F1 plants from the cross of two Mu lines (Mu x Mu) were tested for Mu activity by the standard Mu outcross tests to determine if such F1's would have a higher mutation rate than plants receiving Mu from only one parent. The results are in Table 1.
In the Mu x Mu crosses there appear to be two populations. One has a higher than standard Mu activity and the other appears to have the standard or lower Mu rate (families 9627 & 9631). These latter crosses may be instances where the Mu x Mu plant received Mu from only one parent (= loss of Mu). If these low rate families are removed from the population, then Mu x Mu plants have significantly higher mutation rates than plants that receive Mu from only a single parent. This is particularly obvious in the "% Different Mutants" class. In this class the rate is twice the control rate suggesting two Mu's have twice the effect of one. However, there is slightly less of a marked difference in the "% Mutants" class. In this case, the Mu x Mu lines have 1.6 times the mutation rate of the control. Additional Mu x Mu outcrosses are needed in order to obtain more reliable figures. Certainly, the preliminary data suggest that Mu effects may be additive in plants with Mu contributions from both parents.
Donald S. Robertson
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