Self-Organization of Multicellular Systems

Giulia Serafini, Maryam Setoudeh*, Marina B. Cuenca*, Charlène Brillard, Matthias Arzt, Pavel Mejstřik, Pierre A. Haas^#, and Pavel Tomančák^#, bioRxiv (2026)

Morphogenetic processes during animal development are remarkably invariant. This stability is established by the interaction between genetic determination of developmental progression and the constraints imposed by the surrounding embryonic environment. We discover that the germ band extension process in Drosophila is rather variable: instead of extending straight towards the head, the germ band tends to twist to the side. Through a combination of experiments and theory, we demonstrate that Scab integrin-mediated attachment to the vitelline envelope stabilizes the germ band and supports its straight extension. Our quantification of germ band extension dynamics also reveals a consistent handedness to the twist of the germ band. We show that this left-right asymmetry can be altered by manipulating the expression of Myo1D, the molecular determinant of chirality in Drosophila. Our data thus suggest that Myo1D expression causes the early gastrulating blastoderm epithelium to already exhibit inherent chirality and that the resulting destabilization of germ band extension is suppressed by Scab-mediated friction between the blastoderm and the vitelline envelope.

Valens Tribet and Pierre A. Haas^#, arXiv (2026)

The processes of morphogenesis that give rise to the shapes of organs and organisms during development are often driven by mechanical instabilities. Can such mechanical bifurcations also drive or constrain the evolution of these processes in the first place? We discover an instance of these constraints in the green algae of the family Volvocaceae. During their development, their bowl-shaped embryonic cell sheet turns itself inside out. This inversion is driven by a simple wave of cell wedging in the genus Pleodorina (16-128 cells) and more complex programmes of cell shape changes in Volvox (~400-50000 cells). However, no species with intermediate cell numbers (256 cells) have been described. Here, we relate this gap to a mechanical bifurcation: Focusing on the inversion of Pleodorina californica (64 cells), we develop a continuum model, in which the cell shape changes driving inversion appear as changes of the intrinsic curvature of an elastic surface. A mechanical bifurcation in this model predicts that inversion is only possible in a subset of its parameter space. Strikingly, parameters estimated for P. californica fall into this possible subset, but those that we extrapolate to 256 or more cells using allometric observations and a model of cell cleavage in Volvocaceae do not. Our work thus suggests that the more complex inversion strategies of Volvox are an evolutionary necessity to obviate this bifurcation and indicates more broadly how mechanical bifurcations can drive the evolution of morphogenesis.