A Role for Exaptation in Sculpting Sexually Dimorphic Brains from Shared Neural Lineages.

Sex differences in behaviours arise from variations in female and male nervous systems, yet the cellular and molecular bases of these differences remain poorly defined. Here, we take an unbiased, single-cell transcriptomic approach to uncover how sex shapes the adult Drosophila melanogaster brain. We show that sex differences do not result from large-scale transcriptional reprogramming but through fine-tuning of otherwise shared developmental templates via the sex-differentiating transcription factors Doublesex and Fruitless. We reveal, with unprecedented resolution, the extraordinary genetic diversity within these sexually dimorphic cell types and find birth order represents a novel axis of sexual differentiation. Neuronal identity in the adult reflects spatiotemporal patterning and sex-specific survival, with female-biased neurons arising early and male-biased neurons arising late. This pattern reframes dimorphic neurons as "paralogous" rather than "orthologous", suggesting sex leverages distinct developmental windows to build behavioural circuits and highlights a role for exaptation in diversifying the brain.