COMPARATIVE VIRUS-RESISTANCE AND FRUIT YIELD OF TRANSGENIC SQUASH WITH SINGLE AND MULTIPLE COAT PROTEIN GENES

Five transgenic squash lines expressing coat protein (CP) genes from c ucumber mosaic cucumovirus (CMV), zucchini yellow mosaic potyvirus (ZY MV), and watermelon mosaic virus 2 potyvirus (WMV 2) were analyzed in the field for their reaction to mixed infections by these three viruse s and for fruit production. Test plants were exposed to natural inocul ations via aphids in trials simulating the introduction of viruses by secondary spread from mechanically infected susceptible border row pla nts. Plants of transgenic line CZW-3 expressing the CP genes from CMV, ZYMV, and WMV 2 displayed the highest level of resistance with no sys temic infection, although 64% exhibited localized chlorotic dots which were mainly confined to older leaves. CZW-3 plants had a 50-fold incr ease in marketable yield compared to controls and the highest predicte d cash returns. Plants of transgenic line ZW-20 expressing the CP gene s from ZYMV and WMV 2 displayed high levels of resistance to these two potyviruses, but 22% became infected by CMV. However, ZW-20 plants pr ovided a 40-fold increase in marketable yield relative to controls and good estimated cash returns. Three transgenic lines expressing single CP genes from either ZYMV (line Z-33), WMV 2 (line W-164) or CMV (lin e C-14) developed systemic symptoms similar to those of controls but s howed a delay of 2 to 4 weeks before the onset of disease. Plants of t ransgenic line Z-33 were highly resistant to ZYMV but not to WMV 2 and CMV. Interestingly, Z-33 plants had a 20-fold increase in marketable yield compared to controls and some predicted cash returns if market s ale prices were high. This study indicates that virus-resistant transg enic lines are economically viable even if they are affected by viruse s other than those to which they are resistant.