STRAIN DIFFERENCE IN THE TIMING OF MEIOSIS RESUMPTION IN MOUSE OOCYTES - INVOLVEMENT OF A CYTOPLASMIC FACTOR(S) ACTING PRESUMABLY UPSTREAM OF THE DEPHOSPHORYLATION OF P34(CDC2) KINASE

Oocytes from eight inbred strains of mice were screened for the timing of germinal vesicle breakdown (GVB) in vitro. This characteristic var ied between strains, reaching most extreme values in oocytes from AKR and BALB/c mice (3.1 and 1.6 h after release from dibutyryl cAMP block , respectively; p < 0.0001). The difference between AKR and BALB/c mic e was confirmed in experiments in which GVB was induced in vivo by sti mulation with exogenous gonadotrophins. Analysis of the rate of GVB in hybrids obtained after fusion of nuclear and cytoplasmic fragments of oocytes from both strains suggests that the factor responsible for th e difference between AKR and BALB/c mice is located in the cytoplasm o f the prophase oocytes. Finally, in oocytes from both strains stimulat ed to resume meiotic maturation with okadaic acid, an inhibitor of pro tein phosphatases types 1 and 2A, the rate of GVB was the same (2.2 h and 2.3 h for AKR and BALB/c, respectively; p = 0.48). This suggests t hat the difference between strains is not related to the amount or qua lity of the pre-MPF (Maturation Promoting Factor) stored in the propha se oocyte, but to the factor(s) acting upstream of the dephosphorylati on of p34(cdc2) kinase in the pathway leading to pre-MPF activation.