Double mutants of Phycomyces with abnormal phototropism

Abstract

Seven genes (madA to madG) are known which effect phototropism in Phycomyces. These genes have been partially ordered with respect to the associated stimulus-response pathway. Mutants affected in these genes serve as useful probes of photosensory transduction processes in this model system. To extend and deepen the analysis of the system, we have constructed a family of 21 double mutants in all combinations for the seven mad genes. A set of seven standard alleles was adopted for this work. The double mutants were isolated from crosses between isogenic single-mutant strains of opposite mating type. After a partial physiologic screening of the progeny, the double mutants were identified by complementation tests using single-mutant strains of known genotype. For all but three of the double mutants, the photogeotropism phenotypes were distinct from those of the respective single-mutant parentals. One triple mutant (madA madB madC) was constructed as part of this work. Various applications of the double mutants and the triple mutant are discussed. Recombination analyses were performed on the progeny from seven mad crosses to complete an earlier study. The results establish that all seven mad genes are unlinked.