Calibration of the maize yg2 assay using gamma radiation and ethylmethanesulfonate

A standard protocol for the yellow-green-2 (yg2) forward mutation assay in Zea mays is proposed. A detailed calibration of the assay using 137Cs gamma rays and ethylmethanesulfonate (EMS) was conducted. Gamma ray-induced mutant sectors in leaves 4 and 5 exhibited one-hit kinetics. The radiation doses ranged from 25 to 500 rads. The mean induced mutation rate per rad of gamma radiation was 4.54 X 10(-6). This value was constant for the primordial cells of leaves 4 or 5. The induction of forward mutation by EMS also exhibited one-hit kinetics in the concentration range 0.25-20 mM (0.33-23.54 mumol EMS/kernel). The mean induced mutation rate per mM EMS was 1.79 X 10(-4), and the mean induced mutation rate per mumol of EMS per kernel was 1.52 X 10(-4). Using the induced mutation rates for gamma radiation and EMS, the rad equivalent was calculated. One rad of gamma radiation is equivalent to the exposure of a 2.53 X 10(-5) M EMS solution or to 2.99 X 10(-8) mol of EMS per kernel.