Submitted to: Environmental Biosafety Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2004
Publication Date: 2/1/2004
Citation: Fuchs, M., Chirco, E.M., McFerson, J.R., Gonsalves, D. 2004. Comparative fitness of a wild squash species and three generations of hybrids between wild x virus-resistant transgenic squash. Environmental Biosafety Research. 3(1):17-28. Interpretive Summary: The movement of transgenes from transgenic crops to wild relatives become of potential concern if plants arising from the cross show added fitness that might make them a threat to the environment. A particular concern is that the transgene movement might make the wild relatives weedier. To address this potential, we compared the relative fitness of the wild squash with hybrids that were formed through the movement of the virus resistant transgene from the transgenic squash to the wild squash. Our results showed that transgenic progeny from wild squash that had crossed with the transgenic squash showed good growth under conditions of high virus pressure when compared to their nontransgenic counterparts. Thus, the interpretation is that virus resistance makes the resistant wild squash more fit than nontransgenic susceptible squash if virus pressure is very high in the vicinity where wild squash are growing. Additionally, a small percentage of progeny from an initial cross of the domesticated squash with the wild squash showed characteristics of the wild squash. This indicates that transgenic progeny with horticultural characteristics of the wild squash can be obtained after a few crosses.
Technical Abstract: We compared some fitness components of the wild squash species Cucurbita pepo spp. ovifera var. texana (C. texana) and three generations of hybrids (F1, BC1, and BC2) between C. texana and commercial transgenic squash CZW-3 over three consecutive years under field conditions of low (LDP) and high disease pressure (HDP) by Cucumber mosaic virus (CMV), Zucchini yellow mosaic virus (ZYMV) and Watermelon mosaic virus (WMV). Transgenic squash CZW-3 expresses the coat protein (CP) genes of CMV, ZYMV, and WMV, and is resistant to these three aphid-borne viruses. Across all HDP trials, transgenic BC1 and BC2 hybrids expressing the three CP genes grew more vigorously, displayed resistance to CMV, ZYMV, and WMV, and produced a greater number of mature fruits and viable seeds than nontransgenic hybrid segregants and C. texana. Transgenic F1 hybrids behaved similarly to BC1 and BC2 hybrids but grew less vigorously than C. texana. In contrast, across all LDP trials, C. texana outperformed the transgenic and nontransgenic hybrid segregants. Further, only one back cross was necessary to recover individuals with most of the C. texana characteristics and yet maintain virus resistance. Our data suggest that C. texana acquiring CP transgenes upon hybridization and introgression could have a selective advantage if CMV, ZYMV, and WMV are severely limiting the growth and reproductibility of wild squash populations.