Alphaviruses are pathogens of humans and livestock with worldwide distribution. The classification of some species as select agents in conjunction with their impact on public health makes understanding their interaction with the host and development of interventions a high priority. Alphaviruses are obligatorily transmitted by a hematophagous arthropod vector in which a lifelong persistent infection is established. This pattern of infection implies an effective but incomplete immune response that preserves host fitness while allowing virus replication to continue thus facilitating virus transmission. A major impediment to understanding the critical virus-host interactions has been the inability to genetically manipulate the host organism. Drosophila melanogaster represents an excellent experimental system for elucidating the genetic, molecular, and biochemical mechanisms underlying numerous physiological and developmental processes. The proposed research aims to exploit the power of Drosophila genetics to define the recently characterized interactions between alphaviruses and two arthropod innate immune response pathways. A system for the analysis of alphavirus replication has been established in which an alphavirus replicon sequence encoding a reporter gene is expressed from the genome of Drosophila under the control of the GAL4-UAS misexpression system. The resulting replication of the viral RNA is observable through reporter gene activity. Mutational analyses of host pathways using this alphavirus replicon fly line have demonstrated an antiviral role for the Imd- and JAK-STAT pathways. This system has been further developed to produce infectious particles. Co-expression in Drosophila of the replicon and viral structural proteins leads to the production of replicon containing particles capable of a single round of infection. These basic tools will be used in combination with Drosophila genetics to (i) determine the viral components responsible for activation of Imd- and JAK-STAT pathways; (ii) identify host genes involved in virus-induced innate immune response; (iii) characterize host requirements for virus spread. The proposed research combines the power of Drosophila genetics with comparative genomics and molecular virology in order to advance our understanding of the interaction between alphaviruses and an arthropod host.