Possible mechanisms for initiating macroscopic left-right asymmetry in animals and plants

Chris Henley, Cornell

How might systematic chiral or left-right (L/R) asymmetry of the body plan originate in multicellular animals and plants? Fundamental principles (of symmetry and statistical mechanics) indicate that usual biological mechanisms -- diffusion and gene regulation -- do not suffice to implement the ``right-hand rule'' for (e.g.) the third body axis in a developing embryo. This requires that, somehow, the microscopic handedness of biological molecules must be brought up to macroscopic scales. Empirically, a semi-macroscopic assembly of molecules is always required on the cellular level: namely, the long stiff polymers which form the ``cytoskeleton''; furthermore forces by molecular motors (i.e. physics!) must come into play. There are many independent examples in varied organisms (human brain, C. elegans, molluscs, Arabidopsis roots, fruit flies, vertebrate internal organs...) Only in the last-mentioned case is there an accepted answer.

In this speculative talk, I will classify the possible scenarios, and survey some of the examples from a physics viewpoint. In two of the cases, the experimental literature suggests a dynamic cortical (membrane-associated) array of actin fibers or of microtubules is involved.