The Neuroproteomic Basis of Enhanced Perception and Processing of Brood Signals That Trigger Increased Reproductive Investment in Honeybee (Apis mellifera) Workers

Abstract

The neuronal basis of complex social behavior is still poorly understood. In honeybees, reproductive investment decisions are made at the colony-level. Queens develop from female-destined larvae that receive particular alloparental care from nurse bees in the form of ad-libitum royal jelly (RJ) secretions. Typically, the number of raised new queens is limited but genetic breeding of “royal jelly bees” (RJBs) for enhanced RJ production over decades has led to a dramatic increase of reproductive investment in queens. Here, we compare RJBs to unselected Italian bees (ITBs) to investigate how their cognitive processing of larval signals in the mushroom bodies (MBs) and antennal lobes (ALs) may contribute to their behavioral differences. A cross-fostering experiment confirms that the RJB syndrome is mainly due to a shift in nurse bee alloparental care behavior. Using olfactory conditioning of the proboscis extension reflex, we show that the RJB nurses spontaneously respond more often to larval odors compared to ITB nurses but their subsequent learning occurs at similar rates. These phenotypic findings are corroborated by our demonstration that the proteome of the brain, particularly of the ALs differs between RJBs and ITBs. Notably, in the ALs of RJB newly emerged bees and nurses compared to ITBs, processes of energy and nutrient metabolism, signal transduction are up-regulated, priming the ALs for receiving and processing the brood signals from the antennae. Moreover, highly abundant major royal jelly proteins and hexamerins in RJBs compared to ITBs during early life when the nervous system still develops suggest crucial new neurobiological roles for these well-characterized proteins. Altogether, our findings reveal that RJBs have evolved a strong olfactory response to larvae, enabled by numerous neurophysiological adaptations that increase the nurse bees’ alloparental care behavior.

Tachykinin signaling inhibits task-specific behavioral responsiveness in honeybee workers

“…Our results support the view that pollen and sucrose are distinct stimuli: While our experimental manipulations of TRP signaling altered the responsiveness of pollen foragers to pollen and sucrose, only responsiveness to sucrose was affected in nectar foragers and only responsiveness to larvae was affected in nurses. The functional significance of the PER in response to larvae is currently unclear, but we show that it is specific to nurses and it has previously been linked to brood provisioning [35]. Thus, our diverse PER results in two species comprehensively support the hypothesis that task-specific response thresholds guide behavioral specialization, leading to division of labor among honeybee workers [19][20][21].…”

“…Response thresholds can be quantified based on the honeybees' reflexive extension of their proboscis in response to stimuli, such as sucrose [18]. The spontaneous proboscis extension reflex (PER) to sucrose has been expanded to other stimuli that bees spontaneously respond to [34,35] and conditioned stimuli to which no spontaneous responses occur [36].…”

Neuromodulation of Behavioral Specialization: Tachykinin Signaling Inhibits Task-specific Behavioral Responsiveness in Honeybee Workers

“…Our results support the view that pollen and sucrose are distinct stimuli: While our experimental manipulations of TRP signaling altered the responsiveness of pollen foragers to pollen and sucrose, only responsiveness to sucrose was affected in nectar foragers and only responsiveness to larvae was affected in nurses. The functional significance of the PER in response to larvae is currently unclear, but we show that it is specific to nurses and it has previously been linked to brood provisioning [35]. Thus, our diverse PER results in two species comprehensively support the hypothesis that task-specific response thresholds guide behavioral specialization, leading to division of labor among honeybee workers [21][22][23].…”

“…Response thresholds can be quantified based on the honeybees' reflexive extension of their proboscis in response to stimuli, such as sucrose [20]. The spontaneous proboscis extension reflex (PER) to sucrose has been expanded to other stimuli that bees spontaneously respond to [34,35] and conditioned stimuli to which no spontaneous responses occur [36].…”

Multifunctionality and intrinsic disorder of royal jelly proteome

“…For a typical colony of honeybees, the annual production of RJ normally amounts to ∼0.5–1 kg, depending on the honeybee species [1]. However, a high RJ‐producing strain of bees (RJBs, A. mellifera ligustica ) has been derived in China from the Italian bees (ITBs), and such RJBs can now produce 10 times as much RJ than ITBs, with a colony of RJBs being able to generate more than 10 kg of RJ per year [38, 44, 108–115]. Although it is recognized now that the enhanced RJ yields produced by RJBs represents an inheritable trait [116, 117], it is not clear how such high performance of RJ production is achieved by the RJBs [115].…”