Somatic mutational transients: response to acute gamma irradiation by two genes in a clone of Nicotiana.

ENETIC analysis implying that mutable alleles at the gene locus V account Gfor flower color variegation in a Nicotiana culture has been presented by SMITH and SAND (1957). These plants also contain stable genes for flower color and have been found useful for the study of interaction between environmental factors and genetic systems of varying stability (SAND 1957, 1962). Other work with this material has suggested that changing metabolic environment influences the mutation rates and expression of both the stable and the mutable genes (SAND 1961). Such interactions, between tissue systems and genetic systems, appear an essential part of the hierarchic set of interactions important for the integrated development of higher organisms (SAND 1965). In exploring differences and similarities between mutable and stable gene systems we exposed our plant material for extended periods to chronic gamma irradiation (SAND, SPARROW, and SMITH 1960). These studies revealed both somatic and germinal effects on a mutable allele of the V gene and on the stable R gene in a clone of Nicotiana. It was shown that for low dose rates the somatic response of the V-system is tenfold greater than that of the R-system, but that the germinal response detected in the progeny is similar for the two systems. The shape of the somatic response curve for the mutable locus is of basic interest, giving greater mutational increments in the low irradiation range than in the high range. However, in these experiments with chronic irradiation the lower total doses were administered at lower intensities. We have now extended the experiments to separate possible intensity effects from effects due to total dose. Data are presented below from acute irradiation treatments administered at one intensity. The transient somatic responses, observed after acute treatments of these growing plants, have permitted refinement of our cell-generation model. In addition, the dose-response curve continues to show hypersensitivity in the low irradiation range: an effect due to total dose is clear. Thus, for this sensitive V gene system the detected mutation rate per roentgen is greater in the 0-60