Are deletions of the y1 locus viable?

Genetic (Robertson and Stinard, Genetics 115:353, 1987) and molecular (L.P. Taylor and V. Walbot, EMBO J. 4:869-876, 1985) evidence indicates that Mutator can induce deletions. In our laboratory, we have used Mutator to induce deletions at the yg2, wd, Bf1 and a1 sh2 loci. We have over 300 putative Mutator-induced mutants at the y1 locus. Preliminary genetic studies of these y1 mutations suggested that some of them may have been the result of small or large deletions involving the y1 locus. These putative deletions were selected on the basis of one of two criteria: 1) The absence of pastel seedlings when kernels from self pollinated ears of y1-Mu/y1 plants were germinated at 37 C; and 2) a low frequency of white seeds on ears of self pollinated Y1 /y1-Mu plants. The first criterion for selecting deletions seemed reasonable because most Mu-induced y1 mutant seedlings are pastel when grown at 37 C. If a large deletion, that could not be transmitted through the male, was induced by Mu involving the y1 locus, self pollinated ears of y1-Mu/y1 plants would have only kernels of the genotypes y1-Mu/y1 or y1/y1 neither of which would give seedlings of pastel phenotype. The second criterion was chosen because a Mu-induced small deletion with reduced male transmission would result in a low frequency of white kernels on self pollinated ears of y1/y1-Mu plants. Other explanations are possible for the occurrence of plants with these patterns of inheritance, but it seemed that these two criteria were the most reasonable ones to use for selecting deletions, if they were being induced.

Using the above criteria, we have selected putative y1 deletion plants and crossed them to stocks homozygous for ms1 or si to produce plants heterozygous for the putative y1 deletions and ms1 or si in repulsion. The ms1 and si loci are very closely linked to y1. Therefore, if the Mu-induced y1 mutations are deletions, and if the deletions include the ms1 or si loci, the heterozygotes with ms1 or si might uncover one or the other of these alleles and exhibit the male sterile or silky phenotype. We have studied 3 different putative deletions as heterozygotes and found none with the male sterile or silky phenotype:
 
  No. of mature plants screened
1986-87 planting nos. Ms1 Si
8679,8680 29 9
8681.8682 19 58
8703,8704 36  68

In a similar analysis, stocks heterozygous for three different putative deletions of the y1 locus were pollinated by plants known to be heterozygous for l10 or l*-4920. Both of these luteus seedling mutants are within two crossover units of y1. Therefore, a deletion involving the y1 locus might also delete one or the other, or both, of these seedling mutant loci. If a deletion included these loci, and if it was female transmissible, some of the progeny of this cross would be expected to exhibit the luteus phenotype in the seedling. For one putative deletion 5 plants were crossed by l*-4920 heterozygotes and one plant by a l10 heterozygote. No luteus seedlings were observed in the progeny of these crosses. For the second and third putative deletions, 3 plants and 1 plant respectively were crossed by l*-4920 heterozygotes. Again, no luteus seedlings were observed in the progeny of these crosses. Thus we have not yet identified any Mu-induced deletions involving the y1 locus.

To further test the possibility of obtaining deletions for the y1 locus, we will attempt to generate y1 deletions from plants heterozygous for 2 reciprocal translocations involving chromosome 6 but with breakpoints bracketing the y1 locus. If such a deletion is transmissible, some of the meiotic products of the heterozygous reciprocal translocation plants will be missing the region between the 2 chromosome 6 breakpoints (i.e., the y1 region). These plants will be pollinated by homozygous y1 testers. If the eggs carrying the deletion are viable, white kernels should be observed in these crosses. If this region is essential for the viability of the female gametophyte, no white seeds will be observed. This winter, T6-9(6270) (6L.19), T6-9(6019) (6L.27) and T6-9(8906) (6L.27) will be crossed with T6-9e (6L.18). The latter translocation is known to have its breakpoint proximal to y1. T6-9(6270) has a breakpoint very close to T6-9e and indeed may not be distal to y1, but if it is, it will generate the shortest deletion possible involving this locus. T6-9(6019) and T6-9(8906) should have breakpoints that are distal to y1.

The inability (to date) to find deletions for the y1 locus is intriguing since many of the apparent mutants we have recovered at the yg2 and Bf1 loci are deletions. If the gene product of the y1 locus is a regulatory element in the beta-carotene biosynthetic pathway, it could be that deletions that include this locus are not viable. This is supported by the fact that no white endosperm albino mutant alleles of the y1 locus have ever been found, in contrast to all other mutants (with the exception of y9) in the carotenoid biosynthetic pathway. If the gene product of Y1 is involved in regulating not only the carotenoid pathway, but other pathways as well that are essential for viability, null alleles at this locus would not result in viable kernels or seedlings.

Brent Buckner, Philip S. Stinard and Donald S. Robertson


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