Session J68: Theory Meets High-Precision Biology: Emergent Simplicity in Stochastic Organismal Dynamics

Self-driven phase transitions in living matter

The soil dwelling bacterium Myxococcus xanthus is an amazing organism that uses collective motility to hunt in giant packs when near prey and to form beautiful and protective macroscopic structures comprising millions of cells when food is scarce. I will present an overview of how these cells move and how they regulate that motion to produce different phases of collective behavior. Inspired by recent work on active matter and the physics liquid crystals, I will discuss experiments that reveal how these cells generate nematic order, how defect structure can dictate global behavior, and how Myxo actively tune the Péclet number of the population to drive a phase transition from a gas-like flocking state to an aggregated liquid-droplet state during starvation.   

Stochastic intergenerational cell size homeostasis: Precision measurements, exact kinematics and emergent simplicities

In this talk I will first establish that intergenerational bacterial cell size homeostasis is maintained under appropriate growth conditions, using our high-precision data on growth and division of individual C. Crescentus cells. Next, the emergent simplicities revealed by these data. To conclude, a theoretical framework consistent with observed scaling laws, sans fitting parameters, which captures the exact kinematics of stochastic intergenerational cell size homeostasis.   

Jamming and dynamic arrest in sheltered microbial communities

Microbes often colonize spatially-constrained habitats, such as pores in the skin or crypts in the colon. The resulting micro-communities are often stable and contribute to the genetic diversity and function of our microbiomes. It is, however, unclear how spatial constraints influence microbial community assembly. By monitoring and modeling microbial populations under controlled microfluidic confinement, we find a rich spectrum of dynamical patterns that are controlled by the competition between density-dependent outflow and population growth. Our results show that density-dependent passive diffusion can drive a reproducing population to a jamming threshold. The resultig loss of mixing and intra-species competition controls the resilience and evolution of these sheltered communities.   

How cell growth triggers cell division

Cell size is fundamental to function in different cell types across the human body because it sets the scale of organelle structures, biosynthesis, and surface transport. Tiny lymphocytes squeeze through tight spaces to reach sites of infection, while the four orders of magnitude larger oocyte divides without growth to form the ~100 cell pre-implantation embryo. Despite the vast size range across cell types, cells of a given type are typically uniform in size because cells accurately couple cell growth to division. While some genes affecting cell size have been identified, the molecular mechanisms through which cell growth drives cell division had remained elusive. While it was expected that growth would act to increase the activities of the cyclin-dependent kinases (Cdk) known to promote cell division, this is not the case. Rather, we found that cell growth acts in the opposite manner. Cell growth triggers division by diluting proteins that inhibit cell division, Whi5 in yeast, and the retinoblastoma tumor suppressor Rb in human cells. Thus, inhibitor dilution provides one long sought mechanism coupling cell growth to cell division.