X-chromosome activity in female mouse embryos heterozygous forPgk-1and Searle's translocation, T(X; 16) 16H

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Abstract
To investigate whether the preferential expression of genes on the translocated X chromosome in female mice carrying the X-autosome translocation T(X;16)16H (Searle''s translocation) is due to non-random inactivation or to cell selection, tissues of mouse embryos heterozygous were investigated for the X-linked gene coding for phosphoglycerate kinase (Pgk-1). From the cross T16HPgk-1b/+Pgk-1b.female. x +Pgk-1a/Y.male., embryos expressing both isozymic forms of PGK-1 in the epiblast, and only the maternally inherited Pgk-1b allele in extra-embryonic tissues, were assumed to be chromosomally balanced, heterozygous female embryos carrying the Searle''s translocation (like the mother). The normal X chromosome in this cross carries a high-expression Xcec locus. At 6 days post-coitum (pc) both isozymes were equally expressed in the epiblast as expected if both X chromosomes are active, but by 7 days pc the PGK-1B contribution was significantly less than 50%, suggesting that X inactivation has occurred with a bias towards inactivation of the translocated X chromosome carrying the lower-expression Xce allele. By 8 days pc the situation was the reverse, with a Pgk-1b contribution of significantly more than 50%, and by 12 1/2 days pc no Pgk-1a expression could be detected. The dramatic change in isozyme expression between 7 and 8 days pc was interpreted as indicating rapid selection against cells that had inactivated the translocated 16x chromosome. Two 7-day pc embryos unexpectedly showed equal expression of both Pgk-1 alleles in both embryonic and extra-embryonic tissues; these were presumably chromosomally unbalanced embryos which had inherited from the mother both an active translocated 16x chromosome carrying Pgk-b and an active normal X chromosome carrying Pgk-1a.
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