Fertilisation calcium signals in the ascidian egg.

The egg of ascidians (urochordate), as virtually all animal and plant species, displays Ca^2+ signals upon fertilisation. These Ca2+ signals are repetitive Ca^2+ waves that initiate in the cortex of the egg and spread through the whole egg interior. Two series of Ca2+ waves triggered from two distinct Ca^2+ wave pacemakers entrain the two meiotic divisions preceding entry into the first interphase. The second messenger inositol (1,4,5) trisphosphate (IP3) is the main mediator of these global Ca2+ waves. Other Ca^2+ signalling pathways (RyR and NAADPR) are functional in the egg but they mediate localised cortical Ca^2+ signals whose physiological significance remains unclear. The meiosis I Ca^2+ wave pacemaker is mobile and relies on intracellular Ca^2+ release from the endoplasmic reticulum (ER) induced by a large production of IP3 at the sperm aster site. The meiosis II Ca^2+ wave pacemaker is stably localised in a vegetal protrusion called the contraction pole. It is probable that a local production of IP3 in the contraction pole determines the site of this second pacemaker while functional interactions between ER and mitochondria regulate its activity. Finally, a third ectopic pacemaker can be induced by a global increase in IP3, making the ascidian egg a unique system where three different Ca^2+ wave pacemakers coexist in the same cell.