A post-mitotic bistable switch actively safeguards mature neurons against reciprocal transdifferentiation in vivo

How mature cells actively defend their identity against alternative fates is poorly understood. Here we show that two types of fully differentiated Drosophila somatosensory neurons, derived from distinct lineages, are continuously prevented from transdifferentiating into one another by a post-mitotic bistable switch. Two homeodomain transcription factors, Hmx and Cut, are co-expressed in both neuronal types but adopt opposite stoichiometric dominances through mutual repression, each defining one identity. Disrupting this balance in mature neurons triggers deterministic, lineage-crossing reprogramming of transcriptome, dendritic and axonal architecture, and sensory behavior without reversion to a progenitor state. This regulatory logic is evolutionarily conserved: Drosophila sensory identities share transcriptomic signatures with distinct human Aδ mechanoreceptors, and human CUX and HMX proteins functionally replace their fly counterparts. Mature neuronal identity is thus not a fixed endpoint but a poised equilibrium actively maintained by ongoing competition between opposing selectors.