A multi-year comparison of the fitness of the virus resistant transgene in wild populations of Cucurbita pepo

Background/Question/Methods

Gene exchange between crops and their wild relatives is common and difficult to contain.  Unlike most traits of cultivated species, there are concerns that transgenes conferring resistance to herbivores or pathogens could enhance the fitness and weediness of wild species and/or have indirect impacts on non-target species such as pollinators, herbivores, predators, soil fauna, and other plants in the community.  Gene flow between cultivated squash and its wild counterpart (Cucurbita pepo ssp. texana) may result in the introduction of a transgene (VRT) that confers resistance to three common viruses in wild populations.  The fitness of the transgene during escape is influenced by the full ecological community as indirect costs associated with increased exposure to herbivory by cucumber beetles and the bacterial wilt disease they transmit can mitigate the fitness benefits of the VRT.  We have extensively studied the interactions among C. pepo, its primary herbivores (cucumber beetles and aphids), and the pathogens they transmit (Erwinia tracheiphila, the causative agent of bacterial wilt, and three mosaic viruses).  From 2007 - 2012 we have conducted a series of field-scale experimental epidemics using the entire Cucurbita pathosystem. 

Results/Conclusions

We consistently find that a higher proportion of transgenic plants remain healthy during the growing season as they are susceptible to only one of the two pathogens present in our system. As a consequence, transgenic plans are both more likely to reproduce and yield a greater number of fruits when they do.  These results would suggest that transgenic plants have a strong selective advantage.  However, we find that the fruit production of transgenic plants is only marginally greater than that of non-transgenic plants. Our observations indicate that the fitness advantage of transgenic plants is lower than we would expect from the reduction in the rate of virus infection.  Whereas viral infection suppresses reproductive output by approximately one-third relative to healthy plants, wilt infection can result in total loss of reproductive output.  Our observations show that transgenic plants get proportionally more wilt disease than non-transgenic plants, which mitigates the selective advantage of the transgenic plants.