Caste-specific development of the dopaminergic system during metamorphosis in female honey bees

Caste-specific differences in the dopaminergic systems of social insects assist in maintaining caste-specific behavior. To determine how caste differences in the honey bee occur during metamorphosis, a number of comparative analyses between castes were performed including comprehensive quantification of: levels of dopamine and its metabolite in the brain, the gene expression levels of enzymes involved in dopamine biosynthesis and conversion as well as expression levels of dopamine receptors and a dopamine transporter. Dopamine levels standardized to the protein contents of a whole brain at the day of eclosion in queens were 3.6-fold higher than those in workers. Dopamine levels increased until eclosion (7 days) in queens, whereas those in workers increased until 5–6 days before eclosion and then maintained until eclosion (10 days). These caste-specific dopamine dynamics in the brain were supported by the higher expression of genes (Amddc and Amth) encoding enzymes involved in dopamine synthesis in queens. The distribution of cells expressing Amddc in the brain revealed that soma clusters of dopaminergic cells were similar between castes at 7–8 days after pupation, suggesting the upregulation of Amddc expression in some cells in queens rather than addition of cell clusters. In contrast, genes encoding dopamine receptors were downregulated in queens or showed similar expression levels between castes. The expression of genes encoding dopamine transporters did not differ between castes. These results reveal the developmental process of caste-specific dopaminergic systems during metamorphosis in the honey bee, suggesting caste-specific behavior and division of reproduction in this highly eusocial species.