Seasonal polyphenism underlies the origin of a sterile caste in aphids

The origin of a sterile caste among eusocial animals has been a fundamental but still unresolved problem in understanding the evolution of biological complexity. At the origin of a sterile caste, recruitment of pre-existing plasticity may lead to produce physiologically, morphologically and behaviorally distinct caste phenotypes. Here, we provide convincing evidence that preexisting seasonal polyphenism has been recruited to generate a sterile soldier caste in host-alternating social aphids. We demonstrate that sterile soldier nymphs of Colophina aphids resemble those of monomorphic defensive nymphs produced in a different host-plant generation. Notably, the two morphs in the basal species show the closest similarity in morphology and gene expression among all morph pairs. Moreover, their evolutionary phenotypic changes along the phylogeny of four Colophina species are significantly correlated positively. These results suggest that they may share the common regulatory mechanisms of development, which underpin the heterochronic expression of monomorphic defenders on the different host plant leading to the evolution of a novel soldier phenotype. We further demonstrate that the monomorphic defenders can increase their inclusive fitness by killing predators eggs on a seasonally different host plant. Taken together, our findings suggest that preexisting plasticity that can gain indirect fitness benefits facilitates the early evolution of a sterile caste.