Single-nucleus resolution of sex-biased expression and dosage compensation in Drosophila melanogaster In many species, sex-biased expression is widespread and thought to contribute to sexual dimorphism. While bulk RNA-sequencing has been instrumental in identifying strongly sex-biased genes, it lacks resolution to assess variation across cell-types and tissue compartments. Using single-nucleus expression data from the Fly Cell Atlas, we investigate sex differences in adult Drosophila melanogaster. We find that differences in cell-type composition between the sexes are not a major source of sex-bias, as for the vast majority of genes, the degree of sex-bias is similar regardless of whether sex differences in cell-type composition are controlled for or not. Our analysis confirms a deficit of X-linked male-biased genes in the body’s somatic tissues that is widespread across cell-types. We also find the excess of X-linked female-biased genes to be associated with nervous system cells in the head but with epithelial cells in the body’s somatic tissues, showing that single-nucleus data crucially resolves sex-bias at the cell-type level. We investigate dosage compensation (DC) across 15 tissues and 17 cell-types. We observe that it varies throughout the body. Surprisingly, we observe a lack of DC in a cluster of main cells within the male accessory glands. This result highlights the importance of understanding context-dependent DC. 293 2063 Royal Society of London application/pdf 1