Sex-chromosome pairing and activity during mammalian meiosis.

Mammalian sex chromosomes exhibit marked sexual dimorphism in behavior during gametogenesis. During oogenesis, the X chromosomes pair and participate in unrestricted recombination; both are transcriptionally active. However, during spermatogenesis the X and Y chromosomes experience spatial restriction of pairing and recombination, are transcriptionally inactive, and form a chromatin domain that is markedly different from that of the autosomes. Thus the male germ cell has to contend with the potential loss of X-encoded gene products, and it appears that coping strategies have evolved. Genetic control of sex-chromosome inactivation during spermatogenesis does not involve pairing or the presence of the Y chromosome or an intact X chromosome, and may therefore be under exogenous control by the gonad. Sex-chromosome reactivation during oogenesis and inactivation during spermatogenesis probably reflect specific meiotic events such as recombination. Understanding these phenomena may help explain other sex-related differences in genetic recombination.