Age-related mushroom body expansion in male sweat bees and bumble bees

Abstract

A well-documented phenomenon among social insects is that brain changes occur prior to or at the onset of certain experiences, potentially serving to prime the brain for specific tasks. This insight comes almost exclusively from studies considering developmental maturation in females. As a result, it is unclear whether age-related brain plasticity is consistent across sexes, and to what extent developmental patterns differ. Using confocal microscopy and volumetric analyses, we investigated age-related brain changes coinciding with sexual maturation in the males of the facultatively eusocial sweat bee, Megalopta genalis, and the obligately eusocial bumble bee, Bombus impatiens. We compared volumetric measurements between newly eclosed and reproductively mature males kept isolated in the lab. We found expansion of the mushroom bodies-brain regions associated with learning and memory-with maturation, which were consistent across both species. This age-related plasticity may, therefore, play a functionally-relevant role in preparing male bees for mating, and suggests that developmentally-driven neural restructuring can occur in males, even in species where it is absent in females.

Figure 1

Mushroom body (MB) expansion occurs with maturation in male bees. Relative volumes of the mushroom body structures as whole brain proportions for (a) M. genalis and (b) B. impatiens. In both species, mature males had larger calyces (lip + collar + basal ring; M. genalis: t =  − 2.23, df = 12, p = 0.046; B. impatiens: t =  − 4.34, df = 16, p = 0.0005) and mushroom body neuropil (peduncles + lobes + calyces; M. genalis: t =  − 2.30, df = 12, p = 0.04; B. impatiens: t =  − 3.84, df = 16, p = 0.001) relative to newly eclosed bees. Dots represent individual data points for newly eclosed (NE; white boxes; M. genalis green dots, N = 8; B. impatiens orange dots, N = 11) and mature (gray boxes; M. genalis purple dots, N = 6; B. impatiens blue dots, N = 7) males. Boxes indicate interquartile range, lines are medians, and whiskers extend to 1.5 the interquartile range. “*” = unadjusted p < 0.05 and “**” = unadjusted p < 0.001.

Figure 2

Age-related neuroplasticity in male bees. Neuropil:Kenyon cell ratios are significantly higher in mature, relative to newly eclosed, (a) M. genalis (t =  − 4.32, df = 12, p = 0.001) and (b) B. impatiens (t =  − 5.12, df = 16, p = 0.0001) bees. Dots represent individual data points for newly eclosed (NE; white boxes; M. genalis green dots, N = 8; B. impatiens orange dots, N = 11) and mature (gray boxes; M. genalis purple dots, N = 6; B. impatiens blue dots, N = 7) males. Boxes indicate interquartile range, lines are medians, and whiskers extend to 1.5 the interquartile range. “*” = unadjusted p < 0.05 and “**” = unadjusted p < 0.001.

Figure 3

Confocal microscope image of a (a) Bombus impatiens and (b) Megalopta genalis male brain. Image captures are individual slices taken from raw image stacks. Volumetric measurements were assessed for mushroom body calyces (C; lip, collar, and basal ring as one structure), Kenyon cells (KC), and mushroom body lobes (MBL; peduncle and lobes as one structure). Solid contour lines include structure-specific boundaries (pink (KC), orange (C), and purple (MBL)), whereas the dotted white line indicates the boundary for a whole brain trace. Scale bar 200 µm.