In previous reports (MGCNL 45:81-87, 1971, 49:73-79, 1975) we have described the effects of a mutator system (Mu) in corn. New mutants were observed in self-pollinated progeny of plants derived from the outcross of plants carrying Mu. Outcrosses of Mu plants showed a mutation rate from 15-30 times higher than that observed in non-Mu lines. In making these outcross tests, the Mu parents were always self-pollinated and scored for the presence of any mutations that might complicate the outcross results. Outcrosses were routinely discarded if the Mu selfs segregated for mutants.
Since selfs and outcrosses of the Mu stocks were made at the same time with a given pollen sample, the same population of mutants might be observed in selfs of the selfed progeny as in the selfs of the outcross progeny. This would only be the case if mitotic (premeiotic) mutations were occurring that might give rise to tassel sectors heterozygous for the mutation. Thus, the appearance of similar mutants in these two progenies would suggest that Mu might be acting during mitosis. This occurrence is not conclusive proof, since we have shown (MGCNL 50: 68-70, 1976) that mutants in a given outcross family with similar phenotypes are not necessarily allelic. Some instances of allelism have been observed, however, suggesting that mitotic mutations can occur.
A 50-seed sample from the selfed ears of Mu lines that had been tested in outcrosses was sown. The resulting plants were self-pollinated and the self progeny were scored for seedling mutants. The array of mutants found in the progeny of the selfed Mu plants was compared with that observed in the outcross progeny of the same Mu plants. The outcross progeny and the self progeny were grown in different years. The person scoring the self progeny was not aware of the mutants that had been found in the outcross progeny, which had been tested in a previous year.
In Table 1, the mutation rates and mutant phenotypes in outcross and self progeny of Mu plants are compared. Fifty-seed samples of both outcross and self progeny were planted. The self progeny had consistently fewer ears than the outcrosses due to poorer germination and weaker plants that failed to produce ears as a result of inbreeding depression and/or weak mature plant mutants (e.g., dwarfs, runts, male steriles, etc.). The total mutation rate of self progeny is slightly higher (but not significantly) than that of the outcross progeny. A priori, the mutation rate of the self progeny would be expected to be twice that of the outcrosses since mutation would be expected in both male and female gametes of selfed Mu plants. The results suggest that Mu induced changes may not occur in female tissue. Tests specifically for female transmission of mutations were made this past summer but the results have not been analyzed as yet.
In all but one stock (1040-1), mutants with similar phenotypes were found in both outcrosses and self progeny. As pointed out above, such results would be expected if Mu is inducing mitotic mutations. Positive allele tests of similar outcross and self-mutants will be needed before definite conclusions can be made in this regard.
Donald S. Robertson
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