Thermal sensitivity of the Spiroplasma-Drosophila hydei protective symbiosis: The best of climes, the worst of climes.

The outcome of natural enemy attack in insects has commonly been found to be influenced by the presence of protective symbionts in the host. The degree to which protection functions in natural populations, however, will depend on the robustness of the phenotype to variation in the abiotic environment. We studied the impact of a key environmental parameter - temperature - on the efficacy of the protective effect of the symbiont Spiroplasma on its host Drosophila hydei, against attack by the parasitoid wasp Leptopilina heterotoma. In addition, we investigated the thermal sensitivity of the symbionts vertical transmission, which may be a key determinant of the ability of the symbiont to persist. We found that vertical transmission was more robust than previously considered, with Spiroplasma being maintained at 25 {degrees}C, 18 {degrees}C and with 18/15 {degrees}C diurnal cycles, with rates of segregational loss only increasing at 15 {degrees}C. Protection against wasp attack was ablated before symbiont transmission was lost, with the symbiont failing to rescue the fly host at 18 {degrees}C. We conclude that the presence of a protective symbiosis in natural populations cannot be simply inferred from presence of a symbiont whose protective capacity has been tested under narrow controlled conditions. More broadly, we argue that the thermal environment is likely to represent an important determinant of the evolutionary ecology of defensive symbioses in natural environments, potentially driving seasonal, latitudinal and altitudinal variation in symbiont frequency, and modulating the strength of selection for symbiotic protective systems compared to defensive systems encoded in the nuclear genomes.