The source of the Mutator system and the first Mu-induced mutants

--Donald S. Robertson

The Source. Figure 1 summarizes what is known about the pedigree of the stock from which the Mutator system was derived. The earliest progenitor of record was a P-VV line propagated at the University of Wisconsin, Madison in Dr. Alexander Brink's pericarp project. A P-RR revertant found in this line was transferred from its original background into that of the inbred W23. After 6 generations of backcrossing, the P-RR stock tested negative for the presence of Mp (Ac). A self-pollinated ear of this conversion line, which segregated for a defective kernel mutant, was given to Dr. Jerry Kermicle. One of the self-pollinated plants produced by kernels from this ear segregated for a pale yellow endosperm mutant. Dr. Kermicle sent this ear to me in 1961, because of my interest in studying mutants affecting carotenoid synthesis. This mutant was mapped to the short arm of chromosome ten and was given the symbol y9. The phenotype of y9 seedlings is quite variable. It ranges from normal green, to green seedlings with pale leaf tips, to pale green, to yellow- green seedlings. A single ear segregating for y9 kernels can have y9 seedlings all of which exhibit just one of these phenotypes, or various combinations of the phenotypes can be observed. None of these phenotypes is consistently transmitted and the conditions responsible for the induction of any of them are unknown.

The First Mutant. The first mutant of record in this stock was undoubtedly the defective kernel mutant that attracted Dr. Kermicle to this line. The second mutant was probably the y9 mutant, which was responsible for the stock coming into my hands. The first mutants induced at Iowa State University were a pair of w3 mutants found in 1963.

The first year the y9 stock was planted at I.S.U. was 1961, and both yellow and pale yellow kernels were sown. All plants were weak, but selfs of plants from both classes were obtained. In 1962, kernels from a homozygous y9 ear were sown and the resulting plants were crossed to a series of waxy marked chromosome nine translocations and to standard and inbred lines. Plant 62-1001-3 was crossed by the inbred N25 (male), while the sibling plant 1001-2 was crossed onto a waxy T3-9c stock (female). Ten kernels from the cross of plant 62-1001-3 were sown, and eight of the resulting plants were pollinated by other heterozygous y9 plants. All ears from these crosses segregated for pale yellow y9 kernels that produced seedlings showing the variable y9 phenotype. One plant from this family was not pollinated. The tenth plant was self pollinated and segregated for pale yellow dormant kernels and pale yellow viviparous kernels. Among the seedlings produced by the pale yellow kernels were those that were pale green, albino, and albino with borders of yellow-green tissue on the leaves. Some or all of the pale green seedlings might represent y9 seedlings but the other classes were not typical of y9 seedlings.

Ten kernels from the cross of plant 62-1001-2 were planted. Seven plants resulted and these were all self-pollinated. Six of the ears segregated for pale yellow kernels that produced y9 seedlings. One ear, however, in addition to segregating for dormant pale yellow kernels also had pale yellow viviparous kernels. The pale yellow dormant kernels segregated for green, pale green and albino seedlings. (Vivipary is not unexpected in y9 material because this is one of the phenotypes occasionally associated with y9).

The green and pale green seedlings found on these two selfed ears might represent y9 seedlings, but the other classes were not typical of y9 seedlings. The albinos were very similar to the white endosperm-albino mutant seedlings (e.g., lw1, vp5, etc.). Were these albino seedlings just an extreme expression of the y9 seedling phenotype or was a second mutant involved? Because the ratio of yellow to pale yellow kernels on these ears was 9:7, the latter possibility suggested itself and indeed proved to be the situation. These two mutants were not y9, but they proved to be allelic to each other and also to w3. The allele found in the progeny plants from the cross to waxy T3-9c (w3-Mus1) was phenotypically very similar to w3 (i.e. pale yellow and/or white kernels, which are frequently viviparous and produce albino seedlings only). However, the other allele (w3-Mum1), unlike w3, had seedlings that were quite variable in phenotype. After these new mutants were crossed out of the y9 background, the mutant phenotype resulting from these alleles could be clearly delineated. The following kinds of phenotypes were observed associated with the pale yellow kernels of the w3-Mum1 allele: 1) vivipary, 2) white seedlings only, 3) pastel (pale green) and albino, 4) pastel seedlings only, 4) pastel, albinos and albinos with varying amounts of yellow-green tissue (albescent-like pattern). Some of these mutant seedlings had the late-occurring mutable pattern typical of Mutator-induced mutants. In crosses to plants with the w3-Mus1 allele and w3 tester stocks, the pastel, mutable and albescent-like phenotypes were expressed. The w3-Mum1 mutant has the characteristics expected of Mutator-induced mutants (i.e., variable mutant phenotype and somatic mutability).

The molecular basis for mutability has been determined for at least 14 different Mutator-induced mutants with the typical late mutability pattern of Mutator mutants. All of these have been found to have an insertion belonging to the Mu family of elements. To date, no mutant with a late sectoring pattern from a Mutator stock has been found that has been shown not to have an insertion belonging to the Mutator family of elements. Also, a stable Mutator mutant, which has been characterized molecularly, was found to have Mu1-like elements present. The phenotypic variability of y9 and w3-Mum1, as well as the somatic instability of w3-Mum1, are typical of the Mutator-induced mutants that have been characterized molecularly. We anticipate that these two mutants, and perhaps w3-Mus1 as well, will be found to contain Mu family inserts at the mutant loci. We are setting up these mutants for molecular analysis.

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