Absence of complementation in non-allelic mutants of Dictyostelium discoideum with defects in the transition from growth to development.

In Dictyostelium discoideum, growth and development are mutually exclusive and the transition between the two phases of the life cycle is regulated by the environment. This regulation is disturbed in HBW3, a chemically induced mutant with an unknown molecular defect. The mutant develops rapidly and expresses developmental markers during growth. Here we show that HBW3 fails to complement another mutant which has a similar phenotype: a targeted knock-out of the gdt1 gene. We further show that both mutations are recessive and that both are located on chromosome III, suggesting that the two mutations might be allelic. Molecular analysis, however, demonstrates that the gdt1 gene is not mutated in HBW3. Thus, although a wild-type copy of each gene is present in diploid cell lines, the defects due to the recessive mutations synergize to produce a detectable phenotype. The phenotypic similarities and differences between the two mutants are discussed.