X chromosome inactivation: theme and variations

My contribution to this special issue on Vertebrate Sex Chromosomes deals with the theme of X chromosome inactivation and its variations. I will argue that the single active X--characteristic of mammalian X dosage compensation--is unique to mammals, and that the major underlying mechanism(s) must be the same for most of them. The variable features reflect modifications that do not interfere with the basic theme. These variations were acquired during mammalian evolution--to solve special needs for imprinting and locking in the inactive state. Some of the adaptations reinforce the basic theme, and were needed because of species differences in the timing of interacting developmental events. Elucidating the molecular basis for the single active X requires that we distinguish the mechanisms essential for the basic theme from those responsible for its variations.