One thing we would like to know about the action of the mutator system we have been studying is its specificity. Is it a general mutator affecting all loci equally or is it locus (loci) specific or specific to a certain chromosome or chromosome segment? In the latter case there may be a select group of loci that respond, confined to one or a few chromosome segments, while the great bulk of the genome would remain insensitive. In an attempt to answer these questions we have systematically tested our mutants with a set of A-B translocations that would permit us to locate many of them to chromosome arm. If we are dealing with a general mutator the mutants should be distributed among the chromosome segments tested. Mutator specificity would be expected if all the mutants occurred in one or a few of the tested segments. The A-B translocations listed below were used in these tests.
The TB parents were known hyperploids except for TB-8a and TB-9c. Pollen samples from families of these two A-B translocations were examined and plants with 15-25% abortive pollen were selected for outcrossing. Each was also crossed to an appropriate tester (i.e., v*-A552 for TB-8a and bf for TB-9c). Most of the mutants were in families in which 2/3 of the plants would be expected to be heterozygous. An attempt was made to pollinate three plants from each mutant family with pollen from each A-B translocation. This was not always possible because of poor germination or weak plants.
Nineteen mutants have been located to date, as listed above. So far the distribution of mutants appears to be random. It is a little surprising that no genes have been located in chromosome one as yet since long chromosome segments are involved. However, the sample of mutants is not sufficiently large at this time to permit one to say that any particular chromosome is insensitive to Mu activity.
Four white-endosperm albino mutants that were found in Mu stocks were tested against all known white albinos for allelism. Two turned out to be allelic to lw (long arm of chromosome 1), one was allelic to w3 (long arm of chromosome 2) and the fourth was allelic to y7(vp9) (long arm of chromosome 7). One blue fluorescent mutant was allelic to bf2 (long arm of chromosome 10).
When the allele test results are considered along with those of the A-B crosses the long arm of chromosome one, the short arm of chromosome 10 and the long arm of chromosome 2 have been added to the list of chromosome arms in which mutations have been induced.
The pattern of distribution of induced mutants, at this time, supports a model of a general mutator as opposed to a more specific mutator.
Donald S. Robertson
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