Combined elevated temperature and CO₂ alter Wolbachia abundance and host fitness but not cytoplasmic incompatibility in a stored-product pest

Coinfecting endosymbiont strains may respond differently to environmental change, with consequences for host performance and reproductive incompatibility. We investigated an isofemale line of Callosobruchus chinensis coinfected with the cytoplasmic incompatibility (CI)-inducing Wolbachia strains wBruCon (Con) and wBruOri (Ori), together with derived Con-only and uninfected sublines. Beetles were maintained under control conditions (30°C, ~500 ppm CO₂) or under combined elevated temperature and CO₂ (32°C, ~1000 ppm CO₂; eT&CO₂). In coinfected adults, Ori relative abundance declined under eT&CO₂, whereas Con abundance increased in females. Con responses also depended on host sex and infection status, with singly infected males showing particularly high abundance under control conditions. However, CI expression remained stable across environmental conditions. Coinfected females laid fewer eggs than uninfected females, whereas host life-history responses to eT&CO₂ varied among traits and generations. eT&CO₂ accelerated development and reduced adult longevity, while reduced egg–adult survival emerged in the F2 generation. It also reduced F1 fecundity but only in the uninfected subline. Strong and partial CI associated with Con and Ori, respectively, remained evident under both environmental conditions. Equilibrium analyses incorporating relative fecundity, CI intensity, and maternal transmission suggested lower infection persistence under eT&CO₂ in the F1 generation but higher persistence in the F2 generation. Sensitivity analysis suggested contribution of improved relative fecundity of infected females in the latter. Overall, the two Wolbachia strains exhibited contrasting, sex-dependent abundance responses to eT&CO₂, while CI expression remained stable. These findings suggest that climate change may alter symbiont infection dynamics in economically important pests, with potential implications for symbiont-mediated traits, including pesticide resistance.