Mmm on 1L simultaneously affects the electrophoretic mobilities of the products of all three of the structural MDH loci, Mdh1 on chromosome 8, Mdh2 on chromosome 6, and Mdh3 on chromosome 3 (Genetics 94:153-168; 95:425-442). The common allele (Mmm or Mmm-M) is present in most inbred lines, genetic stocks, breeding materials and races of both maize and teosinte. A recessive allele (mmm or Mmm-m), found occasionally among both U.S. and Latin American stocks, results in faster migration of all the mitochondrial MDH homodimers and heterodimers. The faster migration appears to be coupled to lower enzymatic activity (K. Newton, personal communication). Another allele, intermediate to Mmm-M and Mmm-m in gene action and in phenotypic response, has been discovered in a collection of Cuzco.
The newly discovered allele, which we are calling m2, is recessive to M, but dominant to m. When Mmm-m2 is homozygous or heterozygous with Mmm-m, all the mitochondrial MDH homodimers and heterodimers no longer migrate as single distinct circular or oval bands. Instead, each such band is elongated, extending from its usual position to approximately the position it would hold if Mmm-m were homozygous (on our gels this is about I mm). No loss in overall activity is evident, in contrast to Mmm-m.
In small-scale linkage tests we have seen no recombination between Mmm-m2 and Mmm-m, and Mmm-m2 maps to the same region of 1L as does Mmm-m (Table 1), approximately 5 map units from Mdh4. Specifically, Mmm-m was mapped to 4.4 + 1.1 recombination units from Mdh4, while these data suggest virtually the same result, 4.8 + 2.1 recombination units.
M. M. Goodman and C. W. Stuber
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