Some anomalies associated with Mutator-induced events involving the Bf1 locus --Donald S. Robertson and Philip S. Stinard Some of the results reported in the previous paper are suggestive that Mutator-induced events involving the Bf1 locus can produce unexpected phenomena (e.g., disappearing mutants). In this report several additional anomalies will be described.

In the 1988 News Letter (MNL 62:25-26), we reported that some putative Mu-induced deletion heterozygotes (Bf1-Mu(del)/+), when pollinated by TB-9Lc, produced a few Bf1 seedlings, which were assumed to be homozygous for the putative deleted segment but hypoploid for the rest of the translocated long arm of chromosome nine. These plants were shorter than normal plants, had narrow leaves, were late maturing and had poor tassel development. Some did not extrude anthers and those that did produced little, if any, viable pollen. Bf1 seedlings from the cross of the same putative deletion stocks by TB-9Lc(male) were transplanted to the field last summer. Bf1 seedlings from nine other putative Mu-induced deletion stocks pollinated by TB-9Lc also were transplanted. In addition, Bf1 seedlings from the cross of Bf1 Bf1 Bm4 Bm4 and Bf1 Bf1 bm4 bm4 plants by TB-9Lc heterozygotes were transplanted as controls. The former Bf1 stock was brought to Iowa State University from the California Institute of Technology in 1957 and the latter stock was obtained from the Maize Genetic Cooperation a few years ago. All of the putative deletion TB hypoploids showed the characteristics described above. However, the hypoploid controls from the I.S.U. Bf1 stock also had the same phenotype. Thus, these traits are a result of hypoploidy per se in the I.S.U. genetic background and have little or nothing to do with the possibility that the putative Bf1-Mu(del) crosses were possibly homozygous for a deleted segment involving the Bf1 locus. The hypoploids from the cross of the Coop Bf1 Bf1 bm4 bm4 stock had much broader leaves and were taller than the I.S.U. hypoploids. They, however, had sterile pollen and brown midribs. Thus, genetic background has a strong influence on the hypoploid phenotype of TB-9Lc.

The TB cross of one putative deletion (546-5) produced two putative Bf1-Mu(del) hypoploid plants. Both of these were smaller than other Bf1-Mu(del) hypoploids, but all other characteristics were the same except these plants had brown midribs. There were putative hypoploid Bf1-Mu(del) plants from three other TB-9Lc crosses of the same Bf1-Mu(del) grown (8 plants total) and none of these had brown midribs. This putative deletion was also tested for its ability to uncover the bm4 locus (See previous report). Plants from eight crosses with bm4 stocks were grown, totaling 144 plants. None of these had brown midribs.

A second anomaly was found in analyzing the results from the reciprocal crosses of progeny plants from selfs of Bf1-Mu(del)/+ plants that did not yield any blue fluorescent seedlings (See previous report for details).

Before considering the anomalous results, a brief reminder of the phenotype of Bf1 mutants is in order. Bf1 is expressed as a recessive trait in the seedling. Only Bf1 Bf1 seedlings are blue fluorescent. However, anthers will fluoresce blue when only one Bf1 allele is present. Thus, in the tassel, Bf1 expression is dominant.

All plants of the selfed progenies that were reciprocally crossed in the previous report, were scored for anther fluorescence. In most of these progenies, there were plants with blue fluorescent anthers and ones without. In most families, reciprocal crosses to Bf1 Bf1 plants of those without fluorescent anthers did not segregate for blue fluorescent seedlings, while those with fluorescent anthers segregated for fluorescent seedlings. However, for three families involving progeny of the independent putative deletions 047-4, 047-9 and 047-11, plants with blue fluorescent anthers did not segregate for blue fluorescent seedlings. For 047-4 there were 6 plants tested, for 047-9, 2 plants and for 047-11, 4 plants. There was one exception for the latter stock. One plant gave no fluorescent seedlings when the fluorescent anther plant was crossed as a female but produced 2 very weak and small fluorescent seedlings when crossed as a male.

It seems that one or more of the Mu-induced Bf1 events has generated a mutant allele that only expresses the dominant anther phenotype. Yet that can not be the whole story, for this mutant was first selected as a blue fluorescent seedling from an isolation plot in which the female plants were + + Mu and the pollen source was Bf1 Bf1. Thus, the original plants of these stocks were of the genotype Bf1-Mu(del)/Bf1, and fluoresced blue as seedlings. These plants were crossed as females to a standard (+ +) line, and it was the progenies of these crosses that were self-pollinated, and from which the ears that did not segregate blue fluorescent seedlings were selected. Thus, these mutants originally expressed fluorescence in the seedling but have lost this ability, while retaining the ability to produce anther fluorescence. Could this be a phenomenon caused by the methylation of the Mu element at this locus similar to that observed by Martienssen et al. for the Mu-induced hcf106 mutant (EMBO J. 8:1633-1639, 1989)? Perhaps the Mu elements in these mutants are differentially methylated in the seedling and anthers. The seedlings might have methylated elements and thus the wild type gene expression, while in the anther the elements might be unmethylated, resulting in the mutant phenotype. There is the remote possibility that the observed anther fluorescence is not due to the presence of Bf1 but rather to bf2, which would explain the lack of fluorescence in the seedlings of the reciprocal crosses. However, many, but not all, of the original Bf1-Mu/Bf1 plants isolated from the progeny of the isolation plot were crossed with bf2 bf2 to make sure that this locus was not involved. Seventy-one plants were tested and none carried bf2. The 047-9 stock was one of those that tested negative with bf2. Mutants 047-4 and 047-11 were not tested but will be this summer. However, it is very unlikely that they carry bf2 in light of the 71 negative tests of plants from this isolation plot. Also, because the plants with the blue fluorescent anthers in these three selfed progenies came from selfed ears that did not segregate blue fluorescent seedlings, it is very unlikely that bf2 is involved because bf2 bf2 seedlings would be blue fluorescent.

The third anomaly associated with Mu-induced alterations involving the Bf1 locus involves the selfed progeny of another Bf1-Mu(del)/+ plant (putative deletion 544-3). The seedlings of a selfed ear did not segregate for blue fluorescent seedlings. Ten mature plants were obtained from this selfed ear. Eight of these plants did not have fluorescent anthers, one had one anther with 2 fluorescent spots, and one had many anthers with fluorescent spots. Four of the plants with no fluorescence in the anthers, when reciprocally outcrossed to Bf1 Bf1 plants, did not give fluorescent seedlings. Three plants without fluorescent anthers gave 50% fluorescent seedlings when crossed as both male and female. One plant without fluorescent anthers was outcrossed only as a male and gave only fluorescent seedlings. The plant with one anther with two fluorescent spots gave 50% fluorescent seedlings in reciprocal crosses, while the plant with many fluorescent spotted anthers produced only fluorescent seedlings in crosses in both directions. This situation seems to be just the opposite of the previous anomaly. The 544-3 allele seems to be one that only expresses in the seedling and weakly or not at all in the anthers. The anther expression would suggest that this mutant allele is mutable, in some plants at least. In others, mutability may not be expressed because of methylation. There is one last important aspect of this mutant to consider. The progeny of the self from which these plants were grown did not segregate for fluorescent seedlings! Yet the original isolate (Bf1-Mu/Bf1) was a fluorescent seedling and the reciprocal crosses of these tests gave fluorescent seedlings, which also are of the genotype Bf1-Mu/Bf1. This would suggest that the Bf1-Mu allele might be lethal when homozygous, yet two of the plants from the progeny of the self when outcrossed to Bf1 Bf1 gave only Bf1 seedlings, suggesting that homozygous Bf1-Mu plants were outcrossed. Thus, it seems likely that this particular Bf1-Mu allele does not give fluorescent seedlings when homozygous but does when heterozygous with the standard Bf1 allele.


Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

Return to the MNL 64 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page