Because approximately 30% of the Mu induced mutants have mutable phenotypes, it is possible that Mu is a very active and mobile controlling element system. If this is the case Mu may be related to one of the previously described controlling element systems. Crosses of Mu stocks with lines possessing only the receptor elements a, Ds and I proved that Mu did not have Dt, Ac or En activity. In 1977 we reported (MGCNL 51:37, 1977) that Ac Ds, En I and a Dt systems did not possess a mutation rate that differed from those of control populations studied in our Mu test.
Gonella and Peterson (Genetics 84:629-645, 1977) reported on a new controlling element system Fcu r-cu. Purple aleurone Mu lines were crossed to an r-cu (non-Fcu) stock and the F1 progeny self-pollinated and scored for Fcu activity. No Fcu activity was observed.
To test the mutator activity of the Fcu r-cu system, plants with both elements were self-pollinated and outcrossed to a standard line. The outcross seeds were planted and the resulting plants self-pollinated and the self-progeny were scored for the occurrence of new mutants (Table 1). Three mutations were observed out of a population of 330. This was more than those observed for the Dt, En and Mp systems combined (see MGCNL 51:37, 1977). The occurrence of a mutable yellow-green mutant is suggestive that these mutants could have been the result of the controlling element system. However, if the Fcu r-cu system is producing mutation it does so at a very low rate which does not differ significantly from the rate found in control populations. Certainly it does not approach the mutator activity of Mu.
In summary, to date, there is no evidence that Mu is related to any of the known controlling element systems. Also, to date, it does not appear that any of the controlling element systems approach Mu in their mutator activity.
Donald S. Robertson
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