Since there was evidence that Mu could induce deletion of the end of the short arm of chromosome 9 (MGCNL 59:16-18, 1985), I thought I would try for deletion of the end of the long arm of chromosome 9. It is known that deletions including the Bf1 locus are transmissible through the female. In 1984 we had an isolation plot in which Mu plants were used as female parents and homozygous Bf1 plants were used as male parents. In control crosses, standard lines were used as females. In the greenhouse this spring and early summer, 442,093 seedlings from the Mu cross were screened and 214 seedlings with blue fluorescence (referred to hereafter as Bf-Mu) were found, for a frequency of 4.8 x 10-4. Five blue fluorescing seedlings were found in a control population of 561,014, for a frequency of 8.9 x 10-6.
The Bf-Mu seedlings were transplanted to the field and the plants were scored for pollen sterility and reciprocally crossed to standard. One hundred eighty-four good Bf-Mu plants were pollinated, as well as three of the control blue fluorescent mutants. All Bf-Mu plants were also pollinated onto homozygous bf2 plants. Some of the original Mu parental stocks had bf2 segregating in them, and thus it was possible in the isolation block for some of the blue fluorescent plants to be bf2 instead of Bf1 if the tassel of a Mu plant carrying bf2 shed pollen before the tassel was pulled. All crosses to bf2 proved negative.
The results of the pollen examination are given in Table 1. A total of 174 plants were scored for pollen sterility. Contrary to what was found for putative deficiencies in the short arm of chromosome 9 (in which very little pollen abortion was found), when Bf1 is the target locus considerable pollen abortion of various types is present. This abortion ranged from 75 percent abortive pollen grains to as low as about 15 percent abortive pollen grains. The type of aborted pollen grains also varied from those that were completely empty to partly filled grains to grains that were completely filled with starch but smaller than normal. It has been observed by others that deficiencies with breakpoints in the neighborhood of Bf1 will produce partly filled pollen grains. Larger deficiencies will result in 50 percent abortive pollen grains, either empty or partly filled or a mixture of both. Low frequencies of abortion might result from very small deficiencies where the deficient pollen grains frequently can make sufficient starch so that all deficient grains do not appear abortive. McClintock reported small pollen grains associated with deficiencies of the short arm of chromosome 9 (Genetics 29:478-502, 1944). All of these abortive classes would be expected if other types of aberrations were present (e.g., semisterility with reciprocal translocation, low sterility level with inversion). Since all sterility here is associated with the expression of the Bf1 phenotype, it seems that most are probably deletions. It is known, however, that plants heterozygous for translocations with breakpoints in the vicinity of Bf1 will have blue fluorescent anthers. Thus, translocation breakpoints in this region can affect Bf1 expression. It is possible, therefore, that some of the observed pollen abortion is due to the induction of such an aberration.
To further substantiate the possible induction of deletions by Mu, the crosses by standard onto the silks of the Bf-Mu plants will have their pollen examined for evidence of sterility and be reciprocally crossed with homozygous Bf1 plants. If a deletion has been induced, there is expected to be a deficiency of seedlings with the Bf1 phenotype in the progeny of the male crosses of such heterozygotes, while the ears of the heterozygous plants will frequently give a one to one ratio. In a large deletion, female transmission of the deleted chromosome may also be reduced but not eliminated completely since, if the affected chromosome is completely female lethal, the blue fluorescent plant would not have been found in the first place. Both plants that showed pollen sterility and those with apparently normal pollen will be analyzed in this fashion.
At the same time, some of the plants from the female cross of the original induced Bf1 plants will also be pollinated by TB-9La plants and by plants carrying genes that have been mapped to the long arm of chromosome 9 (e.g., bm4, ms2, v1). I would very much like seeds of any other mutants known to be on the long arm of nine that might be useful to delineate the extent of these putative deletions.
Donald S. Robertson
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