Experience during early adulthood shapes the learning capacities and the number of synaptic boutons in the mushroom bodies of honey bees (Apis mellifera)

Abstract

The honey bee mushroom bodies (MBs) are brain centers required for specific learning tasks. Here, we show that environmental conditions experienced as young adults affect the maturation of MB neuropil and performance in a MB-dependent learning task. Specifically, olfactory reversal learning was selectively impaired following early exposure to an impoverished environment lacking some of the sensory and social interactions present in the hive. In parallel, the overall number of synaptic boutons increased within the MB olfactory neuropil, whose volume remained unaffected. This suggests that experience of the rich in-hive environment promotes MB maturation and the development of MB-dependent learning capacities.

Figure 1.

Reversal learning performances of 10-d-old bees from the in-hive [N = 40, (A)] or impoverished environment [N = 40, (B)]. The percentage of proboscis extension responses (PER) elicited by odor A (solid line) and B (dashed line) is represented during the first phase (A+B−) and the second phase (A−B+) of the learning task. The odors used as A and B were alternated between the bees and the role of each odor as A or B had no effect on learning (repeated-measures ANOVA: F = 1.41; P = 0.23). (***) P < 0.001; (*) P < 0.05.

Figure 2.

Performances in two consecutive differential conditioning of bees from the in-hive [N = 47, (A)] or impoverished environment [N = 36, (B)]. The percentage of PER elicited by odor A (solid line) and B (dashed line) during the first phase (A+B−) and to odors C (solid line) and D (dashed line) during the second phase (C+D−) is represented. The odors used as A and B (A), and C and D (B), were alternated between the bees and the role of each odor as A or B, and C or D, had no effect on learning (A/B: F = 1.78; P = 0.14; C/D: F = 0.30; P = 0.84). (***) P < 0.001.

Figure 3.

Structural analyses of the MB of 1-d-old bees [N = 11] and 10-d-old bees from the in-hive [N = 9] or impoverished [N = 7] environment. (A) Single frontal confocal section of a central region of the right medial calyx immunolabeled for synapsin, revealing the presynaptic terminals that are part of the MB microglomeruli (scale bar is 100 µm). Inset: enlarged view of the lip and collar (scale bar is 20 µm). Synaptic boutons were counted in boxes of 1000 µm³ (four boxes in the lip, three in the dense collar). (B–D) Boxplots of the volume of the lip and dense collar of the MB right median calyx (B), the number of synaptic boutons per 1000 µm³ (C) and the number of synaptic boutons normalized to the volume of each neuropil compartment (lip or dense collar) of the medial calyx (D). The sample size of each group is displayed under the corresponding boxplot. (***) P < 0.0005; (**) P < 0.005; (*) P < 0.05.