Functional Studies Of The Bursicon Signaling Pathway During Development Of Drosophila Melanogaster

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The protective external cuticle of insects is not practical for accommodating growth during development. To compensate for this, the insect life cycle is punctuated by a series of molts. During the molt, a new and larger cuticle is produced underneath the old cuticle. Replacement of the smaller, old cuticle culminates with ecdysis, a stereotyped sequence of shedding behaviors. Following each ecdysis, the new cuticle must expand and harden. Studies from a variety of insect species indicate that this cuticle hardening is regulated by the neuropeptide bursicon. However, genetic evidence from the fruitfly Drosophila melanogaster only supports such a role for bursicon after the final ecdysis, when the adult fly emerges. The research presented here investigates the role that bursicon has at stages of Drosophila development which precede adult ecdysis. In the first part of this work, I address the mechanism and timing of hormonal release from bursicon-positive motor neurons at the larval neuromuscular junction. The key findings here indicate that these peptidergic motor neuron terminals express the same core exocytotic machinery as is required for classical neurotransmitter release, and that they secrete hormones at the neuromuscular junction in two waves of release which coincide with larval ecdysis. The second part of this work addresses the functional significance of bursicon signaling during development, by disrupting the expression of its receptor, rickets. Importantly, I found that the available rickets mutants are not genetic nulls as previously believed, which necessitated the use of targeted RNA interference (RNAi) to disrupt rickets expression. By eliminating rickets expression in different tissues by RNAi, I determined that rickets is developmentally required in the epidermis and imaginal discs for proper formation of the prepupa and to harden the cuticle before eclosion, respectively. Furthermore, I correlated the timing of lethal events resulting from rickets RNAi with hormonal release studies from bursicon-positive neurons. The combined results indicate that bursicon signaling is an important feature of the development of insects.

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