D3132: (1st Place) Turning up the heat: Investigating symbiont-mediated thermotolerance in pea aphids (Acyrthosiphon pisum)

Widespread bacterial symbionts protect insects against environmental stressors, including high temperatures. Paradoxically, high temperatures can also impair or eliminate some symbionts, raising questions on the fates of symbiont-dependent insects under global climate change. To address such questions, we utilize the pea aphid (Acyrthosiphon pisum), a cosmopolitan crop pest requiring an obligate bacterial symbiont, Buchnera aphidicola, for nutrition. It can also harbor combinations of several non-essential, protective bacterial symbionts. One such symbiont, Hamiltonella defensa, is proposed to defend against high temperatures that are known to impair Buchnera function. To assess the robustness and ubiquity of this phenotype, we studied pea aphid clonal lines varying only in their presence/absence, and strain identities of Hamiltonella symbionts. We compared aphid population sizes following three generations under an alternating and stressful 30℃ /19℃ temperature regime. Two biological replicates of Hamiltonella sub-strain “C9” and a single replicate of Hamiltonella strain “A” sustained higher population sizes than Hamiltonella-free aphids, suggesting thermal tolerance effects for these symbiont variants. Surprisingly, a Hamiltonella sub-strain with high relatedness to C9, sub-strain “C11”, decreased aphid population size relative to Hamiltonella-free aphids. Additionally, population sizes for strains “B” and “D” differed depending on the aphid genotype, suggesting an aphid-strain interaction. Genotyped Hamiltonella from aphids collected during a southeastern Pennsylvania warming event in 2012 showed rapid proliferation of the lab-protective Hamiltonella A strain and C9 sub-strain. These results combine to suggest a real-world relevance for a lab-studied protective phenotype, revealing heat-protective symbionts as adaptive agents of aphids in a warming world.