Living Systems often seem to follow, in addition to external constraints and interactions, an intrinsic predictive model of the world — a defining trait of Anticipatory Systems. Here we study rhythmic behaviour in Caulerpa, a marine green alga, which appears to predict the day/night light cycle. Caulerpa consists of differentiated organs resembling leaves, stems and roots. While an individual can exceed a meter in size, it is a single multinucleated giant cell. Active transport has been hypothesized to play a key role in organismal development. It has been an open question in the literature whether rhythmic transport phenomena in this organism are of autonomous circadian nature. Using Raspberry-Pi cameras, we track over weeks the morphogenesis of tens of samples concurrently, while tracing at resolution of tens of seconds the variation of the green coverage. The latter reveals waves propagating over centimeters within few hours, and is attributed to chloroplast redistribution at whole-organism scale. Our observations of algal segments regenerating under 12-hour light/dark cycles indicate that the initiation of the waves precedes the external light change. Using time-frequency analysis, we find that the temporal spectrum of these green pulses contains a circadian period. The latter persists over days even under constant illumination, indicative of its autonomous nature. We further explore the system under non-circadian periods, to reveal how the spectral content changes in response. Time-keeping and synchronization are recurring themes in biological research at various levels of description — from subcellular components to ecological systems. We present a seemingly primitive living system that exhibits apparent anticipatory behaviour. This research offers quantitative constraints for theoretical frameworks of such systems.

Laura Fenlon (Australia): Time shapes all brains: timing of a conserved transcriptional network underlies divergent cortical connectivity routes in mammalian brain development and evolution; Laurent Nguyen (Belgium): Regulation of cerebral cortex morphogenesis by migrating cells; Carol Ann Mason (USA): Wiring the eye to brain for binocular vision: lessons from the albino visual system. Thomas Perlmann (Sweden): Interrogating dopamine neuron development at the single cell level

Embryo morphogenesis is impacted by dynamic changes in tissue material properties, which have been proposed to occur via processes akin phase transitions (PTs). Here, we show that rigidity percolation provides a simple and robust theoretical framework to predict material/structural PTs of embryonic tissues from local cell connectivity. By using percolation theory, combined with directly monitoring dynamic changes in tissue rheology and cell contact mechanics, we demonstrate that the zebrafish blastoderm undergoes a genuine rigidity PT, brought about by a small reduction in adhesion-dependent cell connectivity below a critical value. We quantitatively predict and experimentally verify hallmarks of PTs, including power-law exponents and associated discontinuities of macroscopic observables at criticality. Finally, we show that this uniform PT depends on blastoderm cells undergoing meta-synchronous divisions causing random and, consequently, uniform changes in cell connectivity. Collectively, our theoretical and experimental findings reveal the structural basis of material PTs in an organismal context.

Over the past five years, our understanding of how mechanical processes act across multiple scales to direct morphogenesis has advanced significantly. Yet, there remain numerous open questions, including the role of mechanics in tissue shaping, cancer dissemination, and cellular aging. The From Molecules to Organs:The Mechanobiology of Morphogenesis conference will bring together world leaders in the fields of mechanobiology and morphogenesis. The three-day conference will span scales, from single molecules up to whole organisms.

Over the past five years, our understanding of how mechanical processes act across multiple scales to direct morphogenesis has advanced significantly. Yet, there remain numerous open questions, including the role of mechanics in tissue shaping, cancer dissemination, and cellular aging. The From Molecules to Organs:The Mechanobiology of Morphogenesis conference will bring together world leaders in the fields of mechanobiology and morphogenesis. The three-day conference will span scales, from single molecules up to whole organisms.

My lab studies the design principles of cytoskeletal materials the drive cellular morphogenesis, with a focus on contractile machinery in adherent cells. In addition to force generation, a key feature of these materials are distributed force sensors which allow for rapid assembly, adaptation, repair and disintegration. Here I will discuss our recent identification of 18 proteins from the zyxin, paxillin, Tes and Enigma families with mechanosensitive LIM (Lin11, Isl- 1 & Mec-3) domains. We developed a screen to assess the force-dependent localization of LIM domain-containing region (LCR) from ~30 genes to the actin cytoskeleton and identified features common to their force-sensitive localization. Through in vitro reconstitution, we found that the LCR binds directly to mechanically stressed actin filaments. Moreover, the LCR from the fission yeast protein paxillin-like 1 is also mechanosensitive, suggesting force-sensitivity is highly conserved. We speculate that the evolutionary emergence of contractile F-actin machinery coincided with, or required, proteins that could report on the stresses present there to maintain homeostasis of actively stressed networks.

My lab studies the design principles of cytoskeletal materials the drive cellular morphogenesis, with a focus on contractile machinery in adherent cells. In addition to force generation, a key feature of these materials are distributed force sensors which allow for rapid assembly, adaptation, repair and disintegration. Here I will describe how optogenetic control of RhoA GTPase is a powerful and versatile force spectroscopy approach of cytoskeletal assemblies and its recent use to probe repair response in actomyosin stress fibers. I will also describe our recent identification of 18 proteins from the zyxin, paxillin, Tes and Enigma families with mechanosensitive LIM (Lin11, Isl- 1 & Mec-3) domains that bind exclusively to mechanically stressed actin filaments. Our results suggest that the evolutionary emergence of contractile F-actin machinery coincided with, or required, proteins that could report on the stresses present there to maintain homeostasis of actively stressed networks.

In this talk, I will describe my lab’s recent efforts to understand the design principles of the active, soft materials that drive cell morphogenesis. In particular, we are interested in how collections of myosin II motors and actin polymers generate, relax, sense and adapt to mechanical force. I will discuss how motor-filament interactions lead to either distributed extensile or contractile stresses as the mechanics of the system changes from fluid to solid. Using optical control of motors, we are now exploring how spatially structured stress can be used to drive local flows and motion. If time, I will also describe how feedbacks between local geometry and activity can be harnessed to drive morphogenetic changes in model systems.