Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagonistic pulling and viscous forces

Abstract

Cellular functions like cell division are remarkably conserved across phyla. However the evolutionary principles of cellular organization that drive it are less well explored. Thus, an essential question remains: to what extent cellular parameters evolve without altering the basic function they sustain? Here we have observed 6 different nematode species for which the mitotic spindle is positioned asymmetrically during the first embryonic division. Whereas the C. elegans spindle undergoes oscillations during its displacement, the spindle elongates without oscillations in other species. We asked which evolutionary changes in biophysical parameters could explain differences in spindle motion while maintaining a constant output. Using laser microsurgery of the spindle we revealed that all species are subjected to cortical pulling forces, of varying magnitudes. Using a viscoelastic model to fit the recoil trajectories and with an independent measurement of cytoplasmic viscosity, we extracted the values of cytoplasmic drag, cortical pulling forces and spindle elasticity for all species. We found large variations in cytoplasmic viscosity whereas cortical pulling forces and elasticity were often more constrained. In agreement with previous simulations, we found that increased viscosity correlates with decreased oscillation speeds across species. However, the absence of oscillations despite low viscosity in some species, can only be explained by smaller pulling forces. Consequently, we find that spindle mobility across the species analyzed here is characterized by a tradeoff between cytoplasmic viscosity and pulling forces normalized by the size of the embryo. Our work provides a framework for understanding mechanical constraints on evolutionary diversification of spindle mobility.

Two‐step nuclear centring by competing microtubule‐ and actin‐based mechanisms in 2‐cell mouse embryos

“…For this, we microinjected passive beads and Next, we used the mean diffusion coefficient of the beads estimated from ensemble-averaged MSDs to calculate cytoplasmic viscosity using the Stokes-Einstein equation and found that 2-cell embryos have a cytoplasmic viscosity of 3.6 Pa.S (Fig 5I). This is broadly consistent with the viscosity reported for C. elegans and Drosophila embryos (0.67-1 Pa.S), mouse and human fibroblasts (1-4.5 Pa.S) derived using similar intracellular measurements (Kole et al, 2004;Daniels et al, 2006Daniels et al, , 2010Wessel et al, 2015;Khatri et al, 2022). Notably, we found that the cytoplasmic viscosity of mouse oocytes was 0.5 Pa.S, over 7 times lower than in 2-cell embryos.…”

“…Unlike oocytes which use a non‐specific, diffusion‐based nuclear centring mechanism based on actin vesicles, we found that 2‐cell embryos employ a nucleus‐targeted and actin‐dependent mechanism involving the motor myosin II rather than myosin Vb. Intracellular microrheology measurements suggest that mouse 2‐cell embryos exhibit higher cytoplasmic viscosity than mouse oocytes and other cell types (Kole et al , 2004; Daniels et al , 2006; Wessel et al , 2015; Khatri et al , 2022). Since myosin Vb has been proposed to fluidise the oocyte cytoplasm (Ahmed et al , 2018), it is possible that its inactivity in 2‐cell embryos contributes to a more viscous cytoplasm.…”

Katanin, kinesin-13 and ataxin-2 inhibit premature interaction between maternal and paternal genomes inC. eleganszygotes

“…The simplest hypothesis is that maternal and paternal cytoplasm might not mix during the 45 min from GVBD to pronucleus formation due to the high viscosity of cytoplasm. Attempts at measuring cytoplasmic viscosity of the C. elegans zygote have revealed values from 0.67 – 1.0 Pa s (32–34) which are similar to the viscosity of 100% glycerol. Alternatively, the sperm contents might be held together by a cytoskeleton-like matrix as proposed for the Balbiani body (35).…”

Deep learning based reconstruction of embryonic cell-division cycle from label-free microscopy time-series of evolutionarily diverse nematodes

“…Eventually a cytokinetic furrow is formed, which divides the cytoplasm, giving rise to two asymmetric daughter cells. The cytoplasm fluid mechanical properties have been characterized to result in an effective viscosity ranging between 0.6 to 1 Pa-s based on either microrheology of tracking passive yolk granule movement [6, 9, 10], single particle tracking of injected fluorescent sub-micron particles [11] and spindle displacement and recoil using magnetic tweezers [12]. AAcross-speciesross-species, some closer and some evolutionarily more distant to C. elegans have demonstrated spindle movements in Anaphase vary with very little correlation to phylogenetic distance [13].…”

“…AAcross-speciesross-species, some closer and some evolutionarily more distant to C. elegans have demonstrated spindle movements in Anaphase vary with very little correlation to phylogenetic distance [13]. In contrast the mechanics of the cortical pulling force (F) acting on the spindle, effective cytoplasmic viscosity ( η eff ) and embryo size ( L ) when combined as ω = F/ ( L· η eff ) is predictive of spindle oscillation onset [10]. Comparative studies on spindle dynamics have revealed universal scaling relationships between embryo size and spindle dynamics in C. elegans strains alone [14].…”