Possible origins of macroscopic left-right asymmetry in organisms

TL;DR

This paper explores how microscopic chiral structures like cytoskeletal filaments can generate macroscopic left-right asymmetry in organisms, emphasizing mechanisms involving shear, rotation, and collective filament behavior.

Contribution

It proposes specific scenarios involving actin/myosin and microtubule layers that could explain the emergence of organism-level asymmetry from cellular chirality.

Findings

Chiral cytoskeletal filaments can induce macroscopic asymmetry.

Shear and rotation in two-dimensional cellular layers facilitate handedness emergence.

Examples include snail and C. elegans asymmetry, plant cell microtubules, and neuronal handedness.

Abstract

I consider the microscopic mechanisms by which a particular left-right (L/R) asymmetry is generated at the organism level from the microscopic handedness of cytoskeletal molecules. In light of a fundamental symmetry principle, the typical pattern-formation mechanisms of diffusion plus regulation cannot implement the "right-hand rule"; at the microscopic level, the cell's cytoskeleton of chiral filaments seems always to be involved, usually in collective states driven by polymerization forces or molecular motors. It seems particularly easy for handedness to emerge in a shear or rotation in the background of an effectively two-dimensional system, such as the cell membrane or a layer of cells, as this requires no pre-existing axis apart from the layer normal. I detail a scenario involving actin/myosin layers in snails and in C. elegans, and also one about the microtubule layer in plant cells. I also survey the other examples that I am aware of, such as the emergence of handedness such as the emergence of handedness in neurons, in eukaryote cell motility, and in non-flagellated bacteria.