History matters: thermal environment before, but not during wasp attack determines the efficiency of symbiont-mediated protection.

The outcome of natural enemy attack in insects is commonly impacted by the presence of defensive microbial symbionts residing within the host. The thermal environment is a factor known to affect symbiont-mediated traits in insects. Cooler temperatures, for instance, have been shown to reduce Spiroplasma-mediated protection in Drosophila. Our understanding of protective symbiosis requires a deeper understanding of environment -symbiont- protection links. Here, we dissect the effect of the thermal environment on Spiroplasma-mediated protection against Leptopilina boulardi in Drosophila melanogaster by examining the effect of temperature before, during and after wasp attack on fly survival and wasp success. We observed that the developmental temperature of the mothers of attacked larvae, and not the temperature of the attacked larvae themselves during or after wasp attack, strongly determines the protective influence of Spiroplasma. Cooler maternal environments was associated with weaker Spiroplasma protection of their progeny. The effect of developmental temperature on Spiroplasma-mediated protection is likely mediated by a reduction in Spiroplasma titre. These results indicate the historical thermal environment is a stronger determinant of protection than current environment. Further, protection is a character with transgenerational non-genetic variation likely to produce complex short term responses to selection. In addition, the cool sensitivity of the Spiroplasma-Drosophila symbioses contrasts to the more common failure of symbioses at elevated temperatures, indicating a need to understand the mechanistic basis of low temperature sensitivity on this symbiosis.