Genotype effect on regulation of behaviour by vitellogenin supports reproductive origin of honeybee foraging bias

In honeybee colonies, food collection is performed by a group of mostly sterile females called workers. After an initial nest phase, workers begin foraging for nectar and pollen, but tend to bias their collection towards one or the other. Although foraging choices of honeybees are influenced by vitellogenin, an egg-yolk precursor protein, workers typically do not lay eggs. The forager reproductive ground plan hypothesis (RGPH) proposes an evolutionary path in which the behavioural bias towards collecting nectar or pollen on foraging trips is influenced by variation in reproductive physiology, such as hormone levels and vitellogenin (vg) gene expression. Recently, the connections between vitellogenin and foraging behaviour were challenged by Oldroyd & Beekman (2008), who concluded from their study that the ovary, and especially vitellogenin, played no role in foraging behaviour of bees. We address their challenge directly by manipulating vg expression by RNA interference (RNAi) mediated gene knockdown in two honeybee genotypes with different foraging behaviour and reproductive physiology. We found that vg affected the food-loading decisions of the workers only in the genotype in which the timing of foraging onset (by age) was also sensitive to vitellogenin levels. In the second genotype, changing vitellogenin levels did not affect foraging onset or bias. The effect of vitellogenin on workers' age at foraging onset is explained by the well-supported double repressor hypothesis (DHR), which describes a mutually inhibitory relationship between vitellogenin and juvenile hormone (JH), an endocrine factor that influences development, reproduction and behaviour in many insects. These results support the RGPH and demonstrate how it intersects with an established mechanism of honeybee behavioural control.