Transgenic Virus Resistance in Crop-Wild Cucurbita pepo Does Not Prevent Vertical Transmission of Zucchini yellow mosaic virus

Zucchini yellow mosaic virus (ZYMV) is an economically important pathogen of cucurbits that is transmitted both horizontally and vertically. Although ZYMV is seed-transmitted in Cucurbita pepo, the potential for seed transmission in virus-resistant transgenic cultivars is not known. We crossed and backcrossed a transgenic squash cultivar with wild C. pepo, and determined whether seed-to-seedling transmission of ZYMV was possible in seeds harvested from transgenic backcrossed C. pepo. We then compared these transmission rates to those of non-transgenic (backcrossed and wild) C. pepo. The overall seed-to-seedling transmission rate in ZYMV was similar to those found in previous studies (1.37%), with no significant difference between transgenic backcrossed (2.48%) and non-transgenic (1.03%) backcrossed and wild squash. Fewer transgenic backcrossed plants had symptom development (7%) in comparison with all non-transgenic plants (26%) and may be instrumental in preventing yield reduction due to ZYMV. Our study shows that ZYMV is seed transmitted in transgenic backcrossed squash, which may affect the spread of ZYMV via the movement of ZYMV-infected seeds. Deep genome sequencing of the seed-transmitted viral populations revealed that 23% of the variants found in this study were present in other vertically transmitted ZYMV populations, suggesting that these variants may be necessary for seed transmission or are distributed geographically via seeds.

449 Cucurbit Viruses in Southern Illinois

Cucurbit viruses are a major hindrance to cucurbit production in southern Illinois, often rendering cucumber and summer squash fruit unmarketable. Specific viruses infecting cucurbits in the region need to be determined since this would enable growers to make better decisions on virus disease management. Leaf samples of various cucurbit vegetables that had symptoms of viral infection were collected from grower fields during the 1998 and 1999 growing seasons to determine the predominant cucurbit viruses present. Samples were assayed for the presence of five individual viruses: cucumber mosaic virus (CMV), papaya ringspot virus (PRSV, formerly watermelon mosaic virus-1), squash mosaic virus (SqMV), watermelon mosaic virus (WMV, formerly watermelon mosaic virus-2), and zucchini yellow mosaic virus (ZYMV). Results from the two years indicated that WMV was the most prevalent virus in southern Illinois. Cucumber mosaic virus was found both years, but only in a low percentage of samples collected each year. Two cucurbit viruses, PRSV and ZYMV, were each identified only in one sample during the 1998 growing season, and neither were found in any of the samples collected during 1999. Squash mosaic virus was not identified in any of the samples collected during 1998; however, for the 1999 growing season, SqMV was identified in 19% of the samples collected, primarily from those samples of transgenic squash that were showing symptoms of virus infection.

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 cucumber mosaic cucumovirus (CMV), zucchini yellow mosaic potyvirus (ZYMV), and watermelon mosaic virus 2 potyvirus (WMV 2) were analyzed in the field for their reaction to mixed infections by these three viruses and for fruit production. Test plants were exposed to natural inoculations via aphids in trials simulating the introduction of viruses by secondary spread from mechanically infected susceptible border row plants. Plants of transgenic line CZW-3 expressing the CP genes from CMV, ZYMV, and WMV 2 displayed the highest level of resistance with no systemic infection, although 64% exhibited localized chlorotic dots which were mainly confined to older leaves. CZW-3 plants had a 50-fold increase in marketable yield compared to controls and the highest predicted cash returns. Plants of transgenic line ZW-20 expressing the CP genes from ZYMV and WMV 2 displayed high levels of resistance to these two potyviruses, but 22% became infected by CMV. However, ZW-20 plants provided 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 (line C-14) developed systemic symptoms similar to those of controls but showed a delay of 2 to 4 weeks before the onset of disease. Plants of transgenic 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 sale prices were high. This study indicates that virus-resistant transgenic lines are economically viable even if they are affected by viruses other than those to which they are resistant.

Field Performance Comparison of Two Transgenic Summer Squash Hybrids to Their Parental Hybrid Line

`Pavo', a commercially grown, virus-susceptible squash (Cucurbita pepo L.) hybrid, and two experimental virus-resistant transgenic squash hybrids, XPH-1719 and XPH-1739, were tested for field performance. The two transgenic squash hybrids possess the desired fruit and plant characteristics of their parental line, `Pavo', plus resistance to zucchini yellow mosaic virus and watermelon mosaic virus 2 (XPH-1719), and resistance to zucchini yellow mosaic virus, watermelon mosaic virus 2, and cucumber mosaic virus (XPH-1739). Percent emergence and days to flowering were similar among the three hybrids. XPH-1719 and XPH-1739 were equally effective in producing a high percentage of quality marketable fruit and yields with 90% and 13,800 kg·ha–1 and 87% and 16,500 kg·ha–1, respectively. XPH-1719 and XPH-1739 demonstrated their outstanding virus resistance over `Pavo' by producing only 3% and 14% symptomatic plants, respectively, compared to 53% for `Pavo'. They also produced the lowest percentage of infected fruit, 0% and 7%, respectively, with `Pavo' at 26%.