Further observations on Mu-induced deletions in the short arm of chromosome 9

We have previously described some Mutator-induced deletions in the short arm of chromosome 9 involving the yg2 locus (Genetics 115:353, 1987). In order to isolate these deletions, Yg2 Yg2 Mu females were pollinated by yg2 yg2 males. The yellow-green offspring recovered from this cross were pollinated by standard (Yg2 Yg2) pollen. To determine whether a deletion of the yg2 locus was involved, plants from the standard cross (Yg2 yg2­Mu(del) I were reciprocally crossed to yg2 testers. Isolates in which there was a deficiency of yg2 transmission through the male, and either normal or reduced transmission of yg2 through the female, were considered as being possible deletions. Further testing confirmed that deletions were indeed present in at least some of these isolates. Since a large control (non-Mu) population produced no yg2 events, the yg2 events occurring in our Mutator population are presumably Mu-induced.

McClintock (Genetics 29:478, 1944) has shown that some small deletions that include the yg2 locus do not interfere with male transmission. If these deletions include yg2 and a small region proximal to this locus (the wd, or white-deficiency deletion), then plants heterozygous for one of these deletions, when self-pollinated and seedling tested, will segregate for homozygous deficient seedlings, which have an albino phenotype. McClintock observed 2 types of ratios when she selfed heterozygous wd plants: 1) 3:1 ratios of green:albino seedlings, and 2) ratios greater than 3:1 of green:albino seedlings. The former are wd deletions with undiminished transmission through the egg and pollen, while the latter are wd deletions with reduced male transmission.

In 1986, plants heterozygous for 5 Mu-induced deletions, which had been shown by genetic tests to be the shortest of the 12 deletions published upon in our 1987 paper, were self-pollinated. One did not segregate for albino seedlings, and 4 did. Of those 4, one segregated for approximately 25% albinos and 2 segregated for less than 25% albinos.

Also in 1986, plants heterozygous for 110 different putative yg2 mutant isolates were self-pollinated and the selfed ears seedling tested. These heterozygous plants were produced by outcrossing the original yellow-green isolates as males to standard (Yg2 Yg2) lines. By using crosses in this direction, all large deletions were selected against, and only small male-transmissible wd deletions, yg2-only deletions, and mutations of the yg2 locus would be transmitted. The following patterns of segregation would be expected in the selfed ears of the isolates: 1) If an isolate segregated albino seedlings in some selfs, and yellow-green seedlings in the other selfs, then a maletransmissible wd deletion is present. 2) If some selfs of an isolate segregate for yellow-green mutable seedlings, and the other selfs segregate for yellow-green stable seedlings, then a mutable Mutator-induced yg2 mutation (yg2-Mum) is present. 3) If all selfs of an isolate segregate for stable yellow-green seedlings, then either a yg2-only deletion, a stable yg2 mutation, or a non-male-transmissible deletion is present. 4) If half of the selfs of an isolate produce only green seedlings, and the other half segregate for yellow-green seedlings, then the isolate is not a yg2 mutant at all, and must have been misclassified during the original screening. When the data from this selfing regimen are combined with the data reported in our 1987 Genetics paper, the following results are obtained: Of the 110 isolates examined, 64 proved to involve the yg2 locus. The remaining 46 were either not allelic to yg2, or the data were insufficient to determine allelism. Of the 64 proven yg2 events, 14 (21.9%) were male-transmissible wd deletions; of these 14, 3 were fully male transmissible and 11 were partially male transmissible. Seven (10.9%) of the yg2 events were larger non-male transmissible deletions including at least the wd locus. Twenty events (31.3%) were Mutator-induced mutable yg2 mutations. The remaining 23 events (35.9%) fell under pattern 3 listed above; there was insufficient evidence to determine whether these events were yg2-only deletions, stable yg2 mutations, or non-male transmissible deletions.

Thus, at least one third (32.8%) of the events induced at the yg2 locus in our Mutator stocks are wd deletions. In the original Mutator population that we screened, at least 64 good yellow-green seedlings were isolated out of a total of 779,213 seedlings, giving a frequency of 8.21 x 10-4. The results reported here indicate that the frequency of wd deletions in this population is 2.70 x 10-5, and the frequency of Mu-induced mutable mutations is 2.57 x 10-5.

In the summer of 1987, kernels from 6 different wd-Mu deletion isolates were sown, and all possible intercrosses were made to test for "allelism" of the deletions. If the albino wd phenotype in all instances is due to the presence of a small deletion involving the chromosomal region just proximal to the yg2 locus, all of these allelism tests should be positive. This indeed turned out to be the case. The same wd-Mu plants used in the allelism tests were crossed as males to yg2 tester plants, and all crosses except one produced green and yellow-green seedlings in ratios very close to 1:1. The wd-Mu3 line had outcrosses that were deficient in yellow-green seedlings. Out of a total of 1,180 seedlings scored, only 290 (24.6%) were yellow-green. Thus, the wd-Mu3 deletion must be slightly larger than the other 6 wd-Mu deletions.

To date, our studies of Mu-induced events involving the yg2 locus have demonstrated that a substantial number of these events are deletions. The deletions are not uniform in size, but can range from quite small ones, which are fully male and female transmissible, to larger ones, which have reduced or no male transmission but are fully female transmissible, to the largest class, which shows no male transmission and reduced female transmission. These results establish that Mutator can induce deletions in this region and that many different sites of breakage appear to be involved in generating these deletions. Other reports from our laboratory in this News Letter establish that Mutator can apparently induce deletions in other regions of the maize genome as well. Thus, the production of deletions seems to be a frequent and important aspect of Mutator activity.

Donald S. Robertson and Philip S. Stinard

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

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