A modifier affecting the migration of the mitochondrial MDH bands was discovered independently by M. Goodman, et al. (MNL 52:99) and myself. The modifier is specific for the mitochondrial bands, effecting a slightly faster anodal migration. Simultaneously, the mitochondrial bands stain less intensely--indicating that modifier has a slight effect on the catalytic activity of these isozymes.

Several crosses with our homozygous modifier line have been analyzed. In one series of crosses, a normal pattern (A in Fig. 3-1) was crossed by the modified pattern (B in Fig. 3-1). All tested F1 kernels were "normal." When these kernels were selfed, a 3:1 ratio was observed (i.e. 58 normals: 20 modifieds). When they were backcrossed to the modifier line a 1:1 ratio (58:57) resulted; whereas, when backcross ears to the normal line were tested, no modified kernels were found. These data indicate that modifier is a recessive allele of a locus whose product is somehow involved in the processing of mMDH. Unlike most previously studied modifiers, this locus must be homozygous for the mutant allele in order for the abnormal phenotype to be observed. This suggests that when the normal functioning of the locus is impaired in this way, the mitochondrial MDH's migrate faster and are less active. F2 data from crosses between the modifier line and lines with mitochondrial MDH structural gene variants indicate that these structural loci are genetically independent from the modifier locus.

The standard B-A translocation method for locating recessive mutants (most recently reviewed by J. B. Beckett, J. Hered. 1978) greatly facilitated the localization of the modifier gene. Twenty-two B-A translocations covering 14 of the chromosome arms were used; only those containing a certain segment of the long arm of chromosome 1 "uncover" the modified MDH pattern.
 

	Breakpoints in 1L	Uncovers modifier?
TB-1La-3L4759-3	0.2-0.38	No
TB-1La-4L4692	0.2-0.46	No
TB-1La-3Le	0.2-0.58	No
TB-1La-3L5267	0.2-0.72	Yes
TB-1La-5S8041	0.2-0.80	Yes
TB-1La-3L5242	0.2-0.90	Yes
TB-1La	0.2-1.00	Yes

Provided that the reported breakpoints are accurate, it would appear that the MDH modifier locus lies between 0.58 and 0.72 on the long arm of chromosome one. The compound B-A 1L-3L translocation series was constructed by J. Birchler, who generously supplied these stocks to me.

Since the use of hyperploid plants (A A^B B^A B^A) as pollen parents facilitates recovery of hypoploid (hemizygous) progeny, the following protocol was instituted. Kernels bearing hyperploid scutella were identified on the basis of small size and/or mutant (hypoploid) endosperm phenotype. When possible, the anthocyanin marker systems (Birchler, MNL, 1979) were used to distinguish the hyperploid scutella. With TB-3La, and with the compound TB-1L-3L series, crosses to Birchler's a-m-1 R-scm tester line had previously been made, which facilitated the choice of the partially trisomic kernels. Plants grown from such kernels were crossed reciprocally with the modifier line. When testing modifier x TB-1La (1 1^B B¹ B¹), alcohol dehydrogenase (ADH) phenotypes could be used to distinguish the doses of 1L. Birchler had incorporated an Adh1-S allele onto the particular TB-1La used in these studies and crossed it to an Adh-C line. The modifier line is Adh1-F. Preliminary data from this cross are as follows: 1) Approximately 50% of the kernels on the ear were small and none of the fourteen which were tested exhibited the modified mMDH phenotype. 2) Twenty-five large kernels were tested--sixteen of them showed the modified pattern, the other nine kernels showed a normal migration for mitochondrial MDH's. 3) ADH phenotypes confirmed the fact that the kernels exhibiting the modified phenotype were hypoploid since they were all Adh-F/- and the presumptive euploids were F/S or F/C. In addition, kernels from the exact reciprocal cross (TB-1La x modifier) did not show the modified mMDH.

Although the numbers collected so far are small, the localization of modifier seems fairly certain. Contamination could not account for the "uncovering" of modifier: 1) The modifier line is su/su and none of the hypoploid embryos had sugary endosperm; 2) It should be noted that the uncovering of modifier occurs in 30-40% of the tested kernels, when hyperploid pollen parents carrying TB-1La or TB-1La-3L5267 were used.

Goodman et al. have determined that one of the sMDH(MdhD) genes is tightly linked to their modifier of mMDH mobility locus. If their modifier is allelic to the one described here, this would also place one of the duplicate soluble MDH genes on 1L. It is more difficult to map these duplicate genes since most of the B-A translocation-carrying stocks have both loci active and one must rely on distinguishing relative isozyme band ratios. Nonetheless, we are employing double variant sMDH tester lines and making exact reciprocal crosses with the TB-A's in our continuing efforts to localize the sMDH genes.

Figure 3-1.

Kathleen J. Newton

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