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

The single-celled green algae Chlamydomonas reinhardtii with its two flagella is a useful model organism for studying size control. Numerous experiments have shown that length control is coupled between the two flagella, and have identified motor-driven transport of tubulin to the flagellar tips as a key component. Here we consider a model of flagellar length control whose key assumption is that proteins responsible for the intraflagellar transport (IFT) of tubulin are present in limiting amounts. We show that this limiting-pool assumption leads to an inverse relationship between the rate at which a flagellum grows and its length, which has been observed experimentally. By considering models in which tubulin or IFT are shared between the two flagella, we identify coupling mechanisms capable of reproducing the experimentally observed length dynamics and rule out some of the previously proposed models. Finally, we make quantitativepredictions for experiments that could differentiate between candidate models.