Chromosomally derived sterile mice have a 'fertile' active XY chromatin conformation but no XY body.

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.