Experience- and Age-Related Outgrowth of Intrinsic Neurons in the Mushroom Bodies of the Adult Worker Honeybee

Abstract

A worker honeybee performs tasks within the hive for approximately the first 3 weeks of adult life. After this time, it becomes a forager, flying repeatedly to collect food outside of the hive for the remainder of its 5-6 week life. Previous studies have shown that foragers have an increased volume of neuropil associated with the mushroom bodies, a brain region involved in learning, memory, and sensory integration. We report here that growth of the mushroom body neuropil in adult bees occurs throughout adult life and continues after bees begin to forage. Studies using Golgi impregnation asked whether the growth of the collar region of the mushroom body neuropil was a result of growth of the dendritic processes of the mushroom body intrinsic neurons, the Kenyon cells. Branching and length of dendrites in the collar region of the calyces were strongly correlated with worker age, but when age-matched bees were directly compared, those with foraging experience had longer, more branched dendrites than bees that had foraged less or not at all. The density of Kenyon cell dendritic spines remained constant regardless of age or behavioral state. Older and more experienced foragers therefore have a greater total number of dendritic spines in the mushroom body neuropil. Our findings indicate that, under natural conditions, the cytoarchitectural complexity of neurons in the mushroom bodies of adult honeybees increases as a function of increasing age, but that foraging experience promotes additional dendritic branching and growth.

Specialization and group size: brain and behavioural correlates of colony size in ants lacking morphological castes

“…Dinoponera [41]) may be useful in understanding whether this is a general trend among social insects. In contrast, the observed increase in those same MB regions for foragers is congruent with findings in other social insects, such as Camponotus ants and honeybees [17,42].…”

Developmental and dominance‐associated differences in mushroom body structure in the paper wasp Mischocyttarus mastigophorus

“…The pattern of expansion of the calyx and reduction in the Kenyon cell body layer suggests that increases in arborization may be supported by the transfer of cytoplasm into the calyx. In Apis, foraging causes increased dendritic branching of Kenyon cells in the MB collar, which may contribute to the volume expansion of the calyx (Farris et al, 2001). However, we did not quantify Kenyon cell changes at the neuron level, and reductions in MB intrinsic neuron densities, or changes in the packing density of Kenyon cell bodies, are other possible explanations for the volume changes that we observed.…”

“…In both Apis and Polybia, which are highly eusocial species, the MB calyx:Kenyon ratios are greater in foragers than in nest workers (Farris et al, 2001;O'Donnell et al, 2004). This pattern has been interpreted as reflecting the greater cognitive demands associated with foraging (Fahrbach and Robinson, 1996), and possibly with higher light levels and other stimuli outside the nest.…”

“…This is a pattern expected if increasing light levels are a major determinant of MB calycal development. Honey bee workers' MB calyces increase in volume with foraging experience, suggesting that task complexity can drive brain plasticity (Withers et al, 1993(Withers et al, , 1995Farris et al, 2001).…”

What arthropod brains say about arthropod phylogeny

“…The mushroom bodies are the insect brain structure best known to non-entomologists because of their functional association with learning and memory (15) and their structural plasticity in adult insects (16). A set of lobed neuropils originating from dorsal clusters of numerous small neurons (globuli or Kenyon cells), they are typically the most prominent features of the insect protocerebrum.…”