Extragenic bypass suppressors of mutations in the essential gene BLD2 promote assembly of basal bodies with abnormal microtubules in Chlamydomonas reinhardtii

bld2-1 mutant Chlamydomonas reinhardtii strains assemble basal bodies with singlet microtubules; bld2-1 cells display flagellar assembly defects as we ll as positioning defects of the mitotic spindle and cleavage furrow. To fu rther understand the role of the BLD2 gene, we have isolated three new bld2 alleles and three partially dominant extragenic suppressors, rgn1-1, rgn1- 2, and rgn1-3. bld2 rgn1-1 strains have phenotypes intermediate between tho se of bld2 and wild-type strains with respect to flagellar number, microtub ule rootlet organization, cleavage furrow positioning, and basal body struc tural phenotypes. Instead of thr triplet microtubules of wild-type cells, b ld2 rgn1-1 basal bodies have mixtures of no, singlet, doublet, and triplet microtubules. The bld2-4 allele was made by insertional mutagenesis and ide ntified in 3 noncomplementation screen in a diploid str ain. The bld2-4 all ele has a lethal phenotype based on mitotic segregation in diploid strains and in haploid strains generated by meiotic recombination. The lethal pheno type in haploid strains is suppressed by rgn1-1; these suppressed strains h ave similar phenotypes to other bld2 rgn1-1 double mutants. It is likely th at BLD2 is an essential gene that is needed for basal body assembly and fun ction.