Thermal sensitivity in a symbiont: effect of elevated cyclic temperature exposure on Wolbachia density in Culex quinquefasciatus (Diptera: Culicidae) embryos.

Temperature influences insect reproduction and the density of the maternally inherited bacterium Wolbachia, which in turn manipulates host reproduction and alters arbovirus transmission. Wolbachia is used in mosquito control strategies, and thermal conditions may shape Wolbachia densities. Species-specific thermal tolerances have been documented in mosquitoes but little is known of thermal impacts on embryos. This study investigated Wolbachia density under elevated cyclical temperatures in Culex quinquefasciatus Say embryos, specifically to determine whether high temperatures reduced Wolbachia density. Experiments used cyclic thermal regimes that simulated high summer temperatures similar to those observed where Cx. quinquefasciatus occurs. The relationship between temperature and Wolbachia density was determined through Fluorescent in-situ hybridization (FISH) and relative quantification based on qPCR. The embryo developmental time and Wolbachia density were reduced at the high cyclic temperature (25 to 40 °C) when compared to the control (25 °C). The time for the embryonic maturation at the high cyclic temperature (25 to 40 °C) was reduced by 8 h compared to development at 25 °C. Integrated fluorescence density of FISH-labeled Cx. quinquefasciatus embryos was significantly lower under the elevated cyclic 25 to 40 °C temperature than at control 25 °C. Relative Wolbachia density by qPCR and ΔΔCt of the wsp gene was also lower under the cyclic elevated temperature. Elevated temperatures which reduce Wolbachia loads may disrupt cytoplasmic incompatibility or antiviral protection. The results herein suggest that environmental thermal regimes could disrupt Wolbachia density in mosquito embryos. This could have implications for Wolbachia‑based disease control strategies and merits further investigation.