DSSS - The evolutionary fluidity of sex differences

Sexual dimorphism in phenotypes is largely driven by genes with sex-biased expression, spanning from key regulators to numerous organ-specific effectors. Current understanding is limited regarding the rapid evolutionary dynamics of these genes in somatic tissues, although these trigger the diversity of phenotypes encountered between the sexes. In a model study across somatic organs in mice, we found an extremely fast evolutionary turnover of somatic sex-biased genes, exceeding even the turnover in gonadal organs. To describe this variation, we introduce a sex-biased gene expression index (SBI) to quantify individual variances. Our findings suggest that adult individuals exhibit a mosaic of sex characteristics across somatic tissues, challenging the simple binary classification of sexes. Our study underscores the complexity of sex differentiation and highlights the need for nuanced approaches in evolutionary biology and medical research.