X chromosome inactivation in diploid parthenogenetic mouse embryos

Abstract

THE mechanism by which one X chromosome in normal cells of female mammals becomes inactive while the other, apparently identical, chromosome remains active is not known. In kangaroos the paternally derived X chromosome is preferentially inactivated in most tissues¹, and in mice and rats similar preferential inactivation of the paternal X chromosome occurs in the cells of the extraembryonic membranes^2,3. Paternal X inactivation in kangaroos led Cooper⁴ to postulate that passage of the X chromosome through male gametogenesis was an important factor in its inactivation. He⁴ and Brown and Chandra⁵ suggested that paternal X inactivation was a primitive form and that random X inactivation as seen in adult eutherian mammals had evolved from it. Brown and Chandra further suggested that passage of one chromosome set through male gametogenesis or fertilisation led to chromosomal imprinting. A locus concerned in the control of X chromosome activity was postulated to become inactive when imprinted, and one active copy of this gene could maintain the activity of one X chromosome. In marsupials this gene was postulated to lie on the X chromosome itself, but on an autosome in euitherians. Thus, in eutherians the number of active X chromosomes in any animal should be equal to the number of maternally derived (and therefore non-imprinted) autosome sets. In this context it is interesting to study X inactivation in artificially formed parthenogenetic embryos, in which all chromosomes are of maternal origin. We report here that diploid parthenogenetic mouse embryos, at the late egg cylinder stage, showed a single late replicating chromosome, indicating that X inactivation had occurred normally. Because in the production of these embryos polar body formation had been suppressed, it seems that neither passage of chromosomes through male gametogenesis, nor fertilisation, nor probably the effects of peripheral egg cytoplasm are required for normal X inactivation in the mouse.