Further studies with the bz-mut, Mut system

During the past year, we have accumulated considerably more data from testcrosses involving ws3 lg and gl2, which confirm the location of Mut between lg and gl2 on chromosome 2. Plants of Ws3 Lg Mut Gl2/ws3 lg mut gl2; bz-mut/bz-mut genotype were crossed by ws3 lg mut gl2; bz-mut pollen pollen parents. The kernels were scored for mutability (due to interaction of bz-mut and Mut), and the ws3, lg and gl2 traits were classified in the seedling bench. The composite data are presented below:


Certain classes of seedlings were transplanted to the field for further tests for the presence of Mut. Doubles (2-3) and triples (1-2-3) were of special interest because they are likely candidates for Mut transposition. The expected frequency of doubles and triples within a region of 30 map units is so low that a considerable number of the apparent multiple crossovers are probably due to transposition of Mut to another chromosome, followed by segregation away from the original chromosome at anaphase I. Such an event, combined with an absence of crossing over in the ws3 lg gl2 region of chromosome 2, gives ws3-lg-gl2 (+ Mut) and Ws3-Lg-Gl2 (- Mut) phenotypes. The former plants may then be selected for further testing to reveal the new location of Mut.

Due to the poor growing conditions in the field in the summer of 1983, only 4 of the (2-3) and (1-2-3) crossovers germinated, survived to maturity, and produced ears. Two of these do not carry Mut and cannot be studied further. Two, however, are potential transpositions; one is ws3 lg (Mut) gl2 and the other is Ws3 lg (Mut) gl2. They have been crossed with Ws3 Lg Gl2; bz-mut stocks and the F1 will be testcrossed in the greenhouse to determine whether Mut is still in chromosome 2.

In our previous report, the phenotype of Mut, bz-mut kernels was not fully characterized. The Bz dots on a bronze background are usually small, comparable in size to the A dots produced in Dt kernels. However, a single kernel may show considerable variation in size of dots and at least two intermediate pigment levels occur along with the full colored Bz. One aspect that remained unanswered was whether or not Mut shows a dosage effect, similar to Dt and Ac2, which give an exponential increase in dots with extra doses, or to Ac where both time and frequency of mutations are affected. Exact reciprocal-crosses of related strains were made:

bz-mut/bz-mut; Mut/Mut X bz-mut/bz-mut; mut/mut

All resulting kernels have three bz-mut responding alleles but one cross produces kernels with two doses of Mut, while the reciprocal cross gives kernels with one dose of Mut. The number of dots on 20 randomly selected kernels from each cross was determined. To ensure accuracy of counts, the kernels were soaked overnight in water and the pericarp was removed. No difference was found in the time of mutation; the size of dots, though somewhat variable, was similar in Mut Mut mut and Mut mut mut kernels. To our surprise, the number of dots also did not differ greatly . An average of 32.6 dots was found on kernels with one dose of

Mut, while kernels with two Mut had an average of 27.4 dots per kernel. Closely related kernels from the Mut/Mut; bz-mut/bz-mut stock possessing three doses of Mut averaged 34.6 dots per kernel. This value was based on 30 kernels from three sibbed ears. These data, though preliminary, clearly indicate that Mut is unlike many regulators in that no dosage effect is observed.

M. M. Rhoades and E. Dempsey

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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