A series of conditional mutants of the algal, biflagellate Chlamydomonas reinhardtii with temperature-sensitive defects in flagellar assembly and function were isolated. The genetics and phenotypes of 21 mutants displaying a rapid alteration in flagellar function upon shift from the permissive (20 degrees C) to the restrictive (32 degrees C) temperatures are described. These mutants designated as "drop-down" or dd-mutants have been placed in four categories on the basis of their defective phenotypes: (a) dd-assembly mutants - the preformed flagella are resorbed at 32 degrees C and reassembly of flagella is inhibited; (b) dd-fragile flagella mutants - the flagella are lost by detachment at 32 degrees C, but can be reassembled; (c) dd-motility mutants - the flagella are retained at 32 degrees C, but are functionally defective; (d) dd-lethal mutants - display combined defects in flagellar function and cell growth. Tetrad analysis of the mutants back-crossed to wild-type, recombination analysis of intermutant crosses, and complementation tests in the construction of heterozygous diploid strains indicate that at least 14 nuclear genetic loci are represented among 21 mutants. The availability of temperature-sensitive mutations affecting the assembly and function of the flagellum suggests that the morphogenesis of this complex eukaryotic organelle is amenable to genetic dissection.

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