Evidence for Mutator activity in the male gametophyte

It has been demonstrated conclusively that Mutator does induce mutants premeiotically (Genetics 94:969-978, 1980; MGCNL 58:11-12, 1984). This article will consider some evidence for activity of the Mutator system in the gametophyte. Last year (MGCNL 58:11-12, 1984) we presented the results of a large experiment to determine the Mu-induced mutation frequency at the wx and y1 loci. In this test, over 300 putative y1 mutants were found. In 1983 and 1984, seeds of these putative mutants were sown and the resulting plants were self-pollinated. Out of 283 selfed ears, 25 (8.8%) segregated for yellow seeds. We have not finished classifying these 25, but some are known to have a lower frequency of yellow seeds than would be expected if only Y1 was involved. The Mu parent used in this test was segregating for modifiers of y1. These modifiers, when present, produce a pale yellow (instead of white) endosperm. Thus, many mutant seeds were in reality pale yellow rather than white. Most of these proved to be y1 upon selfing since no yellow seeds occurred on the selfed ears, but they did segregate for pale yellow and white seeds.

There are several possible explanations for the seeds with apparent y1 y1 y1 endosperms that produce plants that segregate for Y1: 1) Environmental factors resulted in a pale yellow endosperm in the original isolate. 2) The occurrence of a pollen grain with a dominant white or white cap allele. 3) The original Mu-induced mutant was unstable and reverted to Y1 early in the development of the embryo. 4) Simultaneous mutation of Y1 to y1 in the two polar nuclei (but not the egg nucleus). Some of these events are more likely than others but, in view of the possibilities, it is not surprising that some ears segregating Y1 are found. The important point, however, is that such exceptional seeds are rare.

In 1983 we tested for the induction of y1 mutants in the reverse direction (i.e., using Mu plants as males). This test was on a much more limited scale. Twenty-four pale yellow and white seeds were obtained which produced plants that were selfed last summer. Of these, 15 were homozygous for y1 while nine (37.5%) segregated for Y1.

Thus there is a marked difference between the results obtained when mutants are induced in a male Mu parent compared to those produced in a female Mu parent. In the latter instances, very few putative mutant seeds proved to have embryos carrying Y1 (8.8%). On the contrary, when the Mu parent is used as a male, 37.5% had such discordant seeds. Such a high frequency of discordant seeds in this latter instance would be expected if Mu can mutate in the time span between the DNA replication prior to the formation of sperm and the first DNA replication of the triple fusion nucleus giving rise to the endosperm. Mutations may also occur in the female gametophyte but its more complex development, especially that involved in the production of the polar fusion nucleus, insures that such discordant seeds are rarely observed as a result of Mu activity in female Mu plants.

It may be too early to say with certainty that Mu-induced mutants are occurring in the gametophyte but the data are suggestive. We have a large number of additional putative Mu-induced y1 mutants from male Mutator plants that were produced this summer. These will be selfed next summer and should give definitive results.

It would be possible to test for mutations in the female gametophyte that produce the reciprocal class of discordant seeds (i.e., homozygous y1 embryo in yellow seeds) by growing large numbers of yellow seeds from these crosses in an isolation plot where the male rows would be homozygous for y1. If such mutations are occurring, some of the yellow seeds should have embryos with a Mu-induced y1 mutant allele, and homozygous y1 y1 ears would result.

The same discordant class should be found in the male crosses. In the test with Mu plants used as males, the two types of discordant seeds (i.e., y1 y1 y1 endosperm, Y1 y1 embryo and Y1 y1 y1 endosperm, y1 y1 embryo) should occur in equal frequency. When Mu plants, however, are used as females the discordant seeds (i.e., y1 y1 y1 endosperm, Y1 y1 embryo and Y1 Y1 y1 endosperm, y1 y1 embryo) would not be expected in equal frequency. Yellow seeds with homozygous y1 embryos are expected in a much higher frequency and should be about equal to the equivalent class in the male test.

Unfortunately, all yellow seeds from both the male and female tests were discarded. These tests will be repeated using Mu2 per se stocks as the Mu parent and y1 wx gl8 or y1 wx gl1 plants as the y1 parent. The use of Mu2 per se will result in a much higher mutation frequency, permitting the use of smaller populations. Hand pollinations, therefore, will be practical, and exact reciprocal crosses can be made so that comparisons between results of using Mu plants as males and females will not be complicated by differences in genetic background.

Donald S. Robertson


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