Linkage relationship between aminopeptidase and alcohol dehydrogenase

Maize aminopeptidase exists in multiple forms. One aspect of our studies is to elucidate further the genetic background of the aminopeptidase isozymes. Previous studies have shown that the genes which code for Aminopeptidase-A and Aminopeptidase-D variants are linked but the chromosomal location is unknown (Beckman, et al., Genetics 50:899, 1964). In Table I, we present evidence of a linkage relationship between Lp (the gene that codes for Aminopeptidase-A) and a structural gene for alcohol dehydrogenase, Adh.

A backcross of the type F/S F/S S/F x F/F F/F S/S was examined, where the genotypes for Lp, Adh, and Cat (catalase) are listed in that order. F and S refer to fast or slow electrophoretic variants for the given enzyme. Kernels from three different ears (each ear represents an independent cross) were scored. The procedure was as follows. Electrophoresis of liquid endosperm from individual kernels was performed as described previously (Scandalios, Biochem. Genet. 3:37, 1969) with a starch-gel medium and a Tris-citrate buffer system. After electrophoresis the gel is sliced horizontally into three 2 mm slices. The gel slices are stained for aminopeptidase, alcohol dehydrogenase, and catalase. Thus, each kernel is genotyped with respect to these three enzymes.

Table 1 gives the number of each genotypic class found for aminopeptidase with respect to alcohol dehydrogenase, aminopeptidase to catalase, and alcohol dehydrogenase to catalase. In all cases the genotype of the first listed enzyme is written first. If the genes coding for the two enzymes are unlinked or are very far apart on the same chromosome, then a 1:1:1:1 ratio is expected. As is seen, the outer two genotypes listed are parental types while the inner two genotypes are "new" types, i.e., possible recombinants.

Table 1.

In each cross examined Lp exhibits a loose linkage relationship with Adh and the chi-square value is significant at the 0.001 level. It can be seen from the data in Table 1 that the deviations from the expected allelic segregation ratios are not the cause of the large chi-square values. Only in Ear 1 is there significant deviation of aminopeptidase (71 fast:46 hybrid) and ADH (75 fast:42 hybrid) from expected 1:1 ratios.

On the other hand, the relationship of aminopeptidase to catalase and of ADH to catalase does not deviate significantly from the expected 1:1:1:1 ratio except in Ear 3 which reflects a distortion in catalase segregation ratios (31 slow:51 hybrid).

Linkage of Lp to Adh thus places both Lp and Lp2 (which, as mentioned, is known to be linked to Lp) on Chromosome 1 of maize since the ADH gene is located on chromosome 1. We are presently using other phenotypic markers on chromosome 1 in order to find the exact position of Lp and Lp2.

L. A. Ott and J. G. Scandalios


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