414 The Journal of Cell Biology | Volume 155, Number 3, 2001 set to 10 m, the normal length of a Chlamydomonas flagel- lum. Therefore, the total pool size was decreased by the equivalent of one flagella worth of protein. The pool was modeled as regenerating at a rate sufficient to regenerate two flagella to half length in 30 min. To obtain the time course of flagellar length after severing (Fig. 8 F), the differential equations for the two flagella were integrated using the Euler method with a step size in time of 0.1 s. Simulation of length equalization following fusion of wild-type and lf mutant cells. To simulate experiments in which recessive lf mutants are fused with wild-type cells to form dikaryons (Fig. 8 G), two flagella were set to an initial length of 20 m and two were set to the wild-type length of 10 m. ,v, N, and D, were set to the same values used for wild-type cells, assuming the recessive lf mutation is com- pletely complemented after cell fusion. Doug Cole and Dennis Diener were instrumental in designing the turnover visualization assay described here, and the authors gratefully acknowledge their contribution. We thank Nedra Wilson, Dennis Diener (Yale Univer- sity, New Haven, CT), and the Chlamydomonas Genetics Center for strains, Doug Cole (University of Idaho, Moscow, ID) and James Deane for antibodies, Jennifer Fung for advice on digital imaging microscopy, and Nedra Wilson, Pete Lefebvre, and Michael Melkonian for helpful discus- sions. We also thank Diane Casey, Dennis Diener, Jennifer Fung, Eun-Jin Hong, Naomi Morrissette, Lotte Pedersen, Ray Stephens, and Hongmin Qin for careful reading of the manuscript. This work was supported by National Institutes of Health grant GM14642 to J.L. Rosenbaum. W.F. Marshall was supported by a Helen Hay Whitney Foundation postdoctoral fellowship and a Leukemia and Lymphoma Society Special Fellowship. 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