Evidence for expression of the paternal genome in the two-cell mouse embryo

Abstract

Although there is strong evidence that the paternal genome is activated at the four- to eight-cell stage of mouse embryogenesis^1–12, studies to date have been restricted because of their dependence on the manifestations of gene products, such as enzyme activity and cell-surface expression, rather than on direct assay of their synthesis. β₂-microglobulin (β₂M), a 12,000-molecular weight (M_r) peptide associated with several of the gene products encoded by the H–2–Tla complex in the mouse^13–17, is synthesized by early cleavage stage embryos (J.A.S., T.M. and C.J.E., unpublished). The existence of an electrophoretic variant of β₂M in inbred strains has recently been described^18–20, and tryptic peptide mapping¹⁹ and amino acid sequencing data₂₁ indicate that the primary sequences of these variants differ by a single amino acid, thus establishing the genetic basis for this variation. Mice heterozygous at the β₂m locus synthesize both forms, β₂M^a and β₂M^b (refs 18, 20). Using direct immunoprecipitation and two-dimensional gel electrophoresis, we have used this variation at the β₂m locus to distinguish between maternal and paternal β₂M and have now established that the synthesis of paternally derived β₂M is first detectable at the two-cell stage. This is the earliest stage at which expression of the mammalian embryonic genome has been established²².