Effects of translocations on the interallelic interactions in the alcohol dehydrogenase (ADH) system in maize

Isozymes as markers of gene activity provide an approach to some genetical problems such as interallelic and intergenic interactions. An advantageous enzyme is maize alcohol dehydrogenase (ADH) which is controlled by two nonlinked genes, Adh and Adh2 (Schwartz, 1966; Freeling and Schwartz, 1973). In heterozygotes Adh-F Adh-S the relative concentrations of FF, FS and SS isozymes are distributed binomially. This indicates that subunits of ADH molecules associate randomly with equal probability.

However it was also shown in our studies that the isozyme pattern controlled by nonallelic genes Adh and Adh2 is often not binomial (Fig. 1).

The nonbinomial distribution may be the simple sum of patterns yielded by two types of cells, in which one of the genes is more active, either Adh or Adh2.

According to an hypothesis of Henderson (1968) the nonbinomial distribution pattern of isozymes controlled by nonallelic genes is the possible result of the spatial disconnection of the mRNA's of these genes during translation; and according to an hypothesis of Serov (1977) mRNA's of allelic genes have to be located in the same region of cytoplasm during translation to ensure interaction between alleles.

The question is raised whether the spatial disconnection of the translation of the mRNA's of the nonallelic genes is the result of the spatial disconnectedness of the genes themselves.

It is known the gene Adh is located in the 1 chromosome (Schwartz, 1971) and Adh2 is located in the 4 chromosome (M. Freeling, cited after Schwartz, 1976), and the spatial chromosome distribution is very much dependent on the structure of the interphase cell nucleus.

Here, a convenient model would be the interaction of the different alleles of a gene transferred to different chromosomes by translocations. Such a model was used in this study. Maize forms of American origin with breaks in the 1L (long) arm located proximally to the gene Adh were used: T1-9(4995)(1L.19-9S.20); T1-4d(1L.27-4L.30); T1-8a(1L.41-8S.52); T1-9b(1L.50-9L.60). The plants had to be simultaneously heterozygotes for translocations and heterozygotes for the Adh gene. It was first established that all the forms with translocations are homozygous for the F allele. Therefore the plants involved in crosses were a normal line W155 with genotype Adh-S Adh-S and initial forms heterozygous for translocations identified by the criterion of 50% sterile pollen. The semisterile progeny of this cross were the experimental plants and their fertile sibs were controls. Plants of both groups had Adh-F Adh-S genotype.

ADH was analyzed by polyacrylamide gel electrophoresis. This analysis of forms with translocations and their normal sibs demonstrated similar ADH patterns of the scutella of dry seeds with a binomial distribution of staining intensity. There were no differences between the ADH patterns of leaves of control and experimental plants at the age of 10-13 days and at the flowering stage. Deviation from the binomial distribution pattern was observed only in one form, W155 x T1-9b in the flag leaves of plants 20 days after flowering: FF (58.2%):FS (27.6%):SS (14.2%); c2 = 18.1. In normal plants of this group the quantitative relationship of isozymes in the pattern was as follows: FF (57%):FS (31%):SS (12%); c2 = 4.9.

Thus, the distribution pattern in one of the maize forms studied deviates from binomial. The deviation was observed when the activity of the enzyme is very low.

There are two tentative explanations for these deviations: 1) It may be that translocation promotes cell heterogeneity slightly expressed even in normal plants judging by the insignificant deviation of the binomial distribution pattern. 2) As a result of translocation the Adh-F allele is further away from Adh-S than in the other forms studied. It cannot be ruled out that in this form the mRNA coded for by different alleles of the gene Adh passes to the cytoplasm from different nuclear regions.

Nonbinomiality of the interallelic ADH isozyme pattern is expressed very weakly, whereas nonbinomiality of the distribution of the intergenic isozyme pattern is quite clear-cut (Fig. 1).

Based on these observations it was inferred that even if the structure of the interphase nucleus does affect the interaction of nonlinked genes this effect is to a large extent mediated by some specific additional factors.

Figure 1.

E. V. Levites


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