Zucchini yellow mosaic virus: Post-infection impacts on the Cucurbita pathosystem

Most plants are hermaphrodites and achieve, on average, half of their fitness via the male (pollen) function. Environmental stressors, such as pathogen infection, are known to negatively impact seed production. However, few studies have examined the impacts of environmental stress on the male function. Moreover, few empirical studies have examined the complex interactions among multiple biotic stresses of the same host plant. The various parasites (i.e. herbivores and pathogens) that negatively impact the fitness of a host make up a plant pathosystem. Studies examining multiple components of a pathosystem simultaneously suggest that the interactions that result from multiple parasites utilizing the same host cannot be predicted by simple pairwise comparisons alone. This dissertation examines the impacts of a pathogen, Zucchini yellow mosaic virus, on the male function of Cucurbita pepo, and it examines the interactions among various components (plant, pathogens, and herbivores) of the Cucurbita pathosystem in a naturalistic setting.;First, I examine the impact of Zucchini yellow mosaic virus (ZYMV) on the fitness of a virus-resistant transgene (Chapter 2). There are concerns that escape of the virus-resistant transgene into wild populations of Cucurbita pepo could increase the weediness of wild species and/or impact non-target species. Here I show that in the presence of ZYMV, the proportion of seeds sired by transgenic plants is much greater than would expected based solely on staminate flower production, suggesting a stronger fitness advantage to the transgene in the presence of the target pathogen than revealed by previous studies that examined only seed production.;In my second project (Chapter 3) I examine the impact of ZYMV infection on various components of the male function in wild C. pepo. Using large-scale field experiments, I show that viral infection significantly reduces staminate flower production, pollen production per flower, and pollen competitive ability in vivo. Together, these experiments show that transgenic plants sire a greater than expected proportion of seeds due to the significant negative impact of ZYMV infection on the male function of wild C. pepo plants.;Recent evidence shows that vertical infection of ZYMV occurs in wild C. pepo, however, the exact mechanism for vertical transmission has not been determined. Additional studies have shown that vertical infection with ZYMV can also occur in virus-resistant transgenic plants. This suggests the possibility that vertical infection with the virus is a result of pollen transmission. Therefore, for my third project (Chapter 4) I used highly sensitive PCR techniques to determine that while vertical infection of ZYMV can be pollen transmitted at very low rates, the primary mechanism for vertical infection is most likely transmission via the infected maternal parent through the ovule.;Finally, for my fourth project (Chapter 5), I conducted a series of large-scale field and greenhouse experiments to examine the impact of ZYMV infection on subsequent infection with powdery mildew. I show that plants infected with ZYMV are less likely to become infected with powdery mildew and that when infected with powdery mildew, ZYMV infected plants show reduced powdery mildew symptom severity, most likely due to a plant immune response triggered by ZYMV infection. Additionally, I show that because transgenic plants remain virusfree during viral epidemics, they are more likely to be infected with powdery mildew, thereby mitigating the potential benefits of the virus-resistant transgene.;Among the more novel results of this dissertation are A) transgenic plants have a fitness advantage through the male function in the presence of the target pathogen, B) viral infection decreases plant fitness through the male function, C) a possible mechanism for vertical transmission of a virus, and D) a possible immune response triggered by infection with one parasite inhibits infection with a secondary parasite. Taken together, these studies have important implications for the fitness of the virus-resistant transgene during introgression into wild populations of C. pepo.