INTRODUCTION:Honey bees, Apis mellifera, have a diverse community of pathogens. Previous research has mostly focused on bacterial brood diseases of high virulence, but milder diseases caused by fungal pathogens have recently attracted more attention. This interest has been triggered by partial evidence that co-infection with multiple pathogens has the potential to accelerate honey bee mortality. In the present study we tested whether co-infection with closely related fungal brood-pathogen species that are either specialists or non-specialist results in higher host mortality than infections with a single specialist. We used a specially designed laboratory assay to expose honey bee larvae to controlled infections with spores of three Ascosphaera species: A. apis, the specialist pathogen that causes chalkbrood disease in honey bees, A. proliperda, a specialist pathogen that causes chalkbrood disease in solitary bees, and A. atra, a saprophytic fungus growing typically on pollen brood-provision masses of solitary bees.RESULTS:We show for the first time that single infection with a pollen fungus A. atra may induce some mortality and that co-infection with A. atra and A. apis resulted in higher mortality of honey bees compared to single infections with A. apis. However, similar single and mixed infections with A. proliperda did not increase brood mortality.CONCLUSION:Our results show that co-infection with a closely related fungal species can either increase or have no effect on host mortality, depending on the identity of the second species. Together with other studies suggesting that multiple interacting pathogens may be contributing to worldwide honey bee health declines, our results highlight the importance of studying effects of multiple infections, even when all interacting species are not known to be specialist pathogens.

Engineered resistance mediated by RNA interference to control viral diseases in plants has shown great promise. However, the discovery that most known plant viruses encode RNAi suppressors which interfere with RNAi raised the issue to whether this type of engineered resistance can be durable in the presence of heterologous viruses in mixed infection. The overall goal of this study was to investigate the mechanism of suppression of RNAi-mediated resistance in transgenic plants in the presence of a virus carrying a strong suppressor of RNAi. Nicotiana benthamiana plants were transformed with a 1.2 kb from the 5' end of RCNMV RNA-1. Transgenic resistant lines were obtained. Resistance in two different transgenic lines was shown to be mediated by two different types of RNAi: constitutive RNAi in D2 line induced by doubles-stranded (ds) transgene transcripts and virus-induced RNAi in B1 line. We demonstrated that PVY differentially affected RNAi-mediated resistance in the two lines. D2 line is completely immune to RCNMV infection. D2 line contained multiple copies of the 1.2 kb transgene which are rearranged and produced dsRNAs. PVY did not break the resistance in this transgenic line however data showed that PVY interfered with RNAi which correlated to an increase of the 1.2 kb transgene mRNA. In addition, PVY infection induced accumulation of 21 nt siRNAs and did not alter the transcription of the transgene. In contrast, PVY infection suppressed resistance mediated by virus-induced RNAi in B1 line. B1 contains a single copy of the1.2 kb transgene and is initially susceptible to RCNMV infection however became resistant to RCNMV in newly merging leaves after 14 days post inoculation. PVY infection did not affect the accumulation of the 1.2 kb transgene mRNA nor the accumulation of 21 nt siRNA corresponding to the transgene. The differential effect of PVY infection on the two RNAi-mediated resistances in the two transgenic lines suggests that properly designed resistant plants might withstand mixed virus infections and the presence of a strong suppressor of RNAi. In addition, the differential effect of PVY on RNAi suggests that parallel but distinct pathways are involved in dsRNA-induced, virus-induced, and sense RNAi.

Thesis (M.S.)--Ball State University, 2009. / Title from PDF t.p. (viewed on Feb. 08, 2010). Includes bibliographical references (p. 81-94).

Access to abstract permanently restricted to Ball State community only / Access to thesis permanently restricted to Ball State community only / Department of Physiology and Health Science