Date: 10.11.2023, Time: 15:30h

Location: Kleiner Physikhörsaal N020, Faculty of Physics
The talk will also be streamed online.

Prof. David Lubensky
University of Michigan

A fundamental property of any living system is its size, and organisms have correspondingly evolved a variety of mechanisms to assure that they stop growing when they have reached the correct size. In many cases, these mechanisms are quite accurate. For example, a variety of animal organs appear to set their length or area to around 1%, and many cell types across different phyla have of order 10% variation in their size at division. A long-term goal of my lab is to understand what sets these precisions. Here, I will present some initial results in this direction. First, I will introduce a simple phenomenological framework that allows one to consider tradeoffs between noise in the actual growth process and in a system's own estimate of its size. I will argue that, in many circumstances, minimizing variability in cell or organ size amounts to minimizing the estimator error and that, as a result, both forms of noise generically are important to setting the size variance. I will then describe how the phenomenological model can be related to more mechanistic descriptions of division size specification in micro-organisms, notably E. coli and S. pombe. In particular, I will introduce a stochastic model of division size specification in E. coli based on the assumption that division is initiated when FtsZ protein levels reach a critical threshold. After discussing some of the data analysis challenges that arise in trying to infer growth noise from experiments, I will argue that observed size variability is consistent with measured noise in FtsZ expression. Finally, I will touch on some related work on size coordination between different organs in animals, where it is again useful to make a distinction between the actual size and a variable that is in effect a size estimate. Here, a key conclusion is that it is very difficult to coordinate sizes between organs that are not in physical contact, because of potential errors in how the concentration of any signal between them is normalized.