TA/TYR regulates division of labor at both larval and adult stages

How division of labor evolved and how social behavior is regulated in eusocial insects are fundamental questions for evolutionary biologists.  Honey bees as a classic social model have been intensively studied for these questions.  In a honey bee colony, there are one queen and thousands of female workers. The queen only produces eggs, and workers, which are sterile when the queen is present, perform age-related social tasks: young workers feed the queen and brood, construct nest and make honey; older workers forage outside and have bias to collecting pollen or nectar. It has been demonstrated that the regulatory network of division of labor of honey bee workers has reproductive components: both ovariole number and vitellogenin (vg) regulate worker division of labor. Vg, a yolk precursor, is taken up by ovaries and provide nutrition for egg development of workers in queenless colonies, while vg, in queenright colony, is secreted by hypopharyngeal gland as a major content of brood food when the workers fulfill nursing task. In parallel, several quantitative trait loci (QTL) studies have related genomic regions of workers with their social behavior and ovariole number. One genomic region (Pln2) has been mapped repeatedly for both social behavioral traits and ovariole number, suggesting the genes in this region may be selected and play roles in regulating honey bee division of labor through reproductive network. Gene expression studies further identified a gene in this region, called tyramine receptor 1 (TYR1, a receptor for tyramine (TA)), which expression level is associated with both ovariole number and worker social behavior. Here we asked the question: how TA pathway is involved in ovariole number and vg production. We hypothesized that TA pathway determines ovariole number at larval stage, and also regulates vg production in adult workers. Here we performed four studies to test our hypothesis: 1) we fed larvae at the last larval stage with TA and observed the ovariole number in adult bees;  2) we fed newly emerged bees with TA in a queenless condition and observed ovary development; 3) we fed newly emerged bees with TA in both queenright and queenless conditions, and investigated gene expression of vg and other candidate genes;  3) we  knocked down TYR1 and measured gene expression of vg, and candidate genes in insulin and AKH pathways, JH cascade and ecdysteroid cascade. Overall, we found TA pathway regulates worker reproductive system at both larval stage and adult stage: TA determines ovariole number at the last larval stage and regulates ovary uptake of vg in queenless condition. Our gene expression data from TA feeding and TYR1 knockdown experiments suggest that TA pathway does not directly regulate vg production, instead it regulates the sensitivity of fat body to respond to systematic signals such as ecdysteroid, juvenile hormone (JH), insulin (IIS) and adipokenetic hormone (AKH). In addition our data suggest that queen mandibular pheromones (QMP) play an important roles in regulating JH level and Vg production in fat body, and QMP also mediate the response of ecdysteroid to TA in the fat body.