We can now report that we have found linkage between the MdhD locus, which governs the inheritance of the darkest staining, soluble MDH bands, and the migration modifier (MdhM-m) present in such lines as Ky228, Mo24W, and W629A (see MNL 52:99-100). We observed only 3 crossovers among 120 chromosomes carrying MdhD-12 and MdhM-m among testcrosses of Mo24W x Tx325 to Ky228 and W629A. Tight linkage has also been observed between a rare MdhD allele (MdhD-8.5) found in a single Latin American collection and MdhM-m. These two observations have become more significant since in this issue Kathleen Newton (University of Indiana) reports that she has determined the chromosomal location of a very similar, if not identical, migration modifier.
We also have limited data on a seventh MDH locus, also apparently linked to MdhD, discovered in a single collection of the race Pira from Venezuela. Each plant surveyed had the same rare variant, and most plants were homozygous for it, yet we had failed to detect such a variant in any of over 100 other Latin American collections, including several Venezuelan Piras.
Since last year's report we have determined that "null" alleles do exist at MdhC and that MdhE (MdhF in last year's report) is essentially independent of MdhA, MdhB, MdhC, and MdhD. We have also found three additional U.S. lines which may be helpful in studies of maize MDH. B8 from Iowa State and M49 from Pioneer International share the same rare MDHB allele (MdhB-4.5 which, when present in homozygous condition, results in no detectable MDHB locus homodimer on our gels, but which does result in heterodimers with the products of both MdhA and MdhC. When MdhB-4.5 is heterozygous with a more common allele, the homodimer corresponding to the more common allele is present, the expected MDHB locus heterodimer is also present, but the MDHB homodimer corresponding to the MdhB-4.5 allele is absent. For the MdhE locus, Mich77-7, an unreleased Michigan line produced by Dr. Elmer Rossman, may be useful. While the line is still segregating, it appears to have a fast migrating allele (MdhE-16.6) at a locus which is not highly variable among adapted U.S. materials.
Major M. Goodman and Charles W. Stuber
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