Physically asymmetric division of the C. elegans zygote ensures invariably successful embryogenesis

Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division of C. elegans embryos, which yields a large AB cell and a small P1 cell. We equalized AB and P1 sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization, and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P1 descendants, as well as defects in cell positioning, division orientation and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development of C. elegans.
Methods
3D time-lapse recordings of embryos expressing mCherry∷H2B in either wild-type or lin‑5(ev571) background were first pre-processed to enhance the nuclear signal and remove noise with the Noise2Void/CARE machine learning pipeline (Krull et al., 2019). Thereafter, a custom Fiji macro was used to correct the drift using the first polar body as a bright fiducial marker. The lineage was then traced using a level-set image segmentation and model evolution implemented in MATLAB (MathWorks, USA) as described previously (Dzyubachyk et al., 2010; Krüger et al., 2015), and corrected in the WormDeLux Java-based lineage editor (Jelier et al., 2016). Results were exported in the StarryNite format (Bao and Murray, 2011). Cells were then automatically named in the lineage editor according to the canonical lineage (Sulston et al., 1983), and manually checked afterward for possible errors, especially in mutant embryos that often substantially deviated in division orientation and cell positioning from the wild-type model used for naming. 

As mentioned in the manuscript, the source code used for the analysis of deposited data is available at https://github.com/UPGON/worm-rules-eLife. 

R_00_import_Lineages.R was used to load and align data in time and space.

R_01_Analysis_pipeline.R contains code and links to an additional R files in the repo used for statistical analysis of the data.

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