Risk assessment of genetically engineered crops: fitness effects of virus‐resistance transgenes in wild Cucurbita pepo

Abstract

Abstract. The development of crops genetically engineered for pathogen resistance has raised concerns that crop-to-wild gene flow could release wild or weedy relatives from regulation by the pathogens targeted by the transgenes that confer resistance. Investigation of these risks has also raised questions about the impact of gene flow from conventional crops into wild plant populations. Viruses in natural plant populations can play important roles in plant fecundity and competitive interactions. Here, we show that virus-resistance transgenes and conventional crop genes can increase fecundity of wild plants under virus pressure. We asked how gene flow from a cultivated squash (Cucurbita pepo) engineered for virus resistance would affect the fecundity of wild squash (C. pepo) in the presence and absence of virus pressure. A transgenic squash cultivar was crossed and backcrossed with wild C. pepo from Arkansas. Wild C. pepo, transgenic backcross plants, and non-transgenic backcross plants were compared in field plots in Ithaca, New York, USA. The second and third generations of backcrosses (BC2 and BC3) were used in 2002 and 2003, respectively. One-half of the plants were inoculated with zucchini yellow mosaic virus (ZYMV), and one-half of the plants were maintained as healthy controls. Virus pressure dramatically decreased the fecundity of wild C. pepo plants and non-transgenic backcross plants relative to transgenic backcross plants, which showed continued functioning of the virus-resistance transgene. In 2002, non-transgenic backcross fecundity was slightly higher than wild C. pepo fecundity under virus pressure, indicating a possible benefit of conventional crop alleles, but they did not differ in 2003 when fecundity was lower in both groups. We detected no fitness costs of the transgene in the absence of the virus. If viruses play a role in the population dynamics of wild C. pepo, we predict that gene flow from transgenic, virus-resistant squash and, to a much lesser extent, conventionally bred squash would increase C. pepo fecundity. Studies such as this one, in combination with documentation of the probability of crop-to-wild gene flow and surveys of virus incidence in wild populations, can provide a solid basis for environmental risk assessments of crops genetically engineered for virus resistance.

Interrelationships among a Virus-Resistance Transgene, Herbivory, and a Bacterial Disease in a Wild Cucurbita

“…1 The proportion of susceptible plants infected with zucchini yellow mosaic virus on the two unsprayed (virus-present) fields in which viral diseases were allowed to spread. As with all previous studies of transgenic squash-wild gourd hybrids (Fuchs et al 2004;Laughlin et al 2009;Sasu et al 2009), our study shows that the transgene effectively deters a target virus during introgression from a cultivated to a wild Cucurbita pepo. As has been previously observed in both transgenic squash and transgenic squash-wild gourd hybrids (see Fuchs et al 2004;Sasu et al 2009), a few of the tBC4 plants in this study had mild symptoms of and tested positive for zucchini yellow mosaic virus at the end of the growing season in unsprayed (virus-present) fields in which zucchini yellow mosaic virus was epidemic.…”

“…In our virus-present fields, the reproductive output of the ntBC4 plants was only 61% of that of the tBC4 plants. The fitness advantage of the transgenic plants in this study, however, was not as great as that observed in the Fuchs et al (2004) and Laughlin et al (2009) studies. For example, Laughlin et al (2009) hand-inoculated all of the susceptible plants as seedlings with zucchini yellow mosaic virus and removed all wilt-diseased plants from their fields and their analyses, and they found that the susceptible plants produced only 6% of the fruits and 3% of the seeds of the transgenic introgressives in the same fields.…”

“…As with all other studies of the effects of the squash transgene on fitness during introgression into the Texas gourd (Fuchs et al 2004;Laughlin et al 2009;Sasu et al 2009), this study revealed that introgressives with the transgene enjoy a significant fitness advantage when the virus is present in the fields. In our virus-present fields, the reproductive output of the ntBC4 plants was only 61% of that of the tBC4 plants.…”

“…texana (Fuchs et al 2004;Laughlin et al 2009). These studies revealed that the introgressed transgene effectively deters infection by the target viruses, plants with the transgene had a tremendous fitness advantage over the susceptible plants in experimental populations in which the susceptible plants were infected with a target virus as seedlings, and there were no direct costs of the transgene in the relative absence of the target viruses (isolated or insecticide-sprayed fields).…”

A unified approach to the estimation and interpretation of resistance costs in plants

“…Despite its ability to unambiguously identify resistance costs , the NILs protocol has rarely been used (pathogen resistance: Heidel et al, 2004;Korves and Bergelson, 2004;Laughlin et al, 2009;herbicide resistance: McCloskey andHolt, 1990, 1991;Salzmann et al, 2008;Wang et al, 2010) (Table 1; Supplementary Table S1). Several plant generations are required to produce NILs in order to disrupt linkage disequilibrium between resistance and other genes.…”