AmTH Plays an Indispensable Role in Larval Development as Well as Adult Immunity in Apis mellifera

In insects, tyrosine hydroxylase (TH) plays crucial roles in a wide variety of biological processes. However, its function in Apis mellifera remains unknown. Here, AmTH (A. mellifera TH) was cloned and characterized. Expression profiles at different stages showed that AmTH was highly expressed in 2-day-old eggs and during the pupal stage. Tissue expression profiles of newly emerged workers revealed that AmTH was highest expressed in the wings, followed by the legs and antennae. To explore the function of AmTH, gene-edited worker larvae were generated using CRISPR/Cas9 technology, and the edited individuals died at the larval and prepupal stages. In addition, a TH inhibitor (3-IT) was administered via larval feeding and the survival rates were significantly lower in groups fed with 800 μM, 3200 μM, and 12,800 μM of 3-IT in a dose-dependent manner. To investigate the role of AmTH in honeybee immunity, 8-day-old worker bees were fed with 3-IT and subsequently injected with lipopolysaccharide. The results showed that the mRNA expression of Relish, antimicrobial peptide-related genes, TH and DDC were significantly downregulated, whereas laccase 2 and yellow-y were significantly upregulated. These findings indicate that AmTH is indispensable for larval development and adult immunity in honeybees.