Asymmetric Cell Division in the One-Cell C. elegans Embryo: Multiple Steps to Generate Cell Size Asymmetry

Abstract

The first division of the one-cell C. elegans embryo has been a fundamental model in deciphering the mechanisms underlying asymmetric cell division. Polarization of the one-cell zygote is induced by a signal from the sperm centrosome and results in the asymmetric distribution of PAR proteins. Multiple mechanisms then maintain PAR polarity until the end of the first division. Once asymmetrically localized, PAR proteins control several essential aspects of asymmetric division, including the position of the mitotic spindle along the polarity axis. Coordination of the spindle and cytokinetic furrow positions is the next essential step to ensure proper asymmetric division. In this chapter, I review the different mechanisms underlying these successive steps of asymmetric division. Work from the last 30 years has revealed the existence of multiple and redundant regulatory pathways which ensure division robustness. Besides the essential role of PAR proteins, this work also emphasizes the importance of both microtubules and actomyosin throughout the different steps of asymmetric division.

The last-born daughter cell contributes to division orientation of Drosophila larval neuroblasts

“…In the C. elegans zygote, the sperm entry point acts as a cue inducing an actomyosin flow 16 establishing Par complex polarity at the opposite end of the cell (see ref. 55 for a review). Septins 56 , Cindr, Roughest 39 and Flare 37 can be linked in one way or another to the regulation of actomyosin, and at least the maintenance of Baz localization in mitotic NBs is also actin-dependent 57 .…”

The Caenorhabditis elegans gene ham-1 regulates daughter cell size asymmetry primarily in divisions that produce a small anterior daughter cell

“…elegans and Drosophila melanogaster , have led to insights into the molecules and mechanisms that distribute developmental potential to the two daughter cells in an ACD. Many of these molecules control not only the distribution of fate but also the orientation of the mitotic spindle, a process that is critical for the appropriate distribution of fate unequally to daughter cells [ 3 – 5 ].…”

Iterative and Complex Asymmetric Divisions Control Cell Volume Differences in Ciona Notochord Tapering

“…We find that the cell volume differences along the anterior-posterior axis of the Ciona notochord can be fully explained by relatively subtle patterns of asymmetric division that act iteratively over multiple cell cycles to have a cumulatively important effect on organ shape. These subtle and iterative unequal cleavages that occur without obvious segregation of cell fate appear quite different than the canonical models of asymmetric division, such as Drosophila neuroblasts and C. elegans first cleavage [8,9].…”

“…Asymmetric divisions are common in developing embryos, but the canonical models, such as C. elegans first cleavage and fly neuroblasts, all involve highly polarized cells that divide to give daughters of considerably different size and completely different fate [8,9,23,[26][27][28][29]. The asymmetric divisions in the Ciona notochord, however, are relatively subtle and do not involve any obvious segregation of cell identity.…”