MEIOTICALLY INCOMPETENT AND COMPETENT GOAT OOCYTES - TIMING OF NUCLEAR EVENTS AND PROTEIN-PHOSPHORYLATION

The ability of mammalian oocytes to resume meiosis and to complete the first meiotic division is acquired sequentially during their growth p hase. The acquisition of meiotic competence in goat oocytes has been p reviously correlated with follicular size (9). Since protein phosphory lation/dephosphorylation play a key role in oocyte maturation, it coul d be that in meiotically incompetent oocytes, such posttranslational m odifications are inadequate. The aim of this study was to analyze whet her changes in oocyte proteins phosphorylation occured during the acqu isition of meiotic competence. For this propose, goat oocytes were div ided into 4 classes according to follicular size and meiotic competenc e : Class A oocytes from follicles < 0.5 mm in diameter, Class B oocyt es from follicles 0.5 - 0.8 mm; Class C oocytes from follicles 1 - 1.8 mm and class D oocytes from follicles > 3 mm. The protein phosphoryla tion patterns of these classes of oocytes were studied at different ti mes of in vitro maturation; After 4h of culture, when all oocytes were in the germinal vesicle stage, only the oocytes from Class D displaye d the phosphoproteins at 110 kD, 31 kD and around 63 kD. In contrast t o Class D oocytes Classes B and C oocytes were partially competent to mature, they underwent germinal vesicle breakdown later than fully com petent Class D oocytes and remained in early prometaphase I or in meta phase I, respectively. They exhibited the phosphoprotein changes that are associated with commitment to resume meiosis; but the changes occu rred later than in Class D oocytes, which were fully competent to reac h metaphase II. After 27 h of culture, the phosphorylation patterns of Class B, C and D oocytes were identical, whereas the meiotic stages r eached were quite different. The phosphoprotein changes associated wit h oocyte maturation did not occur in meiotically incompetent Class A o ocytes, which were blocked at the germinal vesicle stage. From these r esults it can be concluded that, at the GV stage, meiotically incompet ent and competent goat oocytes display different patterns of protein p hosphorylation. Once oocytes are able to resume meiosis they undergo s pecific phosphorylation changes, but whether these changes are markers or regulators of maturation events remains to be determined.