In 1977 (MGCNL 51:37) we reported on tests of controlling element systems for mutator activity. A variegated pericarp stock with Mp was included in these tests. This active Mp stock did not show any mutator activity. Brink has shown that Mp has Ac activity. Thus this test would suggest that Ac also is lacking measurable mutator activity. To confirm this, an active Ac Ds system was obtained from Dr. Neuffer (Univ. Missouri). The seeds used as a source of Ac Ds were a/A.Ds, Ac/- and had purple or red aleurone sectors. The plants from these seeds were weak and it was not possible to self pollinate them because the silks were delayed until after the pollen had finished shedding. Thus, the Ac Ds plants were sib pollinated and the parents crossed to standards. In the standard test for mutator activity, the tested plants are selfed and outcrossed to standard. The self is necessary to establish that the putative mutator stock is not carrying any mutants. In the standard test, the outcross progeny are self pollinated and the selfs tested for seedling mutants. The frequency of plants segregating for seedling mutants is used to calculate the mutation rate. In these Ac Ds tests, we were not able to use a self to determine if the Ac Ds pollen parents carried a mutant. However, since the pollen parent was crossed with a sibling plant, seeds from these crosses were planted and the resulting plants self pollinated and the self progeny screened for seedling mutants. At the same time seeds of outcrosses of one or both of the sib parents were planted and the resulting plants self pollinated and the self progeny scored for seedling mutants. If one or both of the Ac Ds parents carried mutants they would show up in the progeny of the Ac Ds cross and also in the appropriate standard outcross progeny. Results of these crosses are given in Table 1. It is obvious that these Ac Ds stocks do not have significant mutator activity. The 0.25 percent mutation rate is comparable to those of our control (Non-Mu) stocks. In reality the true mutation rate of this Ac Ds stock is lower than the 0.25 percent value since in the sib crosses tested each parent was Ac Ds. Thus, for each plant selfed in a sib cross progeny, two Ac Ds gametes were tested thus each should be counted twice. The total number of Ac Ds gametes tested in Table 1 thus is 527 and the resulting mutation rate is 0.19 percent.
Table 1. Test of Ac Ds stock for mutator activity.
|1978 Family No.||1977
|New Mutants||Total Plants||% New Mutants|
|1710||Stand/1345-12||v||w*(one self only)||46||2.1|
*v = virescent, w = albino
To date, no controlling element system has a mutation rate that approximates that of Mu. This is an interesting observation in light of the well documented instances of the movement of controlling elements which have resulted in new loci coming under their control. Such movement to a locus controlling a seedling trait would show up as a new mutant in our tests. Since such mutants are rare, it must be concluded that the controlling element systems so far tested are not very efficient mutators.
Donald S. Robertson
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