14/n more generally, if similar principles apply to orientational dynamics in tissues remains to be seen. Comments most welcome. It was an exciting but a very long process, often made smoother and enjoyable by wonderful collaborators.

13/n it makes one think how various processes would be more robust if topological-which change discretely and are not susceptible to fluctuations of shape or size, typical of all living systems. Hopefully this work also brings some clarity to how the fly embryo stays so coordinated for a long time.

12/n The combination of these two ways of looking at this problem allowed us to establish the link between local topological charge and spindle tilt even further.

11/n Using the cell cycle mutants we could change the density of nuclear packing and show that spindle orientation (at a statistical level) does not depend on distance or density--making it scale invariant. Moreover, using the differences in Gaussian curvature we consolidated the link to topology.

10/n which links the torque to topology (num. of neighbors, topological charge=#neighbors -6) via a theorem in integral geometry. This is something we could verify from the data. It was a lucky coincident that I had started learning integral geometry in 2021 for (www.nature.com/articles/s41...)

9/n but how can these alignment effects persist as the asters and spindle push each other to migrate to the cortex. The mystery resolves if you use scaling arguments for total torque with respect to all neighbours. Factors of length scales start dropping off and your left with integral curvature

8/n At this point the only logical thing to look into was possible alignment effects coming out of dynamics of antiparallel microtubules at the aster-aster overlaps. For example the ones known to be present at the SPBs of yeast due to crosslinking. After some and math it seemed like a good idea.

7/n Like many others we thought (mostly M. Vergassola), perhaps nuclear packing and pushing forces between different spindles (later important during cycle 10 onwards) could give rise to some sort of density dependent flip instability. But to @woonyunghur.bsky.social's and my surprise: No.

6/n So what determines which nuclei end up at the cortex of the egg giving rise to the beautiful little fly embryo that we know and love and which ones stay behind to make the yolk nuclei ?

The answer lies in the orientation of nuclear division (the primary driver of motion at this stage).

5/n In many ways this is the first fate decision at the fly embryo. Ziqi Lu looked into it by reanalysing some published data the MZT in the fly.

4/n majority of the fly embryo nuclei must travel from the center of the large embryo to the cortex to give rise to the future embryonic layer but some must stay behind to give rise to the yolk nuclei. @woonyunghur.bsky.social captured this with @npmitchell.bsky.social & @streichan.bsky.social

3/n As a physicist, how in the shared cytoplasm of a syncytial system biochemical identity of nuclei emerge has been always puzzling to me, starting even during my PhD @mpicbg.bsky.social (on C. elegans germline). Generically true for all germlines, slime mold, ashbya etc. Also for fly embryo

2/n It was very exciting theory-driven analysis and cutting age imaging experiment ( @woonyunghur.bsky.social during his time in the lab of @ditalialab.bsky.social ) collaboration across the atlantic between @qbio-ens.bsky.social (M.Vergassola)and @dukepress.bsky.social