Single cell visualisation ofmir-9aandSenselessco-expression duringDrosophila melanogasterembryonic and larval peripheral nervous system development

TheDrosophila melanogasterperipheral nervous system (PNS) comprises the sensory organs that allow the fly to detect environmental factors such as temperature and pressure. PNS development is a highly specified process where each sensilla originates from a single sensory organ precursor (SOP) cell. One of the major genetic orchestrators of PNS development isSenseless, which encodes a zinc finger transcription factor (Sens). Sens is both necessary and sufficient for SOP differentiation.Senselessexpression and SOP number are regulated by the microRNAmiR-9a. However, the reciprocal dynamics ofSenselessandmiR-9aare still obscure. By coupling smFISH with immunofluorescence, we are able to visualize transcription of themir-9alocus and expression of Sens simultaneously. During embryogenesis, we show that the expression ofmir-9ain SOP cells is rapidly lost asSenselessexpression increases. However, this mutually exclusive expression pattern is not observed in the third instar imaginal wing disc, where someSenseless-expressing cells show active sites ofmir-9atranscription. These data challenge and extend previous models ofSenselessregulation, and show complex co-expression dynamics betweenmir-9aandSenseless. The differences in this dynamic relationship between embryonic and larval PNS development suggest a possible switch inmiR-9afunction. Our work brings single-cell resolution to the understanding of dynamic regulation of PNS development bySenselessandmiR-9a.