proteins from the plant itself, EPA should provide guidelines for determining when a similar protein produced by another organism can be considered equivalent to the plant-produced protein and be used in toxicological and environmental testing (see recommendation in chapter 2, section 2.5.1).

Research should be conducted to assess the relevance of using forageand grain-feeding organisms in chronic toxicity studies (see section 2.5.2). Such studies have historically been conducted in evaluating new conventionally-bred forage-crop varieties and have often detected effects of new varieties on animal performance traits (Hanson et al. 1973; Reitz and Caldwell 1974).

3.1.4 Virus-resistant Squash and Papaya

Many crops are damaged by viruses, sometimes to the extent that these pathogens limit the regions where the crop can be grown. Viruses are often transmitted by aphids and other insects that are difficult or impossible to control effectively with pesticides. Conventional breeding for virus-protection is sometimes possible, but naturally occurring protective genes are not always available in the crop or related species. Furthermore, the rapid evolution and spread of new viral strains often thwarts efforts to achieve durable protection in the crop.

Transgenic methods can provide much-needed protection from viruses by transferring pieces of the viral genome into plants (Sanford and Johnston 1985; Powell-Abel et al. 1986). In the future, it might be possible to avoid viral infections by developing plants that have transgenic protection from aphids and other vectors. Here the committee discusses the first transgenic virus-protected crops to be granted nonregulated status and sold commercially: Asgrow's crookneck squash varieties (deregulated in 1994 and 1996) and Cornell University' s papaya (deregulated in 1996). Virus-protected potato has also been approved for marketing, and more than 20 other domesticated species have been field-tested to evaluate transgenic protection against viruses (chapter 1, table 1.4 and table 1.5).

Squash

Varieties of domesticated Cucurbita pepo—commonly known as zucchini, yellow crookneck squash (summer squash), or acorn squash—are widely cultivated in the United States for human consumption. In some regions and some years, viral infections stunt and mottle, or deform the squash, causing major economic losses and even complete failure of the crop. The outbreaks are often intermittent, however, and therefore difficult to predict (for example, Schultheis and Walters 1998). Some of the



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