The Bg-r transposable element system was reported originally by Salamini (Mol. Gen. Genet. 179:497, 1980) to be responsible for controlling the instability of the mutable allele o2-m(r). In this system Bg (Bergamo) is the self-mobile or autonomous element and r (receptor) the nonautonomous element. Though both elements can transpose, only Bg can induce transposition of both itself and r, that is, only Bg has a transacting transposition function. It has also been shown that the o2-m(r)Bg system displays an apparent specificity for the 02 locus (Montanelli et al., Mol. Gen. Genet. 197:209, 1984), and mutations at loci under the control of an autonomous Bg element have not been yet identified. To study the Bg element at the molecular level we set up a transposon tagging experiment to move Bg, located somewhere in the genome, into the waxy (Wx) gene for which molecular probes are available.
In order to induce instability at the Wx locus in the A69Y o2 R Bg-m strain, two experiments were performed in the summer nursery of 1986 and 1987 by crossing homozygous o2 R Bg-m Wx plants to a stable wx tester line. An F1 population of approximately 350,000 seeds was screened for mutable wx (wx-m) kernels, i.e. variegated sectors of Wx over wx background in mature ears. Plants originating from these kernels were selfed to confirm presence of variegated phenotypes in the F2 generations. In two cases the variegated phenotype observed in the F1 kernels was heritable giving rise to the unstable alleles termed wx-m32 and wx-m33. They arose with a frequency of 5.7 x 10-6 (two out of 350,000 kernels).
The wx-m32 mutant appeared interesting because the selfed progenies of plants grown from variegated seeds of the F2 generation gave ears with a great majority of variegated kernels. Therefore, genetic crosses with the wx-m32 allele were initiated to test for the presence of Bg insertion and the autonomous nature of the element present in the wx-m32 strain was confirmed by the following experiments.
Variegated kernels from the 87-7087-9 selfed ear (genotypes wx-m32/wx and wx-m32/wx-m32) were planted in the 1988 winter nursery and the plants resulting from these kernels were selfed and backcrossed to the A69Y wx/wx female tester. Out of the 19 plants considered, upon selfing and backcrossing, 10 ears segregated variegated and waxy kernels, while the remaining 9 ears were found to be homozygously variegated. In the segregating ears, ratios near to 3 variegated: 1 waxy were obtained from such selfed plants, while ratios of 1 variegated: 1 waxy were obtained from the backcross to wx females (Table 1). The conclusion was that wx-m32 is an unstable allele induced by an autonomous transposable element.
Table 1. Results of the backcross of wx-m32/wx plants used as
pollen donor to wx/wx females
|Phenotype of seeds in the backcross to a wx female parent|
(1)Vitreous seeds are counted in the variegated class
Tests were also done by crossing the wx-m32 stock onto o2-m(r)+Bg
tester plants. Results indicated that wx-m32 stock carries an active
Bg element by virtue of its ability to induce 02 sectors
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