Abstract
Bursicon is a heterodimeric neurohormone that acts through a G protein-coupled receptor named rickets (rk), thus inducing an increase in cAMP and the activation of tyrosine hydroxylase, the rate-limiting enzyme in the cuticular tanning pathway. In insects, the role of bursicon in the post-ecdysial tanning of the adult cuticle and wing expansion is well characterized. Here we investigated the roles of the genes encoding the bursicon subunits during the adult cuticle development in the honeybee, Apis mellifera. RNAi-mediated knockdown of AmBurs α and AmBurs β bursicon genes prevented the complete formation and tanning (melanization/sclerotization) of the adult cuticle. A thinner, much less tanned cuticle was produced, and ecdysis toward adult stage was impaired. Consistent with these results, the knockdown of bursicon transcripts also interfered in the expression of genes encoding its receptor, AmRk, structural cuticular proteins, and enzymes in the melanization/sclerotization pathway, thus evidencing roles for bursicon in adult cuticle formation and tanning. Moreover, the expression of AmBurs α, AmBurs β and AmRk is contingent on the declining ecdysteroid titer that triggers the onset of adult cuticle synthesis and deposition. The search for transcripts of AmBurs α, AmBurs β and candidate targets in RNA-seq libraries prepared with brains and integuments strengthened our data on transcript quantification through RT-qPCR. Together, our results support our premise that bursicon has roles in adult cuticle formation and tanning, and are in agreement with other recent studies pointing for roles during the pharate-adult stage, in addition to the classical post-ecdysial ones.
Highlights
The rigid cuticle covering the insects is periodically renewed during the discontinuous body growth
We aimed to characterize whether the process of adult cuticle formation is dependent on bursicon gene expression
Our RT-quantitative PCR (qPCR) data showed that the levels of AmBurs α (Fig 1B) and AmBurs β (Fig 1C) transcripts were significantly higher in the brains of pharate-adults and newly-emerged bees when compared to pupae, suggesting a role in adult cuticle formation
Summary
The aim of the current study was to search for evidence supporting a role of bursicon during the period encompassing the pharate-adult development culminating in the adult ecdysis. We aimed to characterize whether the process of adult cuticle formation is dependent on bursicon gene expression
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