In MGCNL 45:81-87, 1971, we indicated that most outcrosses of lines segregating for heterozygous y9 plants displayed mutability. However, we do have some long standing y9 stocks which seem to have lost the ability to induce mutation (MGCNL 49:73-79, 1975). Since the mutability in the mutator (Mu) y9 lines can be traced back to the original y9 material I received from Dr. Kermicle, and since the nonmutating y9 stocks came from the same source, it is obvious that y9 and mutability can be separated. But how and under what conditions is not known.
Occasionally in our mutator tests an outcross family (test family) will be found that gives no mutations. Do these represent instances where mutability has been lost? They could be. However, the test families are small (50 or less plants) and even at the most the numbers of mutants are few. Therefore, the principles of probability dictate that occasional test families with no mutations will be found even in Mu lines. Additional plants from four test families of Mu lines that gave zero mutation were grown. The results are summarized in Table 1. Two of the putative losses (viz., stocks 3111 and 3127) of Mu on further testing turned out to actually have Mu activity. Therefore, the apparent lack of Mu activity in the original tests was probably due to sampling error. One stock (viz., 3119) gave ambiguous results. The 1.1% mutation rate of the second test is somewhat higher than previously observed control values (range from 0.1% to 0.5%). However, if the results from the first and second tests of this stock are totaled the mutation rate becomes 0.8%, which approximates control values. In the case of the 7501-02 stock there is apparently a bona fide loss of Mu. Both the 3119 and 7501-02 stocks will be outcrossed again, in the first stock to determine if there is a low level or a complete absence of Mu activity and in the second stock to test the permanency of the loss of Mu activity.
Donald S. Robertson
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