The mitochondrial malate dehydrogenase (m-MDH) isozymes are encoded by three independently inherited nuclear genes in seedlings (Goodman et al., MNL 52:99, 1978; Genetics, in press) and in the scutellum of the mature kernel (Newton, MNL 53:16-24, 1979).
Mdh1 is located on chromosome 8. It is linked to the 8L breakpoints of two waxy-marked reciprocal translocations between chromosomes 8 and 9: T8-9d (8L.09; 9L.16) and T8-9 (6673) (8L.35; 9S.31). Mdh1 is not linked to waxy in the absence of these translocations.
Mdh3 is located in the distal region of the long arm of chromosome 3 (Newton, 1979), exhibiting approximately 2.6% recombination with sh2.
Mdh2 was originally reported by Goodman et al. (1978) to be on chromosome 6 following trisomic analyses. Crosses with TB-6Lc confirmed and extended this localization to the long am of chromosome 6 (Newton, 1979). Since linkage of Mdh2 with Y1 (6-17) was NOT observed, we hypothesized that Mdh2 was located in the distal region of the long arm of chromosome 6. TB-6Lb uncovers genes in distal 6L: its breakpoint lies between Pt (6-59) and py (6-68)--see Beckett, J. Hered. 69:27-36. Jack Beckett generously supplied the TB-6Lb stock used to demonstrate uncovering of Mdh2. Thus, we conclude that Mdh2 is located in the distal region of the long arm of chromosome 6.
Using different lines, McMillin, Roupakias and Scandalios (Genetics 92:1241) recently reported evidence which confirms the previous trisomy 6 analysis of Goodman and the TB-6Lc work of Newton. They, however, interpret their results differently--claiming that two linked genes coding for m-MDH isozymes are in this chromosomal region. On the other hand, the results of extensive testcross analyses (Goodman et al., Genetics, in press) and biochemical studies (Newton and Schwartz, Genetics, in press) support the hypothesis that three independently inherited genes encode the m-MDH isozymes. The mapping of Mdh1, Mdh2 and Mdh3 to separate chromosomes is also supportive of the latter model.
Kathleen J. Newton
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