RP gene haploinsufficiency promotes extra sensory organ formation via a threshold effect

Ribosomal protein (RP) gene haploinsufficiency is a conserved form of ribosome dysfunction across species and underlies a class of disorders known as ribosomopathies. In <i>Drosophila</i>, RP gene haploinsufficiency manifests as the Minute phenotype, characterized by thinner and shorter mechanosensory bristles. The development of both bristles and proprioceptive campaniform sensilla (CS) is initiated by the bHLH proneural proteins Achaete (Ac) and Scute (Sc). By analysing genetic interactions between <i>ac sc</i> mutants and <i>Minute</i> mutants of varying severity, we identified a novel bristle-promoting effect that occurs only in the strongly affected <i>Minutes</i> in which the average bristle length is shorter than a threshold. This threshold-dependent effect also promotes ectopic CS formation in the strong <i>Minutes</i>. Transcriptomic analyses comparing the sensory organ - promoting and non-promoting <i>Minutes</i> revealed significant differences in stress-response pathways, including differentially elevated expression of the Xrp1-Irbp18 transcriptional dimer. Notably, mutation of <i>Xrp1</i> suppresses the ectopic CS phenotype, indicating a positive regulatory role. These findings reveal a previously unrecognized threshold effect in RP gene haploinsufficiency, in which excessive Xrp1 activity promotes supernumerary sensory organ formation, suggesting a compensatory mechanism that modulates neurogenesis under severe ribosomal stress.