Relationship between the parental origin of theXchromosomes, embryonic cell lineage andXchromosome expression in mice

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Abstract
The electrophoretic variants of the X-chromosome-linked enzyme phosphoglycerate kinase (PGK-1) have been used to investigate the randomness of X chromosome expression in the fetus and various extra-embryonic membranes of the mouse conceptus. The amnion shows essentially random expression of the maternally derived X chromosome (Xm) and the paternally derived X chromosome (Xp). The parietal endoderm, however, shows exclusive or preferential expression of Xm. The results support the idea that the randomness of X chromosome expression is correlated with embryonic cell lineage such that Xm is preferentially (perhaps exclusively) expressed in derivatives of the primitive endoderm and trophectoderm but that Xm and Xp are randomly expressed in the derivatives of the primitive ectoderm.Experiments involving ovary transplants, embryo transfers or crosses with heterozygous mothers confirm previous findings that Xm is preferentially expressed regardless of the X chromosome expressed in the reproductive tract. Additional experiments show that the preferentially expressed X chromosome in the parietal endoderm and visceral yolk sac endoderm of a normal XmXp conceptus is always Xm regardless of grand-parental origin of Xm and regardless of whether the mother is a normal XX female or an XO female. Xp is, however, expressed in these tissues hi XpO female conceptuses. It is argued that a form of chromosome imprinting occurs at each generation to mark Xm and Xp as different and that this difference influences the choice of which X chromosomes are expressed in each cell lineage.