Rapid evolution of mouse saliva through regulatory rewiring and gene expansions
Mammalian saliva plays essential roles in digestion, immunity, and host-microbiome interactions, yet its protein composition varies widely across species and sexes. This diversity makes saliva a powerful model to study evolution of gene expression. Here, we compared mouse and human salivary glands at the genomic, transcriptomic, and proteomic levels to understand how saliva composition evolves. We found that evolution of gene expression in mouse salivary glands is driven by rapid gene turnover and sexual dimorphism. In the submandibular and sublingual glands, 68% and 73% of expression from genes encoding secreted proteins derives from lineage-specific genes that lack one-to-one human orthologs. This contrasts the prevailing view that gene expression is largely conserved across tissues and highlights saliva as an unusually dynamic system of molecular evolution. Mouse submandibular gland shows striking sexual dimorphism, with 1537 tissue specific sex-biased genes, five times higher than in the liver, a classic model of sex-biased expression. These genes cluster in regions shaped by recent gene duplication, such as the kallikrein gene cluster, a mouse-specific expansion that accounts for ~16.4% of male-biased submandibular gland expression. Our analyses suggest that this bias arises through regulatory changes that are expanded by gene duplication, including the spread of a testosterone-associated regulatory motif and the expansion of a shared chromatin domain that promotes coordinated gene regulation. Our results describe a novel mechanism through which lineage-specific gene duplication and regulatory rewiring drive rapid, sex-specific evolution of the mammalian saliva. Overall design: Bulk RNA-seq was performed on salivary glands (parotid, submandibular, and sublingual), pancreas, and liver from male and female mice of two strains. Each strain included 3 males and 3 females, for a total of 12 individuals.