MAPK is involved in metaphase I arrest in oyster and mussel oocytes

Oocytes of Crassostrea gigas and Mytilus galloprovincialis are arrested in metaphase I when they are spawned and ready to be fertilized. To investigate the role of MAP kinase in maintaining metaphase I arrest, oocytes were exposed to the MEK inhibitor U0126, and the effects on chromosome behavior and MAPK activity were examined by bisbenzimide staining and in immunoblots with anti-phospho MAPK antibodies. Following treatment with 50 microM U0126, active MAPK was undetectable and oocytes resumed meiosis, forming enlarged polar bodies and undergoing chromosome decondensation. Prophase stage oyster oocytes maturing spontaneously in seawater completed germinal vesicle breakdown in the presence of U0126, but failed to arrest in metaphase I, and also formed polar bodies and underwent chromosome decondensation. Treatment of oyster oocytes with the protein synthesis inhibitor, emetine (500 microM), also caused them to resume meiosis, although substantial MAPK activity remained. Levels of phospho-MEK also decreased during emetine treatment. 35 S-methionine incorporation in emetine treated oocytes was reduced to only 5% of control values. These data show that, while active MAPK is necessary to maintain metaphase I arrest, other proteins are also required.