Xist RNA is associated with the transcriptionally inactive XY body in mammalian male meiosis

In eutherian mammals, X inactive-specific transcripts (Xist) are expressed in somatic cells possessing more than one X chromosome, and in germline cells of males, in which the single X chromosome is transcriptionally inactive. In early meiosis of males the sex chromosomes form an inactive XY nuclear compartment (XY body). We show by in situ reverse-transcribed polymerase chain reaction that Xist RNA is concentrated in the XY body. This fine localization suggests that Xist RNA is involved in inactivation of the male X chromosome, and that it has spreading capability, not only in cis but also in a quasi-cis mode, to juxtaposed non-X chromosomes. A hypothetical scheme links the evolution of heteromorphic sex chromosomes to the development of X condensation/inactivation in the male. The mechanism of X inactivation in somatic cells of mammalian females, resulting in male/female dosage compensation, has been recruited from the Xist-activated chromosome condensation machinery that developed in male meiosis earlier in evolution.