Mechanisms regulating caste differentiation in an aphid social system

Abstract

For evolution and maintenance of the social systems of insect colonies, caste production should be controlled in response to external cues so that caste ratio in the colony is kept in an optimal range. Recent developments using artificial diet rearing techniques have revealed an underlying mechanism for adaptive control of caste production in a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the 2nd instar. Aphid density was the proximate cue that acts on 1st instar nymphs and embryos to induce soldier differentiation. The final determination of soldier differentiation occurred postnatally, probably at a late 1st instar stage. Direct contact stimuli from live non-soldier aphids mediated the density effect. While coexisting non-soldiers facilitated soldier differentiation in 1st instar nymphs, coexisting soldiers acted to suppress such differentiation. These results suggest that caste production in aphid colonies is controlled by positive and negative feedback mechanisms consisting of density-dependent induction and suppression of soldier differentiation. Here, we demonstrate the mechanisms that coordinate aphid society, and provide a striking case of clonal superorganism system where simple responses of colony members to local extrinsic stimuli are integrated into a highly organized regulation of the whole colony.