E93-depleted adult insects preserve the prothoracic gland and molt again

Abstract

Insect metamorphosis originated around the middle Devonian, associated with the innovation of the final molt; this occurs after the histolysis of the prothoracic gland (PG; which produces the molting hormone) in the first days of adulthood. We previously hypothesized that transcription factor E93 was crucial in the emergence of metamorphosis, since it triggers metamorphosis in extant insects. This work on the cockroach Blattella germanica reveals that E93 also plays a crucial role in the histolysis of PG, which fits the above hypothesis. Previous studies have shown that the transcription factor FTZ-F1 is essential for PG histolysis. We have found that FTZ-F1 depletion, towards the end of the final nymphal instar, downregulates the expression of E93, while E93-depleted nymphs molt to adults that retain a functional PG. Interestingly, these adults are able to molt again, which is exceptional in insects. The study of insects able to molt again in the adult stage may reveal clues as to how nymphal epidermal cells definitively become adult cells, and if it is possible to revert this process. Summary statementThe prothoracic gland disintegrates after insect metamorphosis. It was believed that the factor FTZ-F1 determines this disintegration. This work reveals that FTZ-F1 action is mediated by the factor E93.