Length regulation of multiple 1 flagella that self-assemble from a 2 shared pool of components 3

The single-celled green algae Chlamydomonas reinhardtii with its two 10 flagella—microtubule-based structures of equal and constant lengths—is the canonical model 11 organism for studying size control of organelles. Experiments have identified motor-driven 12 transport of tubulin to the flagella tips as a key component of their length control. Here we 13 consider a class of models whose key assumption is that proteins responsible for the intraflagellar 14 transport (IFT) of tubulin are present in limiting amounts. We show that the limiting-pool 15 assumption is insufficient to describe the results of severing experiments, in which a flagellum is 16 regenerated after it has been severed. Next, we consider an extension of the limiting-pool model 17 that incorporates proteins that depolymerize microtubules. We show that this “active disassembly” 18 model of flagellar length control explains in quantitative detail the results of severing experiments 19 and use it to make predictions that can be tested in experiments. 20