Date of Award
Doctor of Philosophy (PhD)
Michael R. Schläppi
Gail L. Waring, Rosemary Stuart, Thomas J. Eddinger
Axonemal complexes in flagella are largely prepackaged in the cell body. As such, one mutation often results in the absence of the co-assembled components and permanent motility deficiencies. For example, a Chlamydomonas mutant defective in RSP4 in the radial spoke, which is critical for bend propagation, has paralyzed flagella that also lack the paralogue RSP6 and three additional radial spoke proteins. Intriguingly, recent studies showed that several mutant strains contain a mixed population of swimmers and paralyzed cells despite their identical genetic background. Here we report a cause underlying these variations. Two new mutants lacking RSP6 swim processively and other components appear normally assembled in early log phase indicating that, unlike RSP4, this paralogue is dispensable. However, swimmers cannot maintain the typical helical trajectory and reactivated cell models tend to spin. Interestingly the motile fraction and the spoke head content dwindle during stationary phase. Chemical cross-linking supports a model that RSP4/6 paralogues and the pair of the MORN-motif proteins in the spoke head form a symmetric module. These results suggest that (1) intact radial spokes are critical for maintaining the rhythm of oscillatory beating and thus the helical trajectory; (2) the symmetric module could guide tilt-return cycles of the spoke head against the central apparatus; (3) assembly of the axonemal complex with subtle defects is less efficient and the inefficiency is accentuated in compromised conditions, leading to reversible dyskinesia. Consistently, several organisms only possess one RSP4/6 gene. Gene duplication in Chlamydomonas enhances radial spoke assembly to maintain optimal motility in various environments.