Population size and incidence of virus infection in free-living populations of Cucurbita pepo.

Impact assessments of virus resistance transgene introgression into wild, free-living populations are important for determining whether these transgenes present a risk to agriculture or the environment. Transgenic virus-resistant Cucurbita pepo ssp. ovifera var. ovifera L. (squash) cultivars have been commercialized, and may be cultivated in close proximity to cross-compatible wild, free-living relatives (C. pepo subsp. pepo vars. ozarkana and texana). Therefore, the potential impact of these virus resistance transgenes was studied by surveying the incidence and fluctuations of virus infection (as assayed by ELISA), virus symptoms (which may not be seen in an infected plant) and population size in forty-three free-living C. pepo populations in Illinois, Missouri, Arkansas, Mississippi, Louisiana, and Texas. Ten of these populations were studied over three consecutive seasons. Depending on the year, 61% to 78% percent of the populations had at least one individual infected by at CMV, ZYMV or WMV2, but the median incidence of infection within populations was 13%. The observed infection level in free-living populations was consistent with levels defined as "low" in field plot experiments conducted by others, leading to the conclusion that transgenic virus resistance should not provide a significant fitness advantage to the free-living populations examined. Viral symptoms were detected in only 2% of plants observed, indicating that severity of viral infection was low. CMV, ZYMV, and WMV2 were not the only viruses infecting these populations, further reducing the likelihood that resistance to these viruses would release populations from constraints imposed by virus diseases.