13. Localization of Pgi-1, Sdh-1, Est-9 and Adh-1 on rice chromosomes by trisomic analysis

Availability of primary trisomics offers convenient tools to localize isozyme genes. Two independent sources of evidence can be used to determine which trisomic carries 3 copies of a given gene:

a) Allele dosage effects in F\1\: If a given trisomic carries 3 copies of an allele, its F\1\ progeny with a diploid marker line will have normal diploid plants with one copy of each parental allele, and trisomic plants with two copis of the allele coming from the trisomic parent and one copy of that coming from the diploid parent. These different allele dosages may yield different zymograms.

b) Band segregations in BC\1\ or F\2\: Unequal numbers of alleles in a plant will result in their unequal frequencies in the gametes and a typical subsequent "trisomic" segregation, clearly distorted from a Mendelian segregation. In this case, zymograms are scored regardless of possible allele dosage effects and classified in two types having the seme bands as the initial parents and one type displaying bands of both parents.

We applied both methods to localize Pgi-1, Sdh-1, Est-9, and Adh-1 to respective chromosomes. We crossed the 12 primary trisomics having IR36 background (Khush et al. 1984) with two Indica lines (Acc. 33364 and Acc. 56036). As compared to the trisomics, Acc. 33364 carries a variant allele at Adh-1 locus and Acc. 56036 carries variant alleles at Pgi-1, Sdh-1 and Est-9 loci. We screened the F\1\ progenies for allele dosage effects and the BC\1\ or F\2\ progenies for trisomic segregations.

Allele dosage effects were observed in four F\1\ progenies (Fig. 1). These were with triplo 4 for Pgi-1, triplo 6 for Sdh-1, triplo 7 for Est-9 and triplo 11 for Adh-1. For Sdh-1, one of the triplo 6 plants crossed with Acc. 56036 was already heterozygous and carried another variant allele beside that of IR36. This cross combination produced a few triallelic heterozygotes (Fig.1). Only those progenies which exhibited allele dosage effects in F\1\ were characterized by trisomic segregation in BC\1\ or F\2\ (Table 1).

Thus, BC\1\ and F\2\ segregation data confirm the conclusions drawn from the allele dosage effects. These results localize Pgi-1 on chromosome 4, Sdh-1 on chromosome 6, Est-9 on chromosome 7 and Adh-1 on chromosome 11 of Shastry et al. (1960) designation.

Table 1. Segregation at four loci coding for isozymes in progenies of homozygous primary trisomics (allele T) crossed with homozygous diploid marker lines (allele D); three classes are considered, corresponding to plants carrying allele T only, allele D only and allelles T and D; segregations which differ at 5% significance level from Mendelian ratios are underlined; those giving trisomic segregations are in boxes.

Fig. 1. Zymograms observed in F\1\ progenies of trisomic 4 for Pgi-1, trisomic 6 for Sdh-1, trisomic 7 for Est-9 and trisomic 11 for Adh-1; a corresponds to diploid heterozygotes, b corresponds to trisomic heterozygotes with allele dosage effect and c corresponds to trisomic triallelic heterozygotes

Khush, G.S., R.J. Singh, S.C. Sur and A.L. Librojo, 1984. Primary trisomics of rice: origin, morphology, cytology, and use in linkage mapping. Genetics 107: 141-163.

Shastry, S.V.S., D.R.R. Rao and R.N. Misra, 1960. Pachytene analysis in Oryza. I. Chromosome morphology in Oryza sativa. Indian J. Genet. Plant Breed. 20: 15- 21.