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X chromosome inactivation occurs twice during the mammalian life cycle. In females one of the two X chromosomes of somatic nuclei is inactive, while in males the solitary X chromosome is inactivated during germ cell development. Despite the different properties of the inactivated chromosomes of females and males, the molecular initiation of inactivation may be the same. X inactive-specific transcripts, XIST, are produced from somatic inactivated X chromosomes. We demonstrate here the existence of XIST transcripts in testes of man and mouse. Inactivation of X chromosomes in males, as in females, may thus be mediated through XIST. Conceivably, the silencing of X-linked genes is the price paid for the evolution of successful mechanisms of chromosomal sex determination.

Template-active regions of chromatin are structurally distinct from nontranscribing segments of the genome. Recently, it was suggested that the conformation of active genes which renders them sensitive to DNase I may be maintained even in fixed mitotic chromosomes. We have developed a technique of mitotic cell fixation and DNase I-directed nick-translation which distinguishes between active and inactive X chromosomes. We report here that Gerbillus gerbillus (rodent) female cells contain easily identified composite X chromosomes each of which includes the original X chromosome flanked by two characteristic autosomal segments. After nick-translation the active X chromosome in each cell is labelled specifically in both the autosomal and X-chromosomal regions. The inactive X chromosome is labelled only in the autosomal regions and in a small early replicating band within the late replicating 'original X' chromosome. Our technique opens the possibility of following the kinetics of X-chromosome inactivation and reactivation during embryogenesis, studying active genes in the inactive X chromosome and mapping tissue-specific gene clusters.

We have previously shown that the sex chromosome bivalent of normal, fertile male mice possesses extensive regions of potentially active chromatin, even though, as has been shown by others, certain X-linked genes, and perhaps most of the X chromosome, become inactivated during pachytene. The male meiosis of a fertile (2;11) translocation carrier mouse, a chromosomally derived sterile (11; 19) translocation carrier and that of normal mice is compared. In situ nick translation shows a similar DNase I sensitivity pattern in the sex chromosomes of all examined mice. The X chromosome has four regions of potentially active chromatin conformation, two at the ends of the chromosome and two interstitial ones, coinciding with flexures which become prominent towards late pachytene. The Y chromosome is almost uniformly sensitive to DNase I. The similarity of chromatin conformation patterns in fertile and sterile mice is compatible with the hypothesis that unscheduled transcription of particular genes, possibly included in the active conformation regions, occurs in mice which become sterile. In the sterile (11;19) translocation carrier, a vast majority of all pachytenes are "associated": usually one unpaired segment of chromosome 19 is in end-to-end contact with the X chromosome. The tips of both unpaired segments of chromosome 19 have a thickened axis and display a peculiar chromatin appearance, similar to the modification of the centromeric tip of the X chromosome. Telomeric unpairedness of certain chromosome segments seems to be conducive to autosome-X chromosome association. We suggest that compartmentalization of the nucleus into an autosome mass and a fully developed, protruding, metabolically quiescent XY body, is a precondition for the normal progressing of meiosis. In the associated cells, the autosomal quadrivalent anchors the XY bivalent among the autosomes; as a consequence no XY body is formed. This interference with the course of compartmentalization leads to the abolishment of inactivation of part or all of the potentially active genes and results in meiotic arrest, and hence in sterility.

A microspread, early-mid diplotene nucleus of a man with a normal karyotype and presumably normal meiosis is compared with a similar, earlier described nucleus of a man with meiotic arrest, heterozygous for a (14;21) Robertsonian translocation (Rosenmann et al., 1985). The axes of the XY bivalent of normal diplotene have an extremely tangled configuration, whereas those of the meiotically-arrested cell are straight, recalling the shape of the XY which is normally found in early pachytene. The morphological reversal from the complex configuration to a simpler shape may be associated with reactivation of the sex chromosome(s). Such a reactivation may be responsible for the sterility of the carrier of the Robertsonian translocation which thus may be considered as chromosomally-derived. The diplotene cells shown here have autosomal bivalents with continuous axes and various degrees of focal separation as is typical for diplotene in general. The observations on axis continuity, bivalent segmental dilatations, and XY tanglement in diplotene are compared with findings by others in human ultrathin sectioned material.