Sexual antagonism and X inactivation--the SAXI hypothesis.

X inactivation has evolved in the soma of mammalian females so that both sexes have the same ratio of X:autosomal gene expression. The X chromosome in the germ cells of XY males is also precociously inactivated for reasons that remain unclear. Unlike X inactivation in the soma, this germline X inactivation is not restricted to mammals but has evolved independently in several animal phyla. Thus, germline X inactivation might have been the precursor of somatic X inactivation in mammals. We now propose a hypothesis for the evolution of germline X inactivation. The hypothesis predicts a redistribution of late spermatogenic genes from the X chromosome to the autosomes, leading eventually to germline X inactivation as the X chromosome becomes 'demasculinized'. Sexual antagonism could be the mechanism driving this redistribution. Recent expression and genetic studies in mammals, nematodes and Drosophila support this hypothesis, and expression data on taxa that have not evolved germline X inactivation, such as birds and butterflies, should shed further light on it.