Abstract
The frequency of reversions, of the methionine-requiring mutant strain (4-24) of Ustilago maydis occurred spontaneously as well as by the X-ray irradiation and tris-nitrogen mustard treatment was studied and the nature of the reversions was discussed.
1. Spontaneous reversion takes place around at the rate of 3.0×10-7 per cell per division, when cultured in the complete liquid medium.
The rate of reversional mutation was increased by both X-ray irradiation and nitrogen mustard treatment up to the orders of 10-4. The mutation constant is calculated to be 2.2×10-7 per roentgen.
2. Three types of reversions are discerned:
The pseudo reversion type (PR) appears as a result of residual growth and syntrophism. The complete reversion type (CR) and the inferior reversion type (IR) appear as a result of one gene mutation. The IR-type is distinguished from the CR-type by the restricted growth during early subcultures.
