Impacts of the Hytrosavirus on the Transcriptome of the Housefly, Musca domestica

Hytrosaviridae family members replicate in the salivary glands (SGs) of their adult dipteran hosts and are transmitted to uninfected hosts via saliva during feeding. Despite inducing similar gross symptoms (SG hypertrophy; SGH), hytrosaviruses (SGHVs) have distinct pathobiologies, including sex-ratio distortions in tsetse flies and refusal of infected housefly females to copulate. Via unknown mechanism(s), SGHV replication in other tissues results in reduced fecundity in tsetse flies and total shutdown of vitellogenesis and sterility in housefly females. We hypothesized that vitellogenesis shutdown is caused by virus-induced modulation of hormonal titers. Here, we used RNA-Seq to investigate virus-induced modulation of host genes/pathways in healthy and virus-infected houseflies, and we validated expression of the most significantly modulated genes (n=23) by RT-qPCR. We also evaluated the levels and activities of hemolymph antimicrobial peptides (AMPs), levels of endogenous sesquiterpenoids, and impacts of exogenous hormones on ovarian development in viremic females. Of the 973 housefly unigenes that were significantly modulated (padj = 0.01; log2FC = or = 2.0 ), 446 and 527 genes were downregulated and upregulated, respectively. While the most downregulated genes were related to reproduction (embryogenesis/oogenesis), the repertoire of upregulated genes was overrepresented by genes related to non-self recognition, ubiquitin-protease system, cytoskeletal traffic, cellular proliferation, development and movement, and snRNA processing. Overall, MdSGHV induced upregulation of components of the siRNA, innate antimicrobial immune, and autophagy pathways. MdSGHV reduced hemolymph sesquiterpenoids and completely shut down egg development in viremic females. However, the hormonal rescue of vitellogenesis did not result in egg production. The mechanism underlying MdSGHV-induced sterility has yet to be resolved. Overall design: Housefly pupae, obtained from colonies maintained at the USDA Center for Medical, Agricultural and Veterinary Entomology (CMAVE, Gainesville, FL), were placed in rearing cages, provided with deionized , and incubated at 26 °C, a 12:12 h light: dark photoperiod and 40% relative humidity until adult emergence. Experiments were conducted with MdHV03, the Florida type MdSGHV strain collected in 2005. Both PBS-treated control flies and virus-injected flies were maintained in separate groups in the above-described conditions and provided with food and water ad libitum until used for sample preparation.