412 The Journal of Cell Biology | Volume 155, Number 3, 2001 The model of Eq. 1 also makes predictions about the types of mutations that would cause an lf phenotype. As illustrated in Fig. 8 E, lf mutants could arise in either of two ways. One is for the assembly versus length curve to increase, for exam- ple by increasing the number of functional IFT particles. Al- ternatively, the disassembly rate could be reduced. In both cases, the assembly and disassembly rate curves would inter- sect at a longer length. In the latter case, the turnover should be decreased at steady state. This prediction was demon- strated experimentally in the case of one lf mutant, as illus- trated in Fig. 6 C. Because lf2-5 mutants show decreased mi- crotubule turnover leading to a longer flagellum, we suggest that the lf2 gene product may promote microtubule turn- over in the flagellum, although this effect may be indirect. We note that many lf mutants also show defects in the timing and rate of flagellar regeneration (Barsel et al., 1988). Moreover, certain double mutant combinations of lf muta- tions block flagellar assembly completely. These pleiotropic phenotypes suggest the lf genes might have other cellular functions besides flagellar length control. The fact that a mutation in a single gene can cause multiple phenotypes is in no way surprising. For example, many kinases are in- volved in more than one cellular pathway because they rec- ognize more than one substrate. If, as has been suggested, one or more lf genes encode kinases that regulate flagellar length (S. Berman, N. Haas, C. Asleson, and P. Lefebvre. 2000. Ninth International Conference on Cell and Molecu- lar Biology of Chlamydomonas. 161 (Abstr.); Wilson and Lefebvre, 1999), then unless one proposes that such kinases can recognize only a single substrate, we should in fact ex- pect mutations in such genes to have multiple phenotypes. When the lf genes are cloned and sequenced this may shed some light on the pleiotropic phenotypes of these mutations. One interesting feature of flagellar length control is the fact that the flagella within a given cell will always adjust them- selves to attain equal lengths, even if one of the two is severed (Rosenbaum et al., 1969; Coyne and Rosenbaum, 1970; Bar- sel et al., 1988). To see whether our length control model can account for this observation, we explored the behavior pre- dicted by the model of Eq. 1 in cells with flagella of different lengths, by modifying the equation to include the assumption that the assembly rate, in addition to being length dependent, is also proportional to the size of the available precursor pool, as expected if IFT particles are in binding equilibrium with free axonemal subunits in the cytoplasm (see Appendix for de- tails of the mathematical model and the resulting computer simulations). We then explored the predicted behavior for cells whose flagellar length become unequal, either by severing one flagellum or by mating lf mutants to wild-type cells. First, we considered the case of severing one flagellum. When one flagellum on a biflagellate Chlamydomonas cell is severed while the other remains intact, the severed flagellum immediately begins to regenerate. At the same time, the other, initially intact, flagellum starts to shorten, and continues to do so, as the regenerating flagellum grows, until both reach the same length. In most cases, the shortening of the unsevered fla- gellum stops when the two flagella reach the same length, and then both flagella continue to grow out in parallel until they reach the final equilibrium length (Rosenbaum et al., 1969; Coyne and Rosenbaum, 1970). As illustrated in Fig. 8 F, com- puter simulations based on Eq. 1 (see Appendix for details) re- produce the essential features of this experiment. We note that in flagellar severing experiments, in some cases the shortening flagellum continued to shorten somewhat even after it had reached the same length as the regenerating flagellum, and our simple model does not predict this response. However, this type of behavior only occurred occasionally (Coyne and Rosen- baum, 1970). In any event, the most important feature of the flagellar severing experiments, i.e. the fact that the unsevered flagellum shortens in order to equalize length, is clearly ex- plainable by the simple mechanism proposed here (Eq. 1). Another example of flagellar length equalization occurs when recessive lf mutants are mated with wild-type cells. In the resulting quadriflagellate cells, the lf rapidly shorten until they reach the same length as the wild-type flagella (Barsel et al., 1988). However, as shown in Fig. 8 G, the length-regu- lation model of Eq. 1 is able to recapitulate this behavior in computer simulations (see Appendix for details of simula- tion). Thus, the simple model presented here can account for both shf and lf mutants (Fig. 8, D–E), as well as the ability of flagella to equalize their lengths (Fig. 8, F and G). We con- clude that it is not necessary to invoke a length sensor to ac- count for many aspects of flagellar length control, which can instead by explained by a simple equilibrium between length-dependent IFT and outer doublet turnover. One phenomenon of length control that is not directly ad- dressed by our model is the fact that cells can modify the lengths of their flagella under some circumstances. For exam- ple Chlamydomonas, like many flagellated cells, resorb their flagella prior to cell division and then reassemble them after division. Recent work has revealed some evidence for a sig- nal transduction cascade that regulates flagellar length in Chlamydomonas (Wilson and Lefebvre, 1999). The lack of a length sensor in the model presented here is by no means in- consistent with the existence of a pathway to modify flagellar length. Quite to the contrary, the model makes concrete pre- dictions about possible control points for adjusting length. Perhaps the most obvious would be in controlling IFT itself. Signaling pathways converging on the IFT machinery could modulate the number of active IFT particles in the flagellum, thereby adjusting the assembly versus length curve, leading to stable changes in flagellar length. For example kinesin-II, the motor that drives anterograde IFT, may be subject to regulation via specific phosphorylation events (Reese and Haimo, 2000). IFT need not be the only target for a length modification pathway. Any changes to either the assembly or disassembly rate, such as modifications in the intrinsic stabil- ity of the outer doublet microtubules, is expected to result in changes in the steady state length. Thus, the present model provides a conceptual framework, not only for how flagellar length is set, but also for how it can be changed. Materials and methods Strains Wild-type strain 6145c mt selected for high mating efficiency and strong negative phototaxis was provided by Nedra Wilson (University of Minne- sota, St. Paul, MN) The strain 5C12G12 mt expressing HA epitope– tagged -tubulin was previously described (Kozminski et al., 1993a). The total level of -tubulin in this strain is the same as in wild-type cells, and expression of the HA-tubulin has no effect on cell growth rate, cytoskeletal