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The family Luteoviridae consists of three genera: Luteovirus, Enamovirus, and Polerovirus. The genus Polerovirus contains 32 virus species. All are transmitted by aphids and can infect a wide variety of crops from cereals and wheat to cucurbits and peppers. However, little is known about how this wide range of hosts and vectors developed. In poleroviruses, aphid transmission and virion formation is mediated by the coat protein read-through domain (CPRT) while silencing suppression and phloem limitation is mediated by Protein 0 (P0)—a protein unique to poleroviruses. P0 gives poleroviruses a great advantage amongst plant viruses and diversifies polerovirus species, but the mechanism of suppression is poorly understood. In this thesis, we profiled the genome-wide variability of poleroviruses to understand genome variability and its relation to host adaptation and we experimentally tested P0 to understand the mechanisms of silencing suppression. Results show that P0 and the CPRT are the most variable. P0 and the CPRT also contained the most sites under positive selection, suggesting that these areas provide mutational robustness in an environment that likely includes genetically diverse aphid vectors, host plants, or a combination. P0 was also cloned and tagged for mechanistic analysis. Transient analysis showed P0 is a strong suppressor of transgene silencing. vsiRNA stability, but not biogenesis, was affected when in the presence of P0. In addition, P0 with AGO 1, 2, 5, 7, and 10 were found to degrade together. Our results provide novel insights on the genome-wide variation across the polerovirus genome and the mechanism of siRNA silencing suppression by polerovirus P0.
Advisor: Hernan Garcia-Ruiz