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LBDV laboratory

Fertilization and Meiosis

Project description:

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).

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). Finally we have described the phosphatases opposing MAPK in the ascidian egg and allowing release from cytostatic arrest and exit from meiosis to be PP2A and calcineurin (Levasseur et al., 2013).

Figure 1 : MAPK in ascidian oocyte (in Dumollard et al., 2011)


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; +other ref?). 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. We found that polar body abscission is regulated by Aurora kinase in the ascidian egg (Hebras and McDougall, 2012).

We now studying the mechanisms responsible for meiotic spindle positioning in the egg and what are the force generators underpinning the unequal cleavage resulting in extrusion of a small polar body.


Figure 2 : Rotation of spindle during the 1st meiosis (two top lines) end during the 2nd meiosis (two bottom lines)


Publications :

- Prodon F, Chenevert J, Sardet C (2006) Establishment of animal–vegetal polarity during maturation in ascidian oocytes Developmental biology 290 (2), 297-311. (View or download pdf)


- Prodon F, Sardet C, Nishida H (2008) Cortical and cytoplasmic flows driven by actin microfilaments polarize the cortical ER-mRNA domain along the a–v axis in ascidian oocytes. Developmental biology 313 (2), 682-699. (View or download pdf)


- Dumollard R, Levasseur M, Hebras C, Huitorel P, Carroll M, Chambon JP, McDougall A. (2011) Mos limits the number of meiotic divisions in urochordate eggs. Development. Mar;138(5):885-95. (View or download pdf)


- Levasseur M, Dumollard R, Chambon JP, Hebras C, Sinclair M, Whitaker M, McDougall A. (2013), Release from meiotic arrest in ascidian eggs requires the activity of two phosphatases but not CaMKII. Development.;140(22):4583-93. (View or download pdf)


- McDougall A, Chenevert J, Lee KW, Hebras C, Dumollard R. (2011) Cell cycle in ascidian eggs and embryos. Results Probl Cell Differ. ;53:153-69. (View or download pdf)


- McDougall, A., Chenevert, J. and Dumollard, R. (2012). Cell Cycle Control in Oocytes and during Embryonic Cleavage Cycles in Ascidians. Int Rev Cell Mol Biol. 297, 237-266. (View or download pdf)


- Hebras, C. and McDougall, A. (2012).  Urochordate ascidians possess a single isoform of Aurora kinase that localizes to the midbody via TPX2 in eggs and cleavage stage embryos.  PLOS One 7 (9), e45431. (View or download pdf)


McDougall, A., Hebras, C., Pruliere, G., Burgess, D., Costache, V., Dumollard, R. and Chenevert, J. (2017). Role of midbody remnant in meiosis II creating tethered polar bodies. bioRxiv 191981. (View or download pdf)


- Chambon, J.P., Touati, S. A.,  Berneau, S.,  Cladière, D., Hebras, C., Groeme, R., McDougall, A. and Wassmann, K (2013).  The PP2A Inhibitor I2PP2A is Essential for Sister Chromatid Segregation in Oocyte Meiosis II.  Current Biology 23 (6), 485. (View or download pdf)


Protocols :

Omero, Imaging data base:





Traductions :







Alex McDougall


Rémi Dumollard

Gerard Pruliere 

Janet Chenevert