Effect of intra-specific larval competition on expression of immune genes in Sindbis virus-infected Aedes aegypti

Stressful environmental conditions during mosquito larval development may enhance susceptibility of adult mosquitoes to viral pathogens. While relatively little is known about anti-viral defense system in mosquito, stress-related enhancement of mosquito susceptibility to viral pathogens may be due to alteration of immune signaling pathways such as the Toll and immune deficiency (IMD) pathways. To test the influence of intra-specific larval competition on mosquito Toll/IMD pathways, 100 or 200 Aedes aegypti larvae were reared in 1 L of live oak (Quercus virginiana) leaf infusion and the adults were fed on citrated bovine blood with known titers of Sindbis virus (SINV).  SINV infection status and expression of genes encoding three antimicrobial peptides (cecropin, defensin, diptericin), an iron-binding protein (transferrin), and four modulators of Toll/IMD pathway (caspar, cactus, Rel1A and Rel2) was quantified by RT-qPCR 7 or 14 days post bloodmeal. Virus titers were not significantly influenced by larval density but were significantly higher on day 14 than on day 7. For both larval densities and time intervals, there was significant down-regulation of diptericin and Toll/IMD modulator genes in SINV-infected mosquitoes. In contrast, SINV-infected mosquitoes at both larval densities had significant up-regulation of cecropin on day 14. Thus, we could not attribute enhancement of SINV infection in higher density treatments to competition-mediated alterations in the TOLL/IMD signaling pathways.