Keep cool: The Influence of Symbionts on Thermal Preference

Abiotic environmental factors including temperature and humidity not only determine the survival and distribution of any given species but also influence associated parasites and other symbionts. Cell division rates and cell numbers, for example, are strongly connected to temperature in many microbial symbionts. Ectothermal hosts with limited capacities to physiologically adjust their body temperature, thus have the opportunity to behaviorally regulate the growth of their symbionts. Analogous to physiological fever in humans, they may seek environments with elevated temperatures that are deleterious to parasites. In a recently published study we investigated this phenomenon in the fruit fly Drosophila melanogaster infected with Wolbachia. This bacterium is a very common intracellular symbiont, which not only manipulates the reproduction of its host but also increases anti- viral immunity. Wolbachia can thus not only act as a parasite but also as a mutualist with positive effects to the host. In our experiments, we found that the bacterium influences the thermal preference of its Drosophila host. However, in contrast to our expectations, infected flies exhibited lower thermal preference than uninfected flies. The biological function of this behavioral change remains unclear and its genetic basis unknown. In my proposal, I will address these pending questions, which are of high relevance to our general understanding of the biology and ecology of host-symbiont interactions in natural populations. I will investigate flies from natural populations in Portugal and Finland, which are characterized by highly divergent annual average temperatures, for variation in thermal preference and behavioral changes induced by Wolbachia infections. These experiments will allow to test if behavioral change as observed in our previous analyses also plays a major role in natural populations. Furthermore, I will maintain flies at different developmental temperatures that correspond to the preferred thermal conditions of infected and uninfected flies, respectively. To test if cooler thermal conditions represent an advantage to infected flies, I will investigate the effect of developmental temperature on Wolbachia cell-densities and lifespan, fecundity and body-size of Drosophila hosts. At last, I will employ modern sequencing technology to assess expression differences of all Drosophila genes with respect to temperature and Wolbachia infections. Using transgenic flies, which allow to switch off the function of specific genes, I will furthermore study if genes which are involved in thermal sensing are influenced by Wolbachia. The experiments proposed here will provide a novel insight into the biology and genetics of complex host-symbiont interactions in natural populations. Moreover, these new data are also of high socio-economic relevance. Wolbachia is currently tested for bio- control measures against insect-transmitted viral diseases such as Zika, Dengue and West- Nile virus. Our previous results, however, indicate that the efficiency of such approaches may be strongly influenced by yet unknown behavioral changes of the hosts. 1

The interaction of hosts and their microbial symbionts is strongly influenced by the genetic variation of both partners, but also by environmental conditions, that may asymmetrically favor the life history of host or symbiont. The FWF-funded project P 32275-B: "Keep cool: The Influence of Symbionts on Thermal Preference", allowed us to focus on the vinegar fly Drosophila melanogaster and the bacterial endosymbiont Wolbachia pipientis, which are two of the best-studied model organisms. To test if the previously detected influence of Wolbachia on the thermal behavior of their fly host can not only be found in long-standing laboratory fly strains, but also in flies collected in the wild, we focused on recently sampled flies from Portugal and Finland that were naturally infected with different Wolbachia variants. By employing thermal preference assays based on newly designed experimental devices, we found that Wolbachia has an influence also in wild-caught fly strain and that this effect strongly depends on the Wolbachia strain the flies are infected with. Only wMelCS, but not the more common wMel Wolbachia variant, resulted in significantly reduced thermal preference of the host. We moreover for the first time showed that such effects are potentially influenced by bacterial titer, which may suggest that changes in thermal preference represent an aversion behavior to alleviate potentially costly elevated bacterial titers. We further found that the host genetic background and experimental conditions also strongly contribute to behavioral differences. Follow-up phenotypic assays confirmed a strong effect of temperature on the interaction of Drosophila and Wolbachia. Particularly wMelCS had a strong influence on fecundity, egg maturation and longevity, but not on developmental traits at elevated temperatures. These data further underpin that wMelCS, which is characterized by elevated titer levels in all tissues particularly at elevated temperatures may result in trade-offs that could contribute to costs under specific environmental conditions in natural populations. This effect may have contributed to a recent putative replacement of the wMelCS variant by the nowadays dominant wMel in natural Drosophila populations. Using recently published museomics data of century-old Drosophila melanogaster collected in Northern Europe, we were able to confirm such a recent replacement in Europe by employing a phylogenomic comparison to previously available newly generated genomic data of European fly samples. In summary, this FWF project allowed us to establish that temperature has a strong influence on host-symbiont interactions in the Drosophila-Wolbachia system. Specifically, bacterial titer is strongly influenced by temperature and appears to play a major role on the host fitness which may explain the recent replacement of Wolbachia variants in natural populations.