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Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagonistic pulling and viscous forces.
Mol Biol Cell:mbcE21100532.
Cellular functions like cell division are remarkably conserved across phyla.However the evolutionary principles of cellular organization that drive it areless well explored. Thus, an essential question remains: to what extent cellularparameters evolve without altering the basic function they sustain? Here we haveobserved 6 different nematode species for which the mitotic spindle is positionedasymmetrically during the first embryonic division. Whereas the C. elegansspindle undergoes oscillations during its displacement, the spindle elongateswithout oscillations in other species. We asked which evolutionary changes inbiophysical parameters could explain differences in spindle motion whilemaintaining a constant output. Using laser microsurgery of the spindle werevealed that all species are subjected to cortical pulling forces, of varyingmagnitudes. Using a viscoelastic model to fit the recoil trajectories and with anindependent measurement of cytoplasmic viscosity, we extracted the values ofcytoplasmic drag, cortical pulling forces and spindle elasticity for all species.We found large variations in cytoplasmic viscosity whereas cortical pullingforces and elasticity were often more constrained. In agreement with previoussimulations, we found that increased viscosity correlates with decreasedoscillation speeds across species. However, the absence of oscillations despitelow viscosity in some species, can only be explained by smaller pulling forces.Consequently, we find that spindle mobility across the species analyzed here ischaracterized by a tradeoff between cytoplasmic viscosity and pulling forcesnormalized by the size of the embryo. Our work provides a framework forunderstanding mechanical constraints on evolutionary diversification of spindlemobility.
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