EFFECT OF CA2-SAVIGNYI( ON DEFORMATION, POLAR BODY EXTRUSION AND PRONUCLEUS FORMATION IN THE EGG OF THE ASCIDIAN, CIONA)

Cortical deformation and polar body extrusion are the principal events that occur at fertilization in the ascidian egg. We demonstrated that the intracellular Ca2+ concentration ([Ca2+](i)) in the fertilized eg g of Ciona savignyi increased at egg deformation (main peak) and then several small Ca2+ spikes (Isi spikes) appeared before the first polar body extrusion. Brief Ca2+ spikes (2nd spikes), then appeared in the period between the first and second polar body extrusion. When eggs we re fertilized in Ca2+-free artificial seawater, the main peak and Ist spikes appeared, but the 2nd spikes did not, suggesting that the Ca2required for the main peak and 1 st spikes is released from the intrac ellular store in this species and that extracellular Ca2+ is required for the 2nd spikes. When [Ca2+](i) was clamped at a low level (0.03-0. 13 mu mol/L) by injecting the egg with low-Ca2+ buffers and the egg wa s then inseminated, deformation, polar body extrusion and pronucleus f ormation were suppressed. In contrast, egg deformation and first polar body extrusion were induced without insemination when [Ca2+](i) was 0 .9 mu mol/L. A higher Ca2+ concentration of 1.2-10.1 mu mol/L was requ ired for extrusion of the second polar body and pronucleus formation. These data suggest that sequential Ca2+ increases (i.e. main peak and 1 st and 2nd spikes) are prerequisite for the deformation and polar bo dy extrusion of the egg. Furthermore, in eggs arrested at the second m eiotic metaphase after first polar body extrusion by the injection of Ca2+ buffer, subsequent injection of excess Ca2+ caused formation of a n irregular second polar body-like protrusion, suggesting latent arres t at the second meiotic metaphase in the ascidian egg.