THE ACTIVATION WAVE OF CALCIUM IN THE ASCIDIAN EGG AND ITS ROLE IN OOPLASMIC SEGREGATION

We have studied egg activation and ooplasmic segregation in the ascidian Phallusia mammillata using an imaging system that let us simultaneously monitor egg morphology and calcium-dependent aequorin luminescence. After insemination, a wave of highly elevated free calcium crosses the egg with a peak velocity of 8-9 μm/s. A similar wave is seen in eggs fertilized in the absence of external calcium. Artificial activation via incubation with WGA also results in a calcium wave, albeit with different temporal and spatial characteristics than in sperm-activated eggs. In eggs in which movement of the sperm nucleus after entry is blocked with cytochalasin D, the sperm aster is formed at the site where the calcium wave had previously started. This indicates that the calcium wave starts where the sperm enters. In 70% of the eggs, the calcium wave starts in the animal hemisphere, which confirms previous observations that there is a preference for sperm to enter this part of the egg (Speksnijder, J.E., L.F. Jaffe, and C. Sardet. 1989. Dev. Biol. 133:180-184). About 30-40 s after the calcium wave starts, a slower (1.4 μm/s) wave of cortical contraction starts near the animal pole. It carries the subcortical cytoplasm to a contraction pole, which forms away from the side of sperm entry and up to 50° away from the vegetal pole. We propose that the point of sperm entry may affect the direction of ooplasmic segregation by causing it to tilt away from the vegetal pole, presumably via some action of the calcium wave.