Deletions with the wd phenotype in the short arm of chromosome nine induced by the Mutator system
--Donald S. Robertson

Robertson and Stinard (Genetics 45:353-361, 1986) described the induction of deletions involving the yg2 locus, which had no or reduced transmission through the male. To further characterize these deletions, crosses were made with heterozygous TB-9Sb and wd1 stocks. In these tests, six of these deletions produced albino seedlings, which indicated that they were viable in the hemizygous state and were "allelic" to wd1. ("Allelic" will be used in this report when crosses between two different deletion stocks yield a mutant phenotype. Because deletions do not represent a single locus, the term allele is not technically accurate.) McClintock (Genetics 29:478-502, 1944) gave the symbol wd to terminal deletions of the short arm of chromosome 9, which included the terminal knob, the adjacent chromatin thread, and extended through approximately half of the first chromomere. Plants heterozygous for these deletions and the recessive yg2 allele were yellow green. When plants heterozygous for these deletions were self-pollinated, the progeny segregated for albino seedlings.

Because some of our deletions produced seedlings with the albino phenotype when hemizygous or when crossed to the standard wd1, it was obvious that the Mutator system was producing wd-like deletions. We have available over one hundred putative deletion stocks involving the yg2 locus. To determine if some of these could be short deletions that would give the wd phenotype when homozygous, ten kernels from the male outcrosses of the original isolates (genotype yg2/"deletion") to standard lines (genotype Yg2 Yg2) were sown, and the resulting plants self pollinated. Approximately 50 kernels from each selfed ear were seedling tested and scored for the segregation of albino seedlings, which were expected in half of the outcross plants, if a short deletion had been induced similar to those responsible for the wd phenotype.

Included in these tests were the five deletion stocks that had segregated for seedlings with the wd phenotype in the TB-9Sb and wd crosses, which previously had been described in the 1985 paper in Genetics. As expected, some of the outcross plants of each of these five deletion lines segregated for the wd phenotype.

Seven new heterozygous deletion stocks were found, which segregated for albino seedlings in the progeny of the self pollinated plants. Six of these (given the symbols wd-Mu1 through wd-Mu6) were tested to determine if they were "allelic" to each other. Kernels from the selfed ears of each of the six wd-Mu deletion stocks were sown and the resulting plants of each deletion line were intercrossed in all possible combinations. Sufficient crosses were made to insure that a positive result would be obtained if "allelism" was present. Positive results were obtained in all combinations. Thus, all the deletions had in common the region responsible for the wd phenotype. Crosses of these six deletions with plants heterozygous for the standard wd1 plants deletion proved that they were all "allelic" to this deletion.

Crosses also were made to determine if the six new wd-Mu deletions involved the region of the short arm missing in the pyd deletions. The pyd stocks were generously supplied by Dr. Barbara McClintock. She found these deletions to be shorter than the wd deletions. They included the terminal knob of chromosome 9 and the adjacent chromatin, but did not include the first chromomere. Heterozygous yg2/pyd plants are green. Thus, the pyd deletion does not extend through the yg2 locus (Genetics 29:478-502, 1944). Crosses of plants heterozygous for all six wd-Mu deletions with heterozygous pyd plants resulted in the segregation of pale yellow seedlings, which are expected of pyd/wd heterozygotes.

The five wd-Mu deletions reported in the 1986 paper in Genetics were also tested for "allelism" with pyd and proved to be "allelic". These deletions have been given new symbols as follows: 108-8 - wd-Mu7, 110-9 - wd-Mu8, 110-8 - wd-Mu9, 114-1 - wd-Mu10, 117-8 - wd-Mu11.

It is not known how much of the deleted regions of the pyd and wd1 deletions are responsible for their respective phenotypes. It is known that the pyd deletion is shorter than the wd1 deletion and that the latter deletion uncovers the yg2 locus and includes approximately half of the first chromomere of the short arm of chromosome 9. The genetic evidence indicates that all 11 wd-Mu deletions uncover the regions of chromosome 9 responsible for the wd1 and the pyd phenotypes.

The wd-Mu deletions involve the region of the short arm of chromosome 9 responsible for the wd1 and pyd phenotype. This, however, does not necessarily mean that the wd-Mu deletions are terminal deletions, as are the pyd and wd1 deletions. Cytological analyses are needed to determine the extent of the wd-Mu deletions. Cytological studies of the wd-Mu deletions are being undertaken at this time, but because of the genetic background of these stocks, it is difficult to obtain good cytological preparations. The determination of whether or not some or all of the wd-Mu deletions are terminal awaits further cytological studies. 

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