We have reported the coinduction of Uq activity and the mn*-866248U (miniatures) mutant (MNL 61:6) by 5-aza-2'-deoxycytidine treatment. Our preliminary data in that report indicated that the mn* mutant is dominant and is not male transmissible. It also cosegregates with the activated Uq element. To confirm these initial findings, we have carried out 2 generations of progeny tests. Colorless normal kernels and spotted miniatures of 5 arbitrarily defined classes (a, b, c, d and e) were collected from the ear 866248U/a-ruq and were grown into plants. The plants were crossed reciprocally to a and c-ruq testers. In addition, crosses of a few miniature-derived plants on two other genotypes (r-g tester and inbred HY) were also made to test the male transmission of the mutant gene. The spotted miniatures and the mottled progenies were further tested in another generation. The results of the progeny test are summarized in the Table and the Figure.
The data in the Table clearly prove the following points: 1) The activated Uq is associated with the miniature character; 2) whenever the miniatures are the pollen parent, the mn* and the activated Uq both are lost from the progeny population, and 3) all progeny ears segregate for half normal and half miniature kernels when the spotted miniature class is crossed by a c-ruq tester (test for Uq segregation in this type of cross is in progress). Examination of pollen samples from the colorless normal class and the spotted miniature class has shown no morphological difference (data not shown). However, significant differences in their germination ability may account for the non-male-transmission of mn* (data not shown). In addition, the difference in reducing kernel size has been proved to be due to the expressivity of this mutant gene, since each of the 5 size classes is segregating for all 5 classes of progenies in a maternal cross (Figure).
Based on these results, we propose that mn-866248U is caused by insertion of the activated Uq into a locus that plays an important role in pollen grain and endosperm development. We also strongly believe that once the Uq sequence becomes available, cloning and molecular characterization of this maize developmental gene will turn into a reality.
Figure. The 5 size classes (a. b, c, d, and e) of spotted miniatures produced all 5 classes of offspring when crossed by a sh2 (no Uq) tester (or when selfed; data not shown here). However, the frequency for each class varies within as well as among the 5 size classes tested.
Yong-Bao Pan and Peter A. Peterson
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