In the following article, evidence is presented suggesting that Mu Mutator activity occurs in the male gametophyte. In one of our 1986 reports (MNL 60:12-14), evidence was presented that Mutator activity could occur in early developmental stages of the embryo. The present report will provide some additional support for Mu activity occurring in the development of the embryo.
In most of our tests for Mutator activity, the putative Mu plant is self-pollinated and outcrossed to a standard (non-Mu) plant that has been selfed on the second ear. The kernels from both self-pollinated ears are seedling tested to ensure that no seedling mutants are segregating in either parent. In spite of these precautions to ensure that both the parents are not segregating for a mutant, occasionally in a Mu by standard cross about half of the selfed ears segregate for a seedling mutant of a given phenotype. This would be expected if one of the parents carried a recessive seedling mutant. However, sometimes the test of the selfed ear from both parents indicates that this is not the case (i.e., neither parental selfed ear segregates for a mutant). In the early stages of our study of the Mutator system, we assumed such anomalous results were due to bookkeeping errors or numbering errors in the field. However, after observing similar situations over a period of several years, we began to consider other possible explanations. One possibility is that during the development of the kernel that gave rise to the Mu parent a mutational event occurred in the cell lineage of the tassel after it had separated from the cell lineage of the ear. An alternative explanation is that a mutation occurred in the cell lineage of the ear of the standard parent after it had separated from the cell lineage of the tassel. However, because mutations are extremely rare in standard stocks, while a high mutation frequency is characteristic of Mutator plants, it is much more likely that the mutation occurred in the Mutator parent. How can the hypothesis of a mutation in only the tassel lineage be tested? If this hypothesis is correct, the self-pollinated ear of the Mutator parent would not segregate for the mutant, but at least some of the kernels on this ear would be expected to be heterozygous for the same mutant as that observed in the self-pollinated ears of the Mu by standard cross. If the mutation occurred early in the tassel cell lineage, all tassel tissue would be heterozygous for the mutant and thus half of the kernels on the selfed ear of the Mutator parent would be heterozygous. If, however, the mutation event was somewhat later in the tassel cell lineage, then not all of the tassel could be made up of cells heterozygous for the mutant and thus half of the kernels on the selfed ear of the Mutator parent would be heterozygous. If, however, the mutation event was somewhat later in the tassel cell lineage, then not all of the tassel could be made up of cells heterozygous for the mutant. In such a situation, less than half of the kernels on the selfed ear of the Mu parent would be heterozygous for the mutant. Last spring when it was time to get material out for planting, we searched for past Mutator tests that had half, or a significant number, of the outcross progeny segregating for a given mutant phenotype not seen in the progeny of selfed ears of either parent. Unfortunately, in most cases of this type the selfed ear of the Mu parent either did not have enough kernels left for a test, or ears had been discarded (a practice necessary to provide storage space). We did, however, find one instance suitable for analysis. The outcross progeny of Mu parent 84-2015-1 produced 47 plants, of which 14 (30%) segregated for yellow-green seedlings when self-pollinated. The other 33 plants produced selfed ears that did not segregate for this trait. The self-pollinated ears of the Mu and standard parents of this cross did not segregate for this mutant. In 1986, kernels from the selfed ear of the Mu parent were sown and the plants self-pollinated. Although the selfed Mu parent ear did not segregate for yellow-green seedlings, selfed plants from the kernels produced on this selfed ear did segregate for yellow-green seedlings. Of the 83 plants tested, 24 (29%) segregated for this mutant. This is about the same percentage as that observed in the selfed outcross progeny. When these two results are compared by a contingency chi-square test, they are found not to be significantly different. Thus, results expected if a mutation had occurred solely in the tassel cell lineage are realized. It could be suggested, however, that there is a female gametophytic lethal tightly linked to the yellow-green locus. However, if that were the situation, why do less than half of the outcross and selfed progeny carry the yellow-green mutant? The fact that the frequencies of heterozygous mutant plants are closer to a 2:1 or 3:1 ratio than a 1:1 ratio supports the suggestion that a mutation giving rise to a tassel sector is more reasonable. Another alternative explanation might be that the tightly linked gametophytic lethal mutant is not only completely lethal in the female gametophytes but is also partially lethal in the male gametophytes. Thus no mutants will be seen in the self-pollinated Mu progeny but the mutant allele would be transmitted to less than half of the outcross progeny.
Both of these hypotheses are unlikely because the first ear of the Mu plant, which was outcrossed, had a normal seed set and none of the self-pollinated plants from this outcross progeny segregated for the yellow-green mutant. The fact that the Mu parent had a normal seed set rules out the possibility of a tightly linked female lethal mutant functioning in the gametophyte or earlier in the development. If that were the case something less than a normal seed set would be expected. The fact that the progeny of the first ear also did not segregate for the yellow-green mutant confirms that the mutation occurred solely in the tassel.
We will be analyzing other transmission patterns of this type for what they can tell us about Mu activity in the developing sporophyte.
Donald S. Robertson
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