Last year we presented the results of reciprocal crosses of plants heterozygous for putative Mu-induced deletions involving the yg2 locus near the end of the short arm of chromosome 9 (MGNL 59:17-18, 1985). When putative heterozygous deletion plants (del./+) were reciprocally crossed to homozygous yg2 plants, four kinds of heterozygous plants were found, as determined by the phenotypic ratios in their reciprocal outcross progeny:
1. Those that segregated 1:1 for green and yellow green seedlings when crossed as both female and male.
2. Those that segregated 1:1 for normal seedlings and yellow green when the del./ + plants served as females but fewer than half of the outcross progeny were yellow green when they were crossed as males.
3. Those that segregated 1:1 for normal seedlings and yellow green when the del./ + plants served as females but no yellow green seedlings were observed in the outcross progeny when they were crossed as males.
4. Those that segregated for less than 50 percent yellow green seedlings when the del./+ plants were used as females but produced only green progeny when they were crossed as males.
This past summer, seeds from the latter three kinds of crosses, in which the del./+ plants were used as females, were sown and the yellow green plants (putative genotype = del./yg2) were pollinated by heterozygous wd plants and heterozygous TB-9Sb plants. The wd chromosome has a short deficiency that includes the yg2 locus. This deficiency is male transmissible. If the del./yg2 plants do indeed have deletions of the yg2 locus, the break point could extend proximal to the wd break point. If such is the case, crosses with wd should segregate for white (albino) seedlings. If the deletion is not too extensive, pollination of del./yg2 plants with TB-9Sb pollen might also result in segregation for albino seedlings when the hypoploid sperm fertilized a deletion bearing egg. The albino seedling would be homozygous for the deficient region that extends at least through the wd locus. Of course, both of these types of crosses will also be segregating for yg2 seedlings as well, since both the B-A translocation and wd uncover the yg2 locus carried on the nondeletion homolog. If the deletion is too extensive, it may be lethal in the embryos that are hypoploid for the BA chromosome. The results of these crosses are given in Table 1. Based on the results of the reciprocal crosses of the del./+ plants with yg2 and the crosses of the del./yg2 plants with heterozygous wd and TB-9Sb plants, four classes of deletion plants now can be recognized as indicated in Table 1.
Class 1 probably represents the shortest deficiencies because all yield some yellow green seedlings when del./+ plants are crossed as males. Since they segregate for albinos in the wd crosses, the deletion extends at least through that locus. The fact that all del./yg2 plants segregate for albino seedlings when pollinated with TB-9Sb pollen suggests that a short deletion is involved, since the "homozygous" deletion condition is not lethal. The Class 2 deletions are probably a little longer than those of class one because no yellow green seedlings are observed in the male outcross progeny of del./ + plants to yg2; otherwise they behave like Class 1 deletions. The class 3 deletions are longer yet because no albino seedlings are observed when del./yg2 plants are pollinated by TB-9Sb plants. This would be expected if the "homozygous" deletion condition is lethal.
McClintock suggested that homozygous terminal deficiencies greater than that for wd (i.e., missing more than the first chromomere) would produce inviable embryos (Genetics 29:478-502, 1944). Class 4 deletions are probably the longest since they have reduced female transmission of the deficient chromosome when the del./+ plant is pollinated by yg2; otherwise they behave like Class 3 deletions. Only the relative lengths of these deletions are known, as indicated in Figure 1. Their exact extent and whether they are terminal or internal will have to wait on the results of cytological studies.
It is known that one of the Class 3 deletions (107-2) does not include the sh locus. In this case, the del./yg2 plant carried sh on its normal chromosome linked to yg2 and the heterozygous wd parent had sh linked to wd and Sh on the homologous chromosome. The F1 segregated for shrunken seeds, and when the seedling test was made nonshrunken and shrunken seeds were planted separately. All the seedlings from the shrunken seeds were yellow green except for one green crossover seedling. The nonshrunken seeds gave both green and white seedlings but no yellow green seedling. If the deletion had extended through the sh locus, the seed giving rise to the del./yg2 plant would have been shrunken (which it was not), and white seedlings would have been produced by some of the shrunken seeds in the wd cross.
Table 1. Polliantions of putative heterozygous deletion plant, with heterozygous wd and TB-9Sb plants.
Donald S. Robertson and Philip S. Stinard
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