Cysteine-rich sperm protamines arose independently at least twice on a deeply conserved, intrinsically disordered scaffold in amniotes.
Every amniote must compact and shield the paternal genome, but the chemistry differs. In placental mammals, sperm chromatin is locked by protamine disulfide bridges, and the cysteine-rich form is considered the "mammalian" protamine. Analyzing 46 tetrapod protamine-1 orthologs on a time-calibrated phylogeny, we find it...