Introduction Meiotic sex chromosome inactivation (MSCI) is the process by which the X and Y chromosomes of primary spermatocytes are transcriptionally inactivated; the associated formation of a heteropyknotic XY- or sex body has been viewed as the morphological manifestation of this inactivation process (Solari, 1974; McKee and Handel, 1993). Little is known of the molecular basis or biological role of MSCI. Lifschytz and Lindsley suggested that certain sex-linked genes may be inhibitory to spermatogenesis and that MSCI might serve to silence such genes (Lifschytz and Lindsley, 1972). Others have viewed MSCI as an unfortunate consequence of a need to mask the non-synapsed regions of the sex chromosomes, either in order to avoid the checkpoint that causes germ cell death in response to defective chromosome synapsis (Jablonka and Lamb, 1988; Odorisio et al., 1998), to prevent the initiation of recombination events in the non-synapsed regions of the sex chromosomes (McKee and Handel, 1993), or to ensure efficient sex chromosome synapsis (Turner et al., 2000). It has been suggested that MSCI may have the same molecular basis as the somatic X-inactivation process that ensures X-linked dosage compensation between male (XY) and female (XX) mammals. Somatic X-inactivation requires the cis-acting Xist RNA, that coats one of the two X- chromosomes in each female cell (Penny et al., 1996; Marahrens et al., 1997; Brockdorff, 1998; Jaenisch et al., 1998; Lyon, 1999). The choice of which X chromosome to silence is mediated in-cis by the antisense Tsix RNA (Lee et al., 1999; Lee and Lu, 1999; Luikenhuis et al., 2001). Recent mutation analysis has demonstrated that the domain responsible for X- silencing resides at the 5′-end of the Xist RNA, while localisation of Xist RNA to the X chromosome is mediated by functionally redundant sequences dispersed throughout the rest of the gene (Wutz et al., 2002). In the male, Xist expression is restricted to the testis, the site at which MSCI takes place (Richler et al., 1992; Salido et al., 1992; McCarrey and Dilworth, 1992). Furthermore, it has been reported that Xist transcripts coat the X and Y chromosomal axes in the sex body of pachytene spermatocytes (Ayoub et al., 1997). The inactive somatic X chromosome, and the inactive X and Y chromosomes of spermatocytes, also share other features such as late replication (Priest et al., 1967; Kofman-Alfaro and Chandley, 1970; Odartchenko and Pavillard, 1970) and enrichment for the H2A variant macroH2A1.2 (Costanzi and Pehrson, 1998; Hoyer-Fender et al., 2000; Richler et al., 2000). Ayoub et al. have proposed that MSCI is brought about by Xist RNA-mediated spreading of inactivation from the X chromosome to the Y chromosome, via the region of X-Y pairing (Ayoub et al., 1997). Despite the circumstantial evidence in favour of a common 4097 X chromosome inactivation occurs twice during the life cycle of placental mammals. In normal females, one X chromosome in each cell is inactivated early in embryogenesis, while in the male, the X chromosome is inactivated together with the Y chromosome in spermatogenic cells shortly before or during early meiotic prophase. Inactivation of one X chromosome in somatic cells of females serves to equalise X-linked gene dosage between males and females, but the role of male meiotic sex chromosome inactivation (MSCI) is unknown. The inactive X-chromosome of somatic cells and male meiotic cells share similar properties such as late replication and enrichment for histone macroH2A1.2, suggesting a common mechanism of inactivation. This possibility is supported by the fact that Xist RNA that mediates somatic X-inactivation is expressed in the testis of male mice and humans. In the present study we show that both Xist RNA and Tsix RNA, an antisense RNA that controls Xist function in the soma, are expressed in the testis in a germ-cell-dependent manner. However, our finding that MSCI and sex-body formation are unaltered in mice with targeted mutations of Xist that prevent somatic X inactivation suggests that somatic X-inactivation and MSCI occur by fundamentally different mechanisms. Key words: Xist/Tsix, Germline expression, Xist mutants, MacroH2A1.2, Meiotic sex chromosome inactivation, Sex body, Spermatogenesis Summary Meiotic sex chromosome inactivation in male mice with targeted disruptions of Xist James M. A. Turner 1 , Shantha K. Mahadevaiah 1 , David J. Elliott 2 , Henri-Jean Garchon 3 , John R. Pehrson 4 , Rudolf Jaenisch 5 and Paul S. Burgoyne 1, * 1 Division of Developmental Genetics, National Institute for Medical Research, Mill Hill, London NW7 1AA, UK 2 Institute of Human Genetics, University of Newcastle Upon Tyne, NE1 7RU, UK 3 INSERM U25, Hôpital Necker, 75743 Paris Cedex 15, France 4 Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA 5 Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA *Author for correspondence (e-mail: pburgoy@nimr.mrc.ac.uk) Accepted 12 August 2002 Journal of Cell Science 115, 4097-4105 © 2002 The Company of Biologists Ltd doi:10.1242/jcs.00111 Research Article