Genetics of MDH
Analyses of crosses involving MDH variants have led to the conclusion that the inheritance of the mitochondrial MDH isozymes is largely governed by three nuclear loci (MdhA, MdhB, and MdhC in our temporary, laboratory designation), each with many multiple alleles. Hybrid dimers are formed between the allelic isozymes at each locus and between these three loci. The loci appear to be independent. Several of the hybrid isozymes formed as a result of interaction between various alleles at MdhC and common variants of MdhA and MdhB appear to be the actual basis for the hypothetical loci mdh1 and mdh3 of Yang et al. (P.N.A.S. 74:310-314, 1977). Our MdhA and MdhB appear to be identical to their mdh4 and mdh2, respectively.
It appears that a fourth locus, present in several inbred lines (Ky228, Mc5NT, Mo24W, W629A) and independent of MdhA and MdhB, affects the expression of all three of the mitochondrial Mdh loci (although not that of other allozymes associated with mitochondria), perhaps through a common effect on subunit structure, charge, or configuration or through a direct mitochondrial effect. When the allele involved is homozygous, each mitochondrial isozyme appears to migrate substantially further. In the heterozygous state, the effect is scarcely discernible.
Two nuclear loci govern the inheritance of the soluble Mdh isozymes, but less variation is present for these loci. They appear to be independent of the mitochondrial MdhA, MdhB, and MdhC loci, although the evidence in the case of one of the two soluble Mdh loci (MdhF) is much more convincing than the data accumulated thus far for the second (MdhD).
Our work has been based on coleoptiles homogenized in a sucrose-sodium ascorbate solution and run mostly with L-histidine-citric acid buffers ranging from pH 5.0 to pH 5.7. When ascorbic acid rather than its sodium salt is used in the homogenization process, the soluble MDH bands are selectively eliminated.
We have thus far demonstrated that at least 9 alleles exist at each of the MdhA and MdhB loci, including a "null" allele for each locus. Among U.S. materials, only 3 alleles are commonly found at either locus. Several commercial inbred lines are of special interest since they possess relatively rare allelic combinations. Ky21 and 4Co82 are each "null" for MdhB, the former possessing MdhA-1, resulting in a slow migrating allozyme; the latter having MdhA-10.5, which results in a fast migrating allozyme migrating close to the positions of the common soluble Mdh variants. H25 is "null" for MdhA but has MdhB-3 (equivalent to mdh2-m5 of Yang et al., 1977) and MdhC-18, an allele resulting in a very fast migrating band which almost reaches the edge of our 18.5 cm long, pH 5.7 gels. MdhC-18 is the less frequent of the two common MdhC alleles; we have rarely found three additional alleles at this locus among a wide array of Latin American collections. While we suspect that a "null" allele exists at this locus, we have not demonstrated that this is so.
The common, dark staining soluble MDH band is often found to be governed by the alleles of two loci. The most common variant (MdhD-12) of the darker staining of the loci is present in almost all common U.S. inbred lines except Tx303 and Tx325, which both have MdhD-14.5, a faster migrating variant, and which appear to be "null" for MdhF, the lighter staining soluble Mdh locus. Two additional rare variants of MdhD have been found among Latin American collections, but no null alleles have been detected. At present, it is uncertain whether the most frequent allele at the MdhF locus, which governs the lighter staining soluble MDH bands, is a "null" allele, MdhF-12, or MdhF-15, due to the almost universal occurrence of MdhD-12, which makes differentiation between MdhF-12 and MdhF-null difficult. Certainly MdhF-15 is not common among U.S. inbred lines, but is found in T232, F2 (a French line), Ill-12E (a component of the Stiff Stalk Synthetic), and Ill-13b (a sweet corn line). Three additional rare alleles of MdhF have been detected among Latin American collections.
By using trisomic stocks supplied by Dr. Lambert of the University of Illinois, we have been able to locate MdhB on chromosome 6. We were able to generate several trisomic plants having different sets of 3 different alleles at that locus. The assistance of Miss Wilma Hu, a research assistant of Dr. D. H. Timothy, Department of Crop Science, was essential to this work. We have yet to determine the chromosomal locations of the other Mdh loci.
M. M. Goodman, C. W. Stuber, C. N. Lee, and F. M. Johnson
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