XIST RNA IS ASSOCIATED WITH THE TRANSCRIPTIONALLY INACTIVE XY BODY INMAMMALIAN MALE MEIOSIS

In eutherian mammals, X inactive-specific transcripts (Xist) are expre ssed in somatic cells possessing more than one X chromosome, and in ge rmline cells of males, in which the single X chromosome is transcripti onally 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 i n the XY body. This fine localization suggests that Xist RNA is involv ed 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 juxtapos ed non-X chromosomes. A hypothetical scheme links the evolution of het eromorphic sex chromosomes to the development of X condensation/inacti vation in the male. The mechanism of X inactivation in somatic cells o f mammalian females, resulting in male/female dosage compensation, has been recruited from the Xist-activated chromosome condensation machin ery that developed in male meiosis earlier in evolution.