Yang et al. (PNAS, 1977) reported that the darkest staining mitochondrial band, m-3, in a common pattern consists of the product of two independent loci. In some lines, there is a variant, m-5, of the "stronger" of these genes (MdhB). Band m-4 is known to represent a heterodimer between the two different gene products.
In an attempt to locate the genes encoding mMDH's, we used a pattern (see A of Fig. 4-1) which was lacking a band at the m-3 position in reciprocal crosses with the B-A translocation lines. With TB-6Lc the F1 scutellar MDH patterns differed depending upon the direction of the cross. Our mMDH tester line was homozygous for the y allele, and, therefore, white endosperms in the F1 kernels were associated with hyperploid scutella. Because (1) we are dealing with duplicate genes and (2) the BA chromosome is carrying the m-3 allele of the major locus for mMDH, phenotypes were only distinguishable on the basis of relative band activities. However, since three distinguishable patterns resulted from a cross between two "homozygous" lines, one of which was invariably associated with white endosperms and, due to the fact that pattern E (Fig. 4-1) is lacking in the exact reciprocal cross (where the same TB-6Lc plant is used as the silk parent), we conclude that the gene encoding the most active form of mMDH is located on the long arm of chromosome 6.
This localization is supported by the independent work of Goodman et al. (MNL 52:100), since they demonstrated the simultaneous presence of three alleles of this gene in plants trisomic for chromosome six.
Kathleen J. Newton
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