In inbreeding experiments with Mutator (Mu) stocks (Mol. Gen. Genet. 191:86-90, 1983), by the Mu16 per se generation plants had lost Mutator activity. Molecular studies (unpublished) suggest that at inbreeding levels above Mu8 the copy number of Mu1 levels off and transpositions cease. To test if the Mu1 inserts are permanently altered when this happens or if the loss of transposition is a transitory phenomenon that is only observed when copy number is maintained at a high level, Mu16 per se plants were outcrossed for three generations (1xo.c., 2xo.c., 3xo.c.). By the 3xo.c., if no transpositions are occurring, the level of Mu1 copies on average should be equivalent to that of Mu4 per se, and the Mu activity of the 3xo.c. should approximate that of the Mu4o.c. (15.54%). Table 1 gives the results of such a series of crosses. The results are not clear cut. In the first series of tests (series a), there seems to be a slight increase in mutation frequency. Certainly it does not approach the frequency previously reported for Mu4o.c. However, the single mutants that occurred in 2xo.c. and 3xo.c. are really very questionable. The mutant in the 2xo.c. consisted of one pale green seedling on one ear from which about 50 seedlings were grown. In the 3xo.c. generation, again, one ear had one yellow-green seedling out of about 50 seedlings. There is a definite possibility that both of these "mutant" seedlings are the result of environmental factors rather than Mu activity. Neither seedling was mutable, as is the case with many, but not all, Mu-induced mutants.

In the second series of crosses (series b), the 2xo.c. generation appears to have a sizable frequency of mutation (but not near the 15.14% value observed for Mu4o.c.). As with the "series a" test, the 19 putative mutant events that were observed were, for the most part, not clear cut. The frequencies of mutant seedlings were very low (frequently only one or two per ear) and were of such a type (pale green or slightly yellow-green etc.) that they could be due to environmental factors. There was one mutant, however, that was unambiguous. This was an albino that segregated in a reasonable frequency. None of these mutants in "series b" was mutable. As yet the third outcross generation (3xo.c.) has not been tested.

The results of these tests are not as definitive as one would like. It is uncertain as to whether or not, once Mu1 transposition ceases as a result of high copy numbers, it can again be reactivated. I feel the weight of the evidence is more negative than positive, but this opinion depends upon an admittedly subjective evaluation of what is and is not a Mu-induced mutant. Even if one accepts all the "mutants" in question as truly Mu-induced, the lower-than-expected frequency suggests that the Mu1 elements have not recovered their full transposition potential. Thus, I think it is safe to say that when Mu1 transposition ceases in high copy number lines, the Mu1 element is modified in some manner that has resulted in the loss of the ability to transpose. Once lost, this ability, at best, is regained only slowly or possibly not at all.

Table 1. Frequency of mutations in the first (lxo.c ) second (2xo.c.) and third (3xo.c.) generations of outcrossing of Mu16 per se plants.

Donald S. Robertson

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