ABSTRACT
We have compared mouse embryos of three heterozygous, congenic genotypes (with high, medium and low levels of oocyte-coded glucose phosphate isomerase (GPI-1) activity respectively) to test whether 1) the survival time of oocyte-coded GPI-1 activity in the early embryo is affected by its activity level in the oocyte and 2) whether embryo-coded GPI-1 is detected earlier in embryos that inherit low levels of oocyte-coded GPI-1. The oocyte-coded GPI-1 was entirely GPI-1 A allozyme in the high and medium groups but was the less stable GPI-1C allozyme in the low group. We determined total GPI-1 activity and the ratio of different GPI-1 allozymes in early embryos and calculated the activity of oocyte-coded and embryo-coded GPI-1. In all three groups, the oocyte-coded enzyme activity remained at a more or less constant level for the first 
days. Some oocyte-coded GPI-1 remained in 
day embryos from the high and medium groups but was gone by 
days. Very little remained in 
day embryos that inherited low levels of a less stable form of the enzyme (GPI-1C allozyme). Despite a 4- to 5-fold difference in initial oocyte-coded GPI-1 activity, no differences were seen among the three genotypically distinct groups of embryos in the time of activation of the embryonic Gpi-1s genes. The embryo-coded GPI-1 was first detectable in 
day compacted morulae in all three groups. The level of oocyte-coded GPI-1, in the high group, when embryo-coded GPI-1 was first detected was higher than the level in the low group at any stage prior to detection of embryo-coded GPI-1. These results imply that embryonic Gpi-1s genes are not activated by a mechanism that depends on oocyte-coded GPI-1 enzyme activity falling below a critical threshold. Once both embryonic Gpi-ls alleles were activated. they were usually equally expressed in Gpi-lsa/Gpi-lsb heterozygous embryos. However, the maternal: paternal expression was closer to 2:1 in 5 % (3/60) of the 
and 
day embryos. These could have been digynic triploid embryos.
