(2nd Place) Warmer temperatures disrupt feminization by endosymbionts in Mermessus fradeorum

Many arthropods are infected with maternally-inherited bacterial endosymbionts. These symbionts pass from infected females to their offspring, and some symbionts manipulate host reproduction to favor infected females capable of transmitting the symbiont. One form of manipulation is feminization, which causes genetic males to develop into phenotypic females. While symbiont-conferred phenotypes like feminization can be reliably induced in lab settings, symbionts may also be affected by environmental conditions, especially temperature. Warmer temperatures can destabilize these symbioses by nullifying symbiont-induced phenotypes and causing transmission failure. Using the linyphiid spider Mermessus fradeorum, we investigated the effect of warm temperatures on a heritable symbiont community. A combination of five strains of bacterial endosymbionts— Ricketsiella, Tisiphia, and three strains of Wolbachia— causes feminization in this host species. We compared feminization rates of these quintuply-infected or uninfected spiders following development at either 21°C (standard rearing temperature with reliable feminization) or an elevated temperature treatment (27°C day / 24°C night 16:8 hr cycle). We next mated these F1 female spiders with cured males to determine the sex ratio of their F2 offspring, which developed at 21°C. We found that while warmer temperatures did not affect the feminization of F1 females directly exposed to the temperature treatment, the F2 offspring of warm-reared spiders were not feminized, suggesting that the M. fradeorum symbiont community is temperature-sensitive. Warm temperatures likely inhibit symbiont spread and their ability to feminize spiders in natural populations. These results also indicate that temperature stress can have long-lasting transgenerational effects on heritable symbioses.