Fig. 1. Cyclin E associates with the sperm head following fertilization and remains associated with male chromatin throughout pronuclear fusion. L. pictus (A-C) and S. purpuratus (D-F) eggs were fertilized and cultured in artificial seawater at 15°C. Eggs and embryos were immobilized on glass coverslips, fixed at various timepoints throughout the first cell cycle and processed for double-label immunofluorescence microscopy using antibodies to cyclin E (shown in red) and -tubulin (shown in green). Images were collected using a Zeiss LSM-410 laser-scanning confocal microscope. Cyclin E localization is seen as a punctate stain in the female pronucleus of unfertilized eggs (A). Following fertilization, cyclin E associates with the sperm head (B) and is incorporated into the zygotic nucleus following pronuclear fusion (C). It can be seen that the paternal chromatin remains condensed until pronuclear fusion (D,E), at which time decondensation is completed, and cyclin E disperses throughout the nucleus (F). Bar, 25 µm.