Mechanical and Evolutionary Constraints on Cell Shape

Cells come in a dizzying array of shapes. Often it is clear how shape is related to function, but most of the time it is unclear. I will present an comparative analysis of the morphology of a widespread polyphyletic group of cells, namely tip-growing cells. Using a combination of mechanics theory and experimental cell biology, I will explain that because of their convergent mechanical mechanism of growth, tip-growing cells can only assume a highly limited space of shapes. That is, in this system, shape is "developmentally constrained." However, even within this space of possible shapes, we discovered that cells are excluded from a large range of shapes. We found that the boundary of natural shape space corresponds to an instability in the mechanism of cell growth, and that this instability separates fast-growing shapes from unobserved slow-growing shapes. In line with our theory, we were able to induce cells to assume these slow-growing shapes using pharmacological perturbations to cell growth. These findings indicate that the mechanism of tip growth leads directly to a fitness landscape governed by a mechanical instability. I will argue that this is a paradigmatic example of how natural selection acts on the mechanism of cell morphogenesis to determine the shapes of a vast group of cells.