Minibrain plays a role in the adult brain development of honeybee (Apis mellifera) workers

Polyphenisms arise because of environmental influence on genotype expression. Among the most compelling examples of polyphenism are those allowing social organization in the highly eusocial honeybee, Apis mellifera. In this species, one of the characters displaying phenotypic plasticity is the brain, which is proportionally larger in adult workers than in queens. During larval development, though, as response to the differential feeding favouring prospective queens, the brain of the latter caste develops faster and reaches higher organizational degrees than that of workers. This condition shifts during pharate-adult development. A previous molecular screening pinpointed minibrain (mnb) as one of the key regulators of the shifting process. Here, we used RNAi approach to test the extent of mnb role on brain diphenism development in honeybees. White-eyed unpigmented cuticle pupae were injected with dsRNA for mnb (Mnb-i) or gfp and their phenotypes were assessed two and eight days later using classic histological and transcriptomic analyses. Early after treatment, Mnb-i bees showed 98% downregulation of mnb gene. After eight days, the brain of Mnb-i bees showed reduced total volume as well as the volume of the mushroom bodies and the antennal and optic lobes. Additionally, the nucleus of the Kenyon cells showed signs of compacted chromatin and the cohesion of the brain tissues appeared disturbed. Our transcriptomic analyses revealed 226 genes under the control of mnb, most of which allowing neuron fasciculation. These results suggest the evolutionary conserved mnb has been coopted for promoting hormone-mediated developmental brain morphological plasticity generating caste diphenism in honeybees.