Bhabha Atomic Research Centre

Nature of dominant inhibitor of aleurone color, C1-I
--S. B. Allagikar-Nadiger and N. K. Notani

I is considered allelic (i.e., C1-I) to C1 (colored aleurone) and the recessive allele c1 (colorless aleurone). I, an inhibitor of aleurone color, is dominant over C which in turn is dominant over c. Based on a superficial similarity to a super-repressed mutation in the regulatory gene of I could be a regulatory gene of C (Chandra Mouli and Notani, MNL42:6-10). However, all tests of allelism are consistent with the interpretation that I is allelic to C. We may then ask, in what way is I different from C and what is its origin? In a spontaneous mutation experiment, we showed that out of 35,000 kernels scored in the progeny of I/I crossed by C/C, we got only one mutation and that was towards colorless (i or c) (Chandra Mouli and Notani, MNL41:11-12, 1967). We have reported a mutable I manifesting coloured dots on colorless background. We interpreted this mutability in terms of insertion of a transposable element in the DNA sequence corresponding to a DNA-binding (Operator) region of the protein product (Allagikar et al., J. Genet. 70:33-41, 1991). Saedler's group (Paz-Ares et al., EMBO J. 9:315-321, 1990) showed by heteroduplex mapping that at the DNA level, I (source Ed Coe) differs from C in at least 3 regions, while there is considerable homology in the rest of the DNA stretch. We have recently examined recombination in the I Sh Bz interval in a stock isolated locally which has not only an I allele but also has an enigmatic Sh1 Bombay allele (see accompanying note). Because of the presence of the Sh1 Bombay allele, crossover data could be obtained only for the I-Bz interval.

Table. Recombination from the cross I sh1-B Bz/C sh1 bz x C sh1 bz
Parental Crossover
Colorless Bronze Purple Crossover %
1 226 220 21 x 2 9
2 110 120 7 x 2 6
3 62 50 4 x 2 7
4 40 38 4 x 2 10
5 97 95 6 x 2 6

There is no conspicuous reduction in the crossover values; if anything there is a slight increase--5% (standard value) vs. 7.6%. Our I allele may or may not be similar to C1-I:Coe, however, there is little effect on the recombination frequency in the I-Bz interval. The present recombination data do not allow strong deductions about rearrangements, if any, between I and C alleles. 

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