Mus musculus and Mus spretus sperm methylation. Mus musculus and Mus spretus

The mammalian X chromosome has unusual evolutionary dynamics compared toautosomes. Faster-X evolution of proteins involved in spermatogenesis is welldocumented, but it is unclear if this rapid evolution extends to other forms of moleculardivergence. Here we conduct the first comprehensive test for faster-X evolutionspanning three different forms of molecular evolution - divergence in protein sequence,gene expression, and DNA methylation - across different developmental stages ofspermatogenesis in mice. We use fluorescence-activated cell sorting to generatetranscriptome profiles for specific cell populations across spermatogenesis, therebyovercoming a fundamental limitation of most previous studies on expression evolution.Our novel comparative approach reveals faster-X protein evolution but stage-specificslower-X expression and methylation divergence. Expression levels were mostconserved between species for genes expressed in postmeiotic round spermatidswhile the same group of late-expressed genes tended to show higher rates of proteincodingevolution. We argue that slower-X regulatory evolution is a consequence ofconstraints imposed by chromosome-wide epigenetic processes active during themeiotic and postmeiotic stages of spermatogenesis in mice. These stage-specificconstraints likely have a pervasive impact on sex chromosome evolution and speciation