Spatial and temporal distribution of pathogenic Wolbachia strain wMelPop in Drosophila melanogaster central nervous system under different temperature conditions

The pathogenic Wolbachia strain, wMelPop, of Drosophila melanogaster is propagated in the fly's brain and muscles. To determine how wMelPop spreads in the host's central nervous system (CNS) during its life cycle, we used whole-mount fluorescent in situ hybridization to demonstrate the spatial distribution of wMelPop in D.melanogaster larvae and adults. To assess the effect of temperature on the pattern of wMelPop spread, we performed this analysis under moderate (25°C) and high (29°C) temperature conditions. Wolbachia distribution pattern in the third instar larva and adult brain was similar at both temperatures. wMelPop was generally localized to the subesophageal ganglion and the central brain of the host, whereas optic lobe anlagen cells of third instar larvae and cells of the optic lobe, lamina and retina of adult flies were mostly free of bacteria. Interestingly, high temperature had no significant effect on wMelPop titer or localization in the brain during larval development, but considerably altered it in adults immediately after eclosion. At both temperatures and within all tested stages of the life cycle, the bacterial titer varied only slightly between individuals. The observed differences in wMelPop titers in the central brain, subesophageal ganglion and optic lobe anlagen cells of third instar larva's CNS, together with the observation that these patterns are conserved in the adult brain, suggest that Wolbachia distribution is determined during fly embryogenesis.