Background: The insect neuroendocrine system acts in the regulation of physiology, development and growth. Molecular evolution of this system hence has the potential to allow for major biological differences between insect groups. Two prohormone convertases, PC1/3 and PC2, are found in animals and both function in the processing of neuropeptide precursors in the vertebrate neurosecretory pathway. Whereas PC2-function is conserved between the fly Drosophila and vertebrates , ancestral PC1/3 was lost in the fly lineage and has not been functionally studied in any protostome.
Results: In order to understand its original functions and the changes accompanying the gene loss in the fly, we investigated PC1/3 and PC2 expression and function in the beetle Tribolium castaneum. We found that PC2 is broadly expressed in the nervous system, whereas surprisingly, PC1/3 expression is restricted to specific cell groups in the posterior brain and suboesophageal ganglion. Both proteases have parallel but non-redundant functions in adult beetles’ viability and fertility. Female infertility following RNAi is caused by a failure to deposit sufficient nutritive material to the developing oocytes. Larval RNAi of both genes produced moulting defects where the larvae were not able to shed their old cuticle and became ‘locked-in’. Unexpectedly, PC1/3 RNAi larvae went through supernumerary larval moults despite minimal to zero weight gain.
Conclusions: Our results indicate a yet unknown molecular mechanism for the coupling of moulting and growth in insect larvae. Conservation of the evolutionary ancient PC1/3 gene in most insect groups suggests conserved function and that its loss in dipterans may have been accompanied by major changes in the coordination of larval growth and moulting.