Sexual dimorphism and phenotypic plasticity in the antennal lobe of a stingless bee, Melipona scutellaris

Abstract

Among social insects, the stingless bees (Apidae, Meliponini), a mainly tropical group of highly eusocial bees, present an intriguing variety of well-described olfactory-dependent behaviors showing both caste- and sex-specific adaptations. By contrast, little is known about the neural structures underlying such behavioral richness or the olfactory detection and processing abilities of this insect group. This study therefore aimed to provide the first detailed description and comparison of the brains and primary olfactory centers, the antennal lobes, of the different members of a colony of the stingless bee Melipona scutellaris. Global neutral red staining, confocal laser scanning microscopy, and 3D reconstructions were used to compare the brain structures of males, workers, and virgin queens with a special emphasis on the antennal lobe. We found significant differences between both sexes and castes with regard to the relative volumes of olfactory and visual neuropils in the brain and also in the number and volume of the olfactory glomeruli. In addition, we identified one (workers, queens) and three or four (males) macroglomeruli in the antennal lobe. In both sexes and all castes, the largest glomerulus (G1) was located at a similar position relative to four identified landmark glomeruli, close to the entrance of the antennal nerve. This similarity in position suggests that G1s of workers, virgin queens, and males of M. scutellaris may correspond to the same glomerular entity, possibly tuned to queen-emitted volatiles since all colony members need this information.

Olfactory pathway of the hornet Vespa velutina: New insights into the evolution of the hymenopteran antennal lobe

“…To identify macroglomeruli a quantitative threshold that defines outliers according to 80% of the distribution of glomerular volumes was used: where V U is the upper percentile (90%) and V L the lower percentile (10%) of glomerular volume distribution. We used k = 3 as a conservative value that successfully categorized macroglomeruli in several hymenopteran species (Kuebler et al, ; Streinzer et al, ; Roselino et al, ). Thus, glomeruli whose volume was above this threshold were considered macroglomeruli.…”

“…Another striking example of such neuroanatomical and physiological adaptations is the existence of enlarged glomeruli, called macroglomeruli , dedicated to pheromone detection and processing (Burrows et al, ; Arnold et al, ; Hanson et al, ; Sandoz, ; Nishikawa et al, ; Nishino et al, ). In most cases, macroglomeruli are male dimorphic adaptations devoted to sex pheromone processing, but they have also been discovered in the females of some social Hymenoptera (Arnold et al, ; Kleineidam et al, ; Roselino et al, ). In leaf‐cutting ants, for instance, a macroglomerulus found only in large workers is thought to process trail pheromone information (Kelber et al, ; Kuebler et al, ).…”

Sexual dimorphism in visual and olfactory brain centers in the perfume‐collecting orchid beeEuglossa dilemma(Hymenoptera, Apidae)

“…On average, E. dilemma females had 21 additional glomeruli relative to males. An increased number of glomeruli in females is commonly observed in bees, although the differential can vary substantially (103 vs. 160 in the honey bee [−36%]; 159 vs. 200 in the stingless bee Melipona scutellaris [−21%]; 98 vs. 133 in the long horned bee Eucera berlandi [−27%]; Roselino et al, ; Streinzer, Kelber, et al, ). Interestingly, the difference between sexes in E. dilemma was the lowest measured for any bee species so far (141 vs. 162, −13%), which may indicate that male and female orchid bees have more similar olfactory capabilities compared to other bee lineages.…”

“…Where V G is the volume of the focal glomerulus, V L the 10th percentile, and V U the 90th percentile of glomerular volume distribution. Following Roselino, Hrncir, da Cruz Landim, Giurfa, and Sandoz (), glomeruli with a K value between 1.5 and 3 were defined as mild outliers and glomeruli with a K ≥ 3 were defined as extreme outliers. Only extreme outliers were classified as macroglomeruli.…”

Evolution of the neural sex‐determination system in insects: does fruitless homologue regulate neural sexual dimorphism in basal insects?

“…Although sexually dimorphic structures have been found in the nervous system of non‐ Drosophila insects (Rössler et al , ; Rospars and Hildebrand, ; Berg et al , ; Nishikawa et al , ; El Jundi et al ., ; Mysore et al , ; Nishino et al , ; Baba etisoforms) were identified using al. , ; Nakanishi et al , ; Hu et al , ; Streinzer et al , ; Roselino et al , ; Zhao et al , ), the molecular, neuronal and developmental mechanisms underlying the sexual dimorphism of the brain are not well understood.…”