Genetic interactions among Chlamydomonas reinhardtii mutations that confer resistance to anti-microtubule herbicides

We previously described two types of genetic interactions among recessive mutations in the APM1 and APM2 loci of Chlamydomonas reinhardtii that may reflect a physical association of the gene products or their involvement in a common structure/process: (1) allele-specific synthetic lethality, and (2) unlinked noncomplementation, or dominant enhancement. To further investigate these interactions, we isolated revertants in which the heat sensitivity caused by the apm2-1 mutation is lost. The heat-insensitive revertants were either fully or partially suppressed for the drug-resistance caused by the apm2-1 allele. In recombination tests the revertants behaved as if the suppressing mutation mapped within the APM2 locus; the partial suppressors of apm2-1 herbicide resistance failed to complement apm2-1, leading to the conclusion that they were likely to be intragenic pseudorevertants. The apm2- 1 partial suppressor mutations reversed apm1^-apm2-1 synthetic lethality in an allele-specific manner with respect both to apm1^- alleles and apm2-1 suppressor mutations. Those apm1^- apm2-1(rev) strains that regained viability also regained heat sensitivity characteristic of the original apm2- 1 mutation, even though the apm2-1 suppressor strains were fully heat- insensitive. The Hs⁺ phenotypes of apm2-1 partial suppressors were also reversed by treatment with the microtubule-stabilizing agent deuterium oxide (D₂O). In addition to the above interactions, we observed interallelic complementation and phenotypic enhancement of temperature conditionality among apm1^- alleles. Evidence of a role for the products of the two genes in microtubule-based processes was obtained from studying flagellar assembly in apm1^- and apm2^- mutants.