Numerous alleles have been studied at both the structural loci (Mdh4 and Mdh5 on chromosomes 1 and 5, respectively) encoding the cytosolic MDH isozymes (M. M. Goodman, C. W. Stuber, C-N. Lee and F. M. Johnson, Genetics 94:153-168, 1980; K. J. Newton and D. Schwartz, Genetics 95:425-442, 1980). With one exception (Goodman and Stuber, MGNL 54:100-101, 1980), each of these alleles ultimately results in the sorts of electrophoretic patterns commonly expected for dimeric isozymes. For example, both intralocus and interlocus heterodimers are formed and these have the customary staining intensities and expected migration distances intermediate (usually halfway) between those of their corresponding homodimers. However, in the case of the one exceptional allele, D8.5, which encodes a slow variant at Mdh4, the migration distances of all of the heterodimers which we have observed are skewed in the direction of slower migration. The result becomes most evident as D8.5 is crossed with variants carrying Mdh4 (or Mdh5) alleles which themselves encode slowly migrating isozymes. Then the hybrid band actually migrates more slowly than either of the homodimers. These inferences were all derived from studies of numerous F1, F2 and backcross progeny. They have been reinforced by studies of pollen MDH. In F1 plants homozygous for a null allele at Mdh5 and heterozygous for D8/D8.5 at Mdh4, sporophytic tissues carry the D8 and D8.5 homodimers and a slower migrating band, the apparent D8*D8.5 hybrid band. Pollen samples carry only the D8 and D8.5 homodimers. This constitutes additional evidence that the slowest migrating cytosolic MDH band in D8/D8.5 heterozygotes is indeed the interallelic heterodimer, since MDH intralocus hybrid bands are not found in pollen (Newton, Ph.D. thesis, Indiana University, 1980). In order to eliminate any possible complications arising from comigrating mitochondrial MDH isozymes, anti-mitochondrial MDH antibodies (Newton, Ph.D. thesis) were used to specifically immuno-precipitate those isozymes prior to electrophoresis. The latter treatment allows an even clearer demonstration of the unusual behavior of these cytosolic MDH bands.
M. M. Goodman, K. J. Newton* and C. W. Stuber
*Currently at Stanford University, Stanford, CA
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