Both X chromosomes function before visible X-chromosome inactivation in female mouse embryos

Abstract

ALTHOUGH neither X chromosome of preimplantation female mammalian embryos exhibits the two cytological signs of inactivation and dosage compensation—heteropyknosis and late replication^1,2—it has not been known whether both X chromosomes actually function during this period. Biochemical evidence based on increasing hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity during the mouse morula stage indicates that at least one X chromosome is functional^3,4. This is corroborated by the observation that mouse embryos lacking an X chromosome do not survive beyond the early cleavage stages⁵. The principal approach to determining whether both X chromosomes are functional before inactivation has been to look at the distributions of the activities of known X-linked enzymes in individual embryos^6,7. A bimodal distribution would be expected if dosage compensation had not yet occurred and female embryos with two X chromosomes made twice as much enzyme as male embryos with only one. However, identities of the embryos being assayed are not known, and conclusions based on activity distributions are necessarily inferential. We have circumvented this difficulty by using a method which facilitates determination of the sex of the embryonic material being assayed. This has led to the demonstration that early male and female mouse blastocysts differ twofold in HGPRT activity, a finding indicative of uncompensated X-chromosome dosage dependent gene activity before X-chromosome inactivation.