Early analyses of Mu stocks had suggested that mutability was associated with the y9 locus (MGCNL 45:81-87, 1971). However, later tests (MGCNL 49:73-79, 1975) demonstrated that non-y9 plants could also transmit mutability. The non-y9 plants were siblings to heterozygous y9 plants in the mutator stocks. In such stocks, all plants, y9/+ or +/+, transmitted mutability. There are several explanations for the deviation from the Mendelian pattern of transmission expected if the y9 locus (or something closely linked to it) was responsible for the observed mutability. One of the possibilities is that a paramutagenic-like change had occurred in the heterozygous y9 plant of the previous generation that were used to produce the tested generation. If in the previous generation, Mu on the y9-bearing homologue had induced a change on the non-y9 homologue converting mu to Mu, both y9 and non-y9 bearing gametes would carry an Mu and all the plants of the tested family would exhibit Mu activity. This suggested change of mu to Mu could be a permanent change or a transitory one. In the latter instance, the induced Mu may weaken in its mutator ability with time resulting in lower mutation rates or even a complete loss of the ability to induce mutations in later generations.
To test for the persistence of the Mu activity, plants from the outcross non-y9 mutator plants were again outcrossed to test for mutability in the next generation (second generation). A comparison of first and second generation mutability is given in Table 1. In most instances second generation non-y9 stocks show a somewhat increased mutation rate over the first generation stocks. The exception to this was in the second generation lines derived from the 3123 first generation stock. Three of these second generation lines had no mutants. These three lines as well as some others that showed a substantially lowered mutation rate may possess a weakened induced Mu. However, these instances with zero or low mutation rates may just be due to sampling error. Instances have occurred of Mu outcrosses that have one year given zero mutation rates but on retesting a second year revealed an elevated rate similar to other stocks. Since families are small (50 plants or less) and mutants, even in families with high rates, are few, it is not surprising that as a result of sampling error an occasional Mu family occurs in which no mutations are found (see following report).
In summary, there is no consistent diminution of the mutation rate in non-y9 lines in one generation. There is a suggestion that sporadic reduction may occur in some stocks but these need to be confirmed by further testing.
Donald S. Robertson
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