MicroRNAs and the evolution of insect metamorphosis

Abstract

MicroRNAs (miRNAs) are involved in the regulation of a number of processes associated with insect metamorphosis, either in the less modified hemimetabolan mode (essential adult body plan established during embryogenesis), or in the more modified holometabolan mode (adult body plan formed in the pupal stage). The miR-100/let-7/miR-125 cluster has been studied extensively, especially in relation to wing morphogenesis in both hemimetabolan and holometabolan species. Other miRNAs also participate in wing morphogenesis (bantam, miR-9, miR-7, iab-4/8), as well as in programmed cell and tissue death (bantam, miR-14), neuromaturation, neuromuscular junction formation and neuron cell fate determination (miR-100/let-7/miR-125 cluster), typically during the pupal stage of holometabolan species. A special case is the control of miR-2 over Kr-h1 transcripts, which determines adult morphogenesis in the hemimetabolan metamorphosis. This is an example of how a single miRNA can control an entire process by acting upon a crucial mediator. Nevertheless, this is a quite exceptional mechanism that was apparently lost during the transition from hemimetaboly to holometaboly.