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.

Intimate association between autosomal translocation trivalents and XY bivalents at pachytene was observed in a majority of cells of two men ascertained through primary sterility and found to be heterozygous for a 14;21 Robertsonian translocation. The association, studied by light and electron microscopy of spread first spermatocytes, was between the unpaired short arms of the normal chromosomes of the translocation trivalent and the differential axes of the XY chromosomes. In a minority of cells, this contact was not established, or not maintained, as alternative combinations between the elements available for non-homologous pairing were realized. Following a suggestion of Lifschytz and Lindsley (1972), sterility in these patients was attributed to spermatogenic arrest caused by physical contact of sex chromosomes with autosomal material and consequent interference with the normal metabolism of the sex chromosomes. Autosomal aberrations and polymorphisms, which lead to the presence of unpaired segments at meiosis, may thus play a critical role in a general mechanism of chromosomally-derived male sterility. It is proposed that such a mechanism may also be instrumental in the initiation of reproductive barriers in nature.

Four forms of mole rats with diploid numbers of chromosomes of S2, 54, 58, and 60, respectively, were found in Israel and the vicinity. The differences between the chromosome sets are due to whole-arm (Robertsonian) changes and pericentric inversions. The geographic distribution of the different forms is contiguous. Only a few hybrid individuals have been discovered. These chromosome forms are tentatively considered as sibling species, almost completely isolated by cytogenetic and possibly ethological mechanisms. The weak dispersal powers of mole rats may have-contributed to a rapid fixation of adaptive homozygous chromosomal changes.

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.

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