Fertilization and Meiosis

The aim of this project is to have a better understanding of how the cell cycle has been adapted to meet the specific needs of oocytes and of how the very unequal cleavage generating polar bodies is regulated. The ovulated oocyte is arrested at a specific cell cycle stage and waits for the sperm to activate embryonic development. Ascidian eggs are arrested at metaphase of meiosis I and fertilization triggers the completion of two meiotic divisions before fusion of parental genomes and subsequent embryonic cleavages. We have found that, in ascidians like in most other metazoans, the Mos/MAPK pathway enforces cell cycle arrest. Furthermore it is the timely inactivation of the Mos//MAPK pathway that limits the polar body number in the ascidian to two (Dumollard et al., 2011). In addition active Mos/MAPK also promotes a very unequal cleavage by maintaining a small and peripheral meiotic spindle (Dumollard et al., 2011; McDougall et al;, 2011; McDougall et al 2012; Costache et al., 2014).

2PBs-extrusion phallusia (McDougall et al., 2020)

Polar body extrusion occurs 9 minutes after fertilization (PB1) and 25 minutes after fertilization (PB2). A series of steps accompanies the unequal cleavages that produce polar bodies. First the meiotic spindle migrates in an actin-dependant manner close to the cortex defining the animal pole of the egg and causes the overlying cortex to differentiate (Prodon et al., 2006; Prodon et al., 2008). Following egg activation the membrane overlying the meiotic spindle protrudes and the spindle rotates with one pole in the protrusion. The polar body is formed after anaphase by abscission leaving a midbody remnant in the oocyte (McDougall et al 2020). We found that polar body abscission is regulated by Aurora kinase in the ascidian egg (Hebras and McDougall, 2012).

2PBs-extrusion-Mytilus (McDougall et al., 2020)

We are now studying the mechanisms responsible for meiotic spindle positioning in the egg and what are the force generators underpinning the unequal cleavage resulting in the successive extrusion of two small polar bodies at the animal pole of the egg.