In a survey on the distribution of the Uq element, four inbred lines (B70, C123, 187-2, C103) were crossed by a-ruq testers and the resulting F1 hybrids were then backcrossed by a-ruq testers. The backcrossed progeny kernels derived from each of the inbred lines fell into 3 distinct phenotypic classes: colored (.25), mottled (.25) and colorless (.50). No fully spotted kernels were found. This indicated that none of the 4 inbred lines had an active Uq element in its genome.
On the other hand, sectors of spotting were found on individual spotted kernels. This spotting occurred in 29 (.0008) out of the 37,434 colorless kernels as a single sector of color spots in the aleurone layer (Figure 1). These 29 single sectors showed two features. First, their sizes were different, varying from .05 mm2 to 32.82 mm2 . This simply indicated that various numbers of aleurone cells (from 82 to 54,257) were involved in the sector area (timing of activation). Second, the sectors displayed a spotting pattern that was very similar to the one triggered by a standard Uq element. Therefore, we proposed that these single sectors of color spots resulted from the interaction between the a-ruq allele and a Uq regulatory element which had been activated.
To confirm that this was Uq-activated mutability and not R-mottling, we planted these 29 single-sectored kernels, their few-spot (from 1 to 5 single spots per colorless kernel) sib kernels as well as the colorless sib kernels. Twenty-five plants were grown from the 29 sectored kernels. These plants were first crossed by r testers and then reciprocally backcrossed to a-ruq testers. In addition, 4 tillers were selfed. The resulting progeny kernels were analyzed with the following results:
1. All were homozygous for a -ruq (Fourteen plants were R/R, the other 11 plants were R/r). The homozygosity of the a-ruq allele precludes R-mottling as a basis of sectoring.
2. None had an active Uq element in its genome, indicating that the Uq activation was not a germinal event.
3. Colorless kernels with a single sector of color spots were regenerated when the 25 plants were backcrossed to a-ruq testers reciprocally (Table 1). When they were used as maternal parents, 16 (.0047) out of the 3,398 progeny kernels had single sectors. On the other hand, 30 (.0018) out of the 16,235 progeny kernels of the reciprocal cross were sectored. Selfing of the additional 4 tillers also produced 4 single-sectored kernels with a frequency of .0052.
Similar results were obtained for the few-spot and the colorless sibs. They were all homozygous for a-ruq (the composition of the R locus was not tested) and generated single-sectored kernels when backcrossing to a-ruq testers (Table 1). However, the frequency of single-sectored kernels was lower for the few-spot sibs (.0017 as maternal and .0020 as paternal), and was the lowest for the colorless sibs (.0009 as maternal and .0008 as paternal). Finally, it was noted that no kernels with single sectors appeared in selfs or sibs of the a-ruq testers (Table 1).
Based on these results, we concluded that the presence of the sectors of color spots (sector of mutability) in an otherwise colorless aleurone layer is due to the activation of a Uq element that triggers the mutability at the a-ruq locus at different times during endosperm development. Further, the genomes of the four inbred lines provide certain conditions that stimulate Uq activation. However, we are not able to determine whether the activated Uq is from the ruq receptor element at the A locus or if it is an inactive Uq becoming active in the genome, unless a germinal event can be rescued.
Figure 1. When an active Uq is not present in the genome, a colorless kernel of a-ruq/a-ruq shows a single sector of color spots upon activation of the Uq element during endosperm development.
Table 1. Number and frequency of the single-sectored colorless progeny kernels generated in the reciprocal backcrosses to a-ruq testers of plants derived from different types of sib kernel.
Yong-Bao Pan and Peter A. Peterson
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