Infectious disease dynamics and the direct and indirect effects of an escaped viral-resistance transgene on plant fitness in a wild squash

This dissertation consists of six chapters: an introduction, four data chapters, and a conclusion: in the Introduction, I present background and provide general information about Cucurbita pepo ssp. texana (the wild progenitor of cultivated squashes), the major herbivores that feed on this plant, and the pathogens they transmit.;I explored the possibility of transmission of plant pathogens via the floral nectaries by: (1) Inoculating greenhouse-grown plants by placing a solution containing Erwinia tracheiphila onto the nectaries of the flowers (after removing the nectar), and showing that E. tracheiphila is able to infect plants via the nectaries; (2) Transforming E. tracheiphila cells with a GFP-marker and visually showing the progression of the bacteria through the nectary tissue and into the xylem of the pedicel; (3) Collecting cucumber beetle frass from field collected flowers and showing that more than 50% of flowers contain cucumber beetle frass in or around the nectaries, and (4) Developing E. tracheiphila -specific primers to screen cucumber beetle frass collected from flowers in the field and showing that more that 90% of flowers contained cucumber beetle frass contaminated with the bacteria. The results of these experiments provide strong circumstantial evidence that E. tracheiphila can be transmitted via the floral nectaries in C. pepo ssp. texana.;Chapter three contains a series of experiments to determine if the nectar of C. pepo ssp. texana has antimicrobial properties. In one experiment, I examined the effect of nectar on the growth of Erwinia tracheiphila and Escherichia coli using disk diffusion assays. In a second experiment, I inoculated greenhouse-grown plants via flowers with and without nectar. This experiment showed that plants inoculated through flowers without nectar experienced significantly higher incidence of wilt disease. Together, these findings show that antimicrobial compounds in C. pepo nectar inhibit the growth of E. tracheiphila and can function to retard the pathogen's transmission through the floral tissues, allowing time for an abscission layer to form between the flower and pedicel.;Chapter four examines the direct and indirect effects of a virus-resistance transgene (VRT) on fitness during introgression in C. pepo ssp. texana in the presence of the full range of insect herbivores (vectors) and diseases. These studies revealed that (1) Each year, viral diseases became established in mid-July and spread rapidly through the fields until the end of the season when most non-transgenic (susceptible) plants showed symptoms. This also showed that the introgressed transgene was truly effective against viral diseases. (2) The wild gourds, non-transgenic introgressives and transgenic introgressives did not differ in the amount of beetle damage or wilt disease incidence prior to the spread of viral diseases. (3) Once the viral diseases spread through the fields, cucumber beetles preferred to feed on healthy (transgenic) plants and the transgenic plants experienced increased wilt disease incidence. (4) A series of path analysis models showed that the VRT had a direct beneficial effect on reproductive output (transgenic plants are more fit) but this beneficial effect was mitigated by the indirect effects of concentrated cucumber beetle herbivory and increased wilt disease incidence once viral diseases spread thorough the fields.;Chapter five focuses on the costs and benefits of the VRT during introgression into the wild gourd in the presence and absence of viral diseases, as well as the costs and benefits of cultivar genes during introgression. Our findings also show that there are indirect effects of the VRT on a non-target herbivore (cucumber beetles) and the pathogen it transmits (E. tracheiphila ) when viral diseases are allowed to spread through the fields. These findings suggest that yearly variations in environmental variables such as temperature, rainfall and time of virus introduction, can impact the fitness of the VRT during introgression. In addition, it is important to note that our studies were performed north of the native range of wild gourds and that different populations of organisms (insects, predators, pathogens) as well as competition (both inter and intra-specific) among plants may also play roles in the fitness of the VRT during introgression. (Abstract shortened by UMI.)...