In the previous report we presented evidence that Mu may be active in the zygote or early stages of development. To test this hypothesis, we set up an isolation plot this summer in which the female rows were y1 y1 wx wx Mu plants and the pollen parents were our standard line (Y1 Y1). If, in the triple fusion product giving rise to the endosperm., the Mu system contributed by the female is causing mutations at the Y1 or Wx loci contributed by the sperms from the standard parents, completely white (or pale yellow) or waxy seeds should be observed. If Mu was active in an early stage of endosperm development, white/yellow or starch/waxy sectored seeds should result. The size of the sectors would depend upon how early the mutation occurred and how much of the endosperm arose from the two different cell lineages generated by the mutational event. Mutations in the first division might be expected to give rise to sectors in which half or more of the seed is white (or waxy), while mutations occurring in the second division might give seeds in which about a quarter of the seed is white (or waxy). Mutations occurring later in development would be expected to give sectors involving less than a quarter of the endosperm. The results are given in Table 1.
The waxy mutants do not occur in a high enough frequency for a good test. A fair number of white (or pale yellow) seeds were observed. The frequency of this class was higher in the Mutator population than in the controls, but not significantly higher. The sectored seeds were frequent in both the Mutator tests and the controls, but the Mutator population had a significantly higher number.
The distribution of seeds with various sizes of white (pale yellow) sectors in the white/yellow sectored seeds is given in Table 2. Both Mutator and control populations had the same types of sectored seeds, but the distributions among classes varied.
The results of this test are inconclusive. The numbers are in the direction expected if Mu is inducing mutation in the triple fusion nucleus or early development, but more data are needed.
We did not plant a control for the test in 1985 because this was only a preliminary experiment to determine if there was any indication that Mu was functioning in these stages of seed development. The control used for the data reported here was a control population for a test made in 1984. The y1 y1 wx wx parent in the control was y1 y1 wx wx gl8 gl8, and therefore the female parent was not as well matched to the Mu parent (i.e., y1 y1 wx wx Mu) as it might be. Because these preliminary results are suggestive of Mu activity, we will repeat this work using as the y1 y1 wx wx control the recurrent parent stock used to make the y1 y1 wx wx Mu line.
If these early observations are substantiated, we will look for mutations in the zygote of the embryo. The white seeds obtained in the tests reported here are most likely all discordant seeds, because it is very unlikely that a mutation at the y1 locus would occur simultaneously in the triple fusion nucleus and the zygote of the embryo. To look for mutations that have occurred in the zygote, mature plants from the yellow seeds will have to be grown in an isolation plot and used as females with y1 y1 wx wx plants serving as male parents. Homozygous white ears are expected if a mutation has occurred in the zygote. Such a test will require a very large number of mature plants and is not worth undertaking until there is a very good likelihood of success.
Table 1. The results of tests for Mu-induced mutation in the triple fusion nucleus or the early stages of development of the endosperm.
Table 2. Distribution seeds with various size y1 sectors.
D. S. Robertson
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