THE SPERM ENTRY POINT DEFINES THE ORIENTATION OF THE CALCIUM-INDUCED CONTRACTION WAVE THAT DIRECTS THE FIRST-PHASE OF CYTOPLASMIC REORGANIZATION IN THE ASCIDIAN EGG

Ascidians eggs are spawned with their cytoskeleton and organelles orga nized along a preexisting animal-vegetal axis, Fertilization triggers a spectacular microfilament-dependant cortical contraction that causes the relocalization of preexisiting cytoplasmic domains and the creati on of new domains in the lower part of the vegetal hemisphere, We have investigated the relationship between fertilization, the cortical con traction and the localization of cytoplasmic domains in eggs of the as cidian Phallusia mammillata. We have also examined the link between th is first phase of ooplasmic segregation and the site of gastrulation. The cortical contraction was found to be initiated on the side of the egg where intracellular calcium is first released either by the enteri ng sperm or by photolysis of caged InsP(3). The cortical contraction c arries the sperm nucleus towards the vegetal hemisphere along with a s ubcortical mitochondria-rich domain (the myoplasm), If the sperm enter s close to the animal or vegetal poles the cortical contraction is sym metrical, travelling along the animal-vegetal axis, If the sperm enter s closer to the equator, the contraction is asymmetrical and its direc tion does not coincide with the animal-vegetal axis, The direction of contraction defines an axis along which preexisting (such as the myopl asm) or newly created cytoplasmic domains are relocalized, Two microfi lament-rich surface constrictions, the 'contraction pole' and the 'veg etal button' (which forms 20 minutes later), appear along that axis ap proximately opposite the site where the contraction is initiated, The contraction pole can be situated as much as 55 degrees from the vegeta l pole, and its location predicts the site of gastrulation. It thus ap pears that in ascidian eggs, the organization of the egg before fertil ization defines a 110 degrees cone centered around the vegetal pole in which the future site of gastrulation of the embryo will lie, The cal cium wave and cortical contraction triggered by the entering sperm adj ust the location of cytoplasmic domains along an axis within that perm issive zone. We discuss the relation between that axis and the establi shment of the dorsoventral axis in the ascidian embryo.