaic virus (CMV), zucchini yellow mosaic virus (ZYMV), and papaya ringspot virus (PRSV, formerly WMV1). Each of those viruses has many host species in the curcurbit family and others, and all are transmitted by aphids (Price 1940; USDA 1994b). Infections in any given area are sporadic, and by the time symptoms are visible it is often too late to control the aphids with pesticides (for example, Tricoli et al. 1995).
Protection from viruses in cucurbit crops has been improved somewhat by the use of genes from wild and cultivated species (for example, Gilbert-Albertini et al. 1993). Cucurbits are well known for their ability to hybridize (Wilson 1990), but species barriers have often made it difficult to transfer desirable genes into commercial varieties. Genetic protection from WMV2 and ZYMV was developed nearly simultaneously by both transgenic and conventional breeding. The Harris Moran company released a conventionally bred zucchini known as Tigress with protection from WMV2 and ZYMV (USDA 1994b; Schultheis and Walters 1998), and Upjohn/Asgrow used viral coat protein genes to achieve similar objectives in transgenic yellow crookneck squash. Upjohn/Asgrow's first commercial transgenic product was Freedom II, which was protected from WMV2 and ZYMV and was deregulated in 1994; it exhibits strong protection from the targeted viruses (Fuchs and Gonsalves 1995; Tricoli et al. 1995).
In a 1995 field experiment in North Carolina, both the conventional Tigress and the transgenic Freedom II exhibited better protection from viruses than most other varieties tested (Schultheis and Walters 1998). Asgrow then produced another transgenic pest-protected squash, known as CZW-3 (deregulated in 1996), with a marker gene for resistance to the antibiotic kanamycin and coat-protein genes for protection from three cucurbit viruses. Squash plants with transgenic protection from as many as five viruses have been field-tested (USDA 1999c) and might eventually be considered for deregulation.
The health concerns about transgenic virus-protected squash have been related to both viral and bacterial genes that are expressed in all the plant's cells, including the edible portions. Human or animal consumption of plants with viral coat proteins is widely considered to be safe, on the basis of common exposure to these proteins in nontransgenic types of food. However, Asgrow's 1996 virus-protected squash also had a marker gene for resistance to the antibiotic kanamycin. Some people have proposed that the widespread use of antibiotic-resistance genes as markers for transgenic traits could exacerbate current losses of antibiotic effectiveness due to overuse. However, for markers in current commercial use, that risk is extremely small (Nap et al. 1992; Fuchs et al. 1993), and the