Some thoughts about the nature of Mu-induced Y1 mutants

Previous studies of the genes affecting carotene synthesis in maize have indicated that many, if not all, of the same genes that are responsible for the carotene biosynthetic pathway in the leaves and other green parts of the plant are also involved in carotene synthesis in the endos erm as well.

My analyses of Guatemalan teosintes, which are perhaps the purest teosinte races, indicate that they are white seeded. Crosses with yellow-seeded corn as females give yellow F1 seeds and F2 ears that segregate 3:1 for yellow and white seeds. This white-seeded allele of teosinte is allelic to the standard y1 gene of corn. If, as has been suggested by many, corn evolved from teosinte, early corn was probably white seeded. Thus the genes responsible for carotene synthesis were probably only "turned on" in the leaf but not in the endosperm of the primitive corn. Sometime in the development of modern yellow-seeded corn lines, corn acquired the ability to turn these genes on in the endosperm. Since yellow is dominant, it is likely that the genetic change that permitted carotene synthesis in the endosperm is involved in the regulation of this pathway. Perhaps a mutation of a site that binds an endosperm repressor of a gene regulating this pathway occurred. Thus in the white seeded progenitors of modern yellow-seeded corn, this repressor would turn off this pathway in the endosperm. However, a mutation in or near the site of repressor binding could prevent its binding and thus result in the turning on of the carotene pathway. Such a mutation would be a dominant. There are other possibilities that can be suggested that would result in an apparent dominant mutation (e.g., mutations of the locus producing the repressor substance resulting in a repressor that is no longer able to bind to the repressor site of the gene regulating the pathway).

The y1 locus is the most likely candidate as the locus involved in this regulation because it is the locus that is responsible for the white-seeded condition of teosinte.

Over the last few years we have accumulated several hundred independent Mu-induced mutants at the y1 locus. These all were originally isolated from crosses in which Y1 Y1 Mu stocks were used as either the male or female parents in crosses with y1 y1 wx wx gl1 gl1 (or gl8 gl8) stocks.

Earlier studies (pre-Mutator studies) had revealed two classes of y1 alleles: 1) Those in which the endosperm is white and the plant green and 2) Temperature sensitive y1 alleles in which the endosperm is white but the plant is pale green (pastel) when grown at temperatures about 35C. These latter alleles give zebra type plants when grown in the field. To date we have tested 278 of the Mu-induced y1 mutants and 71.94% have been the pastel type of allele.

A pastel y1 allele found at the California Institute of Technology in the stocks from the post World War II atom bomb tests (y1-wmut) was also mutable. This allele had both mutable endosperms and mutable seedlings (plant). Because Mu is known to induce mutable mutants (e.g. at the al, a2, c2, bz1 and bz2 loci), the Mu-induced y1 mutants were screened for mutable endosperms. To date, no mutable endosperms have been observed. The percentage of mutable mutants varies from one Mutator cross to another. The last determination of the frequency of mutables was made in 1984. Of 395 seedling mutants scored, 193 or 48.86% were mutable. Thus the Mu-induced y1 mutants, at first, seemed to be an exception in that none was mutable. When these Mu-induced y1 mutants were seedling tested, however, 52.5% of them had mutable pastel seedlings. Thus these are not unlike the other Mu-induced mutants. Some indeed are mutable. Yet even those that have mutable seedlings do not exhibit endosperm mutability, even when the seeds are cut and scrutinized under a dissecting microscope. Characteristically, mutable Mu-induced seed mutants have very small revertant sectors. The Mu-induced waxy mutants have this pattern of mutability. In some of these, islands of one or a few cells scattered throughout the endosperm stain blue with the iodine stain in these mutant endosperms. If the same is true for the mutable y1 alleles, it may be very difficult to recognize such isolated revertant cells in an otherwise white endosperm.

If the y1 locus is indeed a gene involved in the regulation of the carotene pathway, how are the phenotypes of the Mu-induced mutants at this locus explained? The white-endosperm-green-plant alleles could be reverse mutations that restored the original regulation of the carotene pathway (i.e., off in the endosperm). For example, perhaps the presence of the Mu insertion changes the configuration of the DNA so that, whereas in the Y1 allele the site of repressor binding was not available to the endosperm repressor, with the Mu insert present it now becomes available.

But how to explain the pastel alleles? The function of this allele in the endosperm seems to have been restored to the pristine condition but now its regulation in the plant is disturbed. Perhaps, for example, this locus has two regulatory receptor regions one involved in endosperm regulation and the other in plant regulation. The Mu insertion in the pastel allele may have restored function to the endosperm regulatory receptor but at the same time interfered with the normal function of the plant regulatory region.

The plant can tolerate a partial shutdown of the plant function of this gene. It seems, however, not to be able to tolerate its complete shutdown because no white-endosperm-albino plant allele of this locus has ever been found in the Mu studies or for that matter, in previous studies of this locus. Nearly all other known mutants in the carotene pathway have alleles that give albino seedlings. It may well be that the y1 locus is involved in the regulation of another pathway (or other pathways) required for the life of the plant. If that is the case, then deletions which include this locus would not be viable. We are currently screening our Mu-induced mutants for putative deletions involving this locus.

Dr. David Morris is presently utilizing the mutable Mu-induced Y1 mutants in a program to isolate the Y1 DNA so that this locus and its regulatory regions can be characterized molecularly.

Donald S. Robertson
 
 


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