Survival strategies and trade-offs among pea aphid color morphs: fitness, tolerance, and immune regulation

Abstract The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), is both an important agricultural pest and a widely used model organism in evolutionary biology. Although only red and green morphs have previously been reported in the wild, we recently identified a wild yellow morph whose biological traits remain uncharacterized. In this study, we examined the newly discovered wild yellow morph, a laboratory-induced yellow mutant, and wild red and green clones as experimental subjects. We evaluated starvation tolerance, predation risk from Harmonia axyridis Pallas (Coleoptera: Coccinellidae), high-temperature tolerance, and immune responsiveness under controlled experimental conditions. The wild yellow clone exhibited significantly greater starvation tolerance than the red and green clones, comparable with that of the laboratory yellow clone. However, both yellow clones were more susceptible to predation by H. axyridis. Following heat shock, although reproductive capacity and symbiont density declined in all clones, the yellow clones exhibited lower mortality rates than the red and green clones. Upon bacterial infection, the wild yellow morph exhibited the lowest mortality and the strongest induction of the JNK gene. These findings suggest that, although the wild yellow morph is disadvantaged in fecundity and predator defense, it exhibits enhanced stress tolerance through distinct physiological strategies. Under environmental conditions characterized by frequent extreme heat events, this morph may therefore achieve relatively higher fitness.