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

The brain of adult honeybee (Apis mellifera) workers is larger than that of queens, facilitating behavioural differentiation between the castes. This brain diphenism develops during the pharate-adult stage and is driven by a caste-specific gene expression cascade in response to unique hormonal milieus. Previous molecular screening identified minibrain (mnb; DYRK1A) as a potential regulator in this process. Here, we used RNAi approach to reduce mnb transcript levels and test its role on brain diphenism development in honeybees. White-eyed unpigmented cuticle worker pupae were injected with dsRNA for mnb (Mnb-i) or gfp, and their phenotypes were assessed two and 8 days later using classic histological and transcriptomic analyses. After 2 days of the injections, Mnb-i bees showed 98% of downregulation of mnb transcripts. After 8 days, the brain of Mnb-i bees showed reduction in total volume and in the volume of the mushroom bodies (MB), antennal, and optic lobes. Additionally, signs of apoptosis were observed in the Kenyon cells region of the MB, and the cohesion of the brain tissues was affected. Our transcriptomic analyses revealed that 226 genes were affected by the knockdown of mnb transcripts, most of which allowing axonal fasciculation. These results suggest the evolutionary conserved mnb gene has been co-opted for promoting hormone-mediated developmental brain morphological plasticity generating caste diphenism in honeybees.