Wolbachia are vertically transmitted endosymbiotic bacteria that are ubiquitous in arthropods, infecting approximately 52% of insect species. Wolbachia form a diversity of relationships with their hosts. Drosophila melanogaster is infected with several closely related genetic variants of the Wolbachia strain wMel: wMel, wMelCS, and wMelPop. All three variants cause weak CI and provide virus protection to varying degrees. The wMel-strain variants wMel and wMelCS infect natural populations of D. melanogaster, whereas wMelPop was isolated from a laboratory Drosophila stock during a survey for genetic mutations and is not found in natural populations. The wMelPop variant is considered a unique example of a pathogenic symbiont. The life-span of D. melanogaster infected with wMelPop is reduced two-fold as a result of the bacteria over-replicating in host tissues, such as the brain, retina, and muscles.
Wolbachia’s replication behavior in terrestrial arthropods is positively temperature-dependent. Most terrestrial arthropods infected with Wolbachia are insect ectotherms. Whether host or symbiont, an organism’s ability to remain within its upper and lower thermal tolerance limits is paramount to its survival. Using a thermal gradient choice assay, our goal was to determine whether Wolbachia-infected D. melanogaster have different preferred temperatures compared to uninfected flies. Moreover, we wanted to know whether flies infected with different genetic variants of the same Wolbachia-strain had different temperature preferences. We hypothesized that Wolbachia-infected flies would prefer lower temperatures compared to uninfected flies. Additionally, we hypothesized that wMelPop infected flies would prefer a lower temperature compared to the other three D. melanogaster Wolbachia combinations because wMelPop actively proliferates and is pathogenic at 25°C and warmer. Finally, we hypothesized that wMel and wMelCS- infected flies would prefer similar temperatures, because the two strains co-exist in the same natural D. melanogaster populations.
Here we compare temperature preferences of D. melanogaster flies infected by different wMel strain variants – wMel, wMelCS, and wMelPop – to temperature preferences of uninfected (w-) flies. We define temperature preference as the temperature along the gradient where the highest proportion of flies aggregate. We show that a significant relationship exists between the temperature preference of D. melanogaster and Wolbachia. In addition, we show temperature preference of D. melanogaster differs depending on the wMel- strain variant infecting the flies. This research elucidates fundamental ecological conflicts between host and symbiont that may arise and affect the success of field applications of Wolbachia.