Maternal Thp lethality in the mouse is a nuclear, not cytoplasmic, defect.

The Thp mutation, an allele of the mouse T/t complex, differs from other known mutations in that its effects are determined by the sex of the parent from which it is inherited; when inherited from the female parent, it is invariably lethal at the embryonic stage, but, most embryos which inherit the mutation from the male parent survive. Thus most heterozygous embryos carrying the maternally derived mutation die in the second half of pregnancy, while the exceptional embryos surviving to parturition give oedematous, cyanotic individuals that die within 24 h. The lethal maternal effect of Thp may be transmitted either through the cytoplasm of the ovum (oogenic defect) or through the female pronucleus (embryogenic defect). Here we have sought to decide between these possibilities by performing reciprocal nuclear transplantations between one-cell embryos from Thp/+ and +/+ females. Our observation that this maternally inherited lethal effect of Thp persists when Thp/+ pronuclei are transplanted into +/+ cytoplasm suggests that the defect responsible for the pattern of inheritance lies in the pronuclei and not the cytoplasm.