Background: Cilia and flagella are often lost in anticipation of mitosis or in response to stress.There are two ways that a cell can lose its flagella: resorption or deflagellation. Deflagellationinvolves active severing of the axoneme at the base of the flagellum; this process is defective inChlamydomonas fa mutants. In contrast, resorption has been thought to occur as a consequence ofconstitutive disassembly at the tip in the absence of continued assembly, which requiresintraflagellar transport (IFT). Chlamydomonas fla mutants are unable to build and maintain flagelladue to defects in IFT.Results: fla10 cells, which are defective in kinesin-II, the anterograde IFT motor, resorb theirflagella at the restrictive temperature (33°C), as previously reported. We find that in standardmedia containing ~300 microM calcium, fla10 cells lose flagella by deflagellation at 33°C. Thistemperature-induced deflagellation of a fla mutant is not predicted by the IFT-based model forflagellar length control. Other fla mutants behave similarly, losing their flagella by deflagellationinstead of resorption, if adequate calcium is available. These data suggest a new model wherebyflagellar resorption involves active disassembly at the base of the flagellum via a mechanism withcomponents in common with the severing machinery of deflagellation. As predicted by this model,we discovered that deflagellation stimuli induce resorption if deflagellation is blocked either bymutation in a FA gene or by lack of calcium. Further support for this model comes from ourdiscovery that fla10-fa double mutants resorb their flagella more slowly than fla10 mutants.Conclusions: Deflagellation of the fla10 mutant at the restrictive temperature is indicative of anactive disassembly signal, which can manifest as either resorption or deflagellation. We proposethat when IFT is halted by either an inactivating mutation or a cellular signal, active flagellardisassembly is initiated. This active disassembly is distinct from the constitutive disassembly whichplays a role in flagellar length control
