Spontaneous Mutation at the R Locus in Maize. II. Race Differences in Mutation Rate

The frequency of seed-color mutation differs widely in different races. The highest rate found was 18.2 x 10-4 in an inbred race designated "Cornell stock." Other stocks gave lower rates, including several with no mutations in populations of 10,000 to 20,000 tested gametes. 2. Plant-color mutation rates, determined on a smaller scale, also vary rather widely. There appears to be no correlation between frequency of plant-color mutation and frequency of seed-color mutation in the same alleles. 3. Differences in seed-color mutation frequency are due in part to intrinsic differences in mutability of the various R alleles. R alleles which yield seed-color mutants distinguishable by their plant-color effects may be compared in mutation rate under strictly comparable conditions in heterozygotes. In these trials significant differences in mutation rate for the two alleles compared are found in some cases. 4. Seed-color mutations in such heterozygotes marked also at the g locus (proximal to R at about 14 cross-over units) are not associated with crossing over in the R-g segment. Among the 14 rr and rg mutants tested, only two showed crossing over in this segment. 5. Differences in mutation rate between different races are also due in large part to the effects of modifying factors. In F1 hybrids produced by crossing the Cornell stock with various other stocks, the Rr: Cornell gametes of the F1 commonly yield mutations at a rate much lower than that found in the Cornell stock. 6. Homozygous Rr: Cornell derivatives of lowered mutation frequency are readily extracted from crosses of the Cornell stock with strains carrying distinguishable R alleles. Among extracted stocks of various Rr alleles, Rr: Cornell stocks are not outstanding in mutation frequency. 7. The varying modifier genotypes occurring in different homozygous Rr: Cornell extractions also sharply modify plant-color mutation rate, apparently independently of their effect upon seed-color mutation rate.