SUMMARY: Mutants of resistant to the antifungal antibiotics, Naramycin or Trichomycin, were isolated after treatment with nitrosoguanidine. When two strains differing in pigmentation and drug resistance were cultured together, amoebae resistant to both drugs appeared in underwater cultures and in liquid shake cultures. Such heterozygotes did not appear during the growth period, but only after cultures reached stationary phase. The appearance of the heterozygotes seems to have some relationship to the state of cells. Studies on the ploidy and the resistance of the progeny of these heterozygous isolates revealed that: (1) diplophase of the original heterozygote is probably transient, (2) all the progeny observed are mononucleate, (3) the ploidy of each progeny settles in a stable state of aneuploidy (between the haploid and the diploid), (4) genetic segregants for drug resistance and pigmentation appear among the progeny in serial subcultures, (5) preferential chromosome elimination occurs during the process of chromosomal reduction. A mechanism of recombination in is discussed in relation to the parasexual cycle.


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