When I was a guest in Dr. Mel Green's laboratory at Davis in the 1980-81 school year, we had a discussion about the possibility of Mutator inducing deficiencies, and he suggested that the short arm of chromosome nine (especially the yg2 locus) might be used to look for deletions in a systematic way. This was just one of those "why didn't I think of that" ideas that was too good not to follow up. In the summer of 1981, we increased our stock of yg2 and in the summer of 1982 we set up a large isolation plot in which Mu plants were used as females and homozygous yg2 plants served as males. In 1983 at planting time, we used all the seedling growing space we could obtain to grow seedlings of this cross. Out of 779,213 seedlings, 125 yellow-green seedlings were obtained (freq. 1.6 x 10-4) . These were transplanted to the field (we lost only one or two plants in transplanting). Most plants were strong enough to get reciprocal crosses to standard stocks. In the 1983-84 winter nursery, seeds from 10 crosses were sown in which the mutant plants (putative yg2-Mu/yg2) were used as females in crosses with standards and the resulting plants were reciprocally crossed to yg2 yg2 plants. If the yg2-Mu mutant involves a deficiency, about half of the plants in a given family carrying the Mu-induced allele could give 1:1 Yg2:yg2 ratios when used as a female, but no yellow-green seedlings or less than 50 percent yellow-green seedlings when used as a male. The other half of the plants should give 1:1 ratios in both directions because they would be carrying the standard yg2 allele. If the original Mu-induced yg2 allele does not involve the production of a deficiency, then all plants will segregate 1:1 in both directions of the reciprocal cross. Of the ten families with Mu-induced mutants tested in 1983-84, two had plants that gave off ratios when the plants heterozygous for the putative deficiency mutants were used as males but not as females (Table 1).
Plant number two of 8626 behaved as would be expected for a deficiency that is not male transmittable. In family 8628 plants 1, 7 and 11 gave crossing patterns expected if there is a deficiency that has reduced transmission through the male.
We must pause to comment on the crosses where the
putative heterozygous yg2-Mud plants used as females gave only green progeny.
In all cases where this occurred, the reciprocal (if made) cross also gave
only green. Such a phenomenon was observed in 5 of the families tested.
The number of such plants in these families were as follows:
|Total plants tested in the family||Number of instances of all green female progeny|
All of these families came from crosses where the female was a mutant (i.e., yg2 -Mu/yg2) that retained a strong yellow-green phenotype at maturity. Thus the "all green" class can not be explained by contamination. In all cases, the yg2 tester parent of the "all green" crosses was used in other crosses that gave positive yg2 tests. Thus the yg2 tester parent was indeed yg2. It could be that the original yellow-green mutant seedling and plant was a pseudo-yellow-green due to some undefined environmental factor(s). If so, this phenotype persisted to maturity. These also could be the result of back mutation of the Mu-induced allele carried by the gametes of the original yg2-Mu/yg2 plant. This explanation seems plausible for most families where this class occurred, but seems unreasonable for family 8626 where there is evidence that a deletion is involved. It is possible that in this family the deletion is outside of the mutated yg2 genic DNA. Larger numbers of sibling plants will need to be tested from these families with "all green" individuals to determine the frequency with which these classes occur.
In 1984, we grew 33 more of the (yg2-Mu/yg2)/stand. crosses to be tested by reciprocally crossing with yg2 (Table 2). There are nine Mu-induced events involving the yg2 locus that appear to involve large enough deficiencies to eliminate male transmission. Six of the Mu-induced events might be smaller deletions. In two families, 6046 and 6061, it appears that the deficiency may be of sufficient size to interfere with female transmission as well. In all other families there appears to be no consistent evidence that female transmission is affected.
There are of course other phenomena that might account for these transmission patterns, such as gametophyte factors or mutations that induce pollen lethals. This winter we are crossing these putative deficient plants from these families with TB-9b, wd and yg2. If the off ratios are due to deficiencies, then the plants giving off ratios might produce albino instead of yellow-green seedlings in crosses with wd and TB-9b.
Cytological samples were taken last summer from the two putative deficiencies found in the 1983-84 crop, but as yet we have not examined them. The 1984 deficiencies will be sampled cytologically next summer.
Table 1. Reciprocal crosses of families in which about half of the plants are heterozygous for a putative Mu-induced yg2 deficiency (i.e., yg2-Mud/Yg2)
Table 2. Results of the 1984 reciprocal test crosses of putative Mu-induced yg2 in which there was evidence of the presence of a deficiency.
Donald S. Robertson
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