A probable codominant gene for tryptophan content of endosperm

--L. H. Perini and J. L. Magoja

As pointed out in other opportunities (MNL 58:120) high free amino acid level (a trait associated with high protein quality) is related to a recessive and spontaneous mutation, which conditions defective kernels (de*-7601) (MNL 56:108). In some cases the association of high protein quality and defective kernel wouldn't fulfill, and this is because it was tentatively deduced that the free amino acid levels seemed to be controlled by a codominant gene, presumably linked to de*-7601. It was also stated that this biochemical modification was produced in normal kernels (red flint phenotype), and because of this it was presumed that the responsible gene of this phenomenon was different from the defective and didn't condition other changes in kernel size or endosperm structure (see MNL 58:120). The probable fact that only one dominant gene could modify the kernel protein quality without affecting its normal phenotype has great practical importance. Because of this, we have recently proposed to give the necessary steps to try to isolate it from the defective gene, and make all the respective tests to characterize and verify its inheritance.

As can be seen in Figure 1, the WKO1 inbred, which segregates for de*-7601, was crossed by a normal (OU) inbred. According to preliminary evaluations (MNL 58:120) it was assumed that only one locus with two alleles (A/a) would be involved in the control of protein quality. The progenies derived from those crossings (OUSD) that do not carry the defective mutation were self-pollinated and reciprocally crossed by the OU line. The resulting materials and their hypothetic genotypes for protein quality are framed in Figure 1.

Three OU, 4 OU x OUSD, 6 OUSD x OU and 9 OUSD ears were available to analyze protein and tryptophan contents in the endosperm, as shown in Table 1. According to the stated hypothesis the reciprocal crossings (OU x OUSD and OUSD x OU) show significant differences for tryptophan content. There is a wide variation for protein quality among the nine ears analyzed in the OUSD progeny. Nevertheless, not a single homogeneous ear (plant) was detected for high tryptophan level according to the previous hope of meeting it in a 1/8 frequency.

In two OUSD ears obtained by self-pollination and with an intermediate average tryptophan content (0.6g/100g prot.) between the normal and high expected values, kernels were individually analyzed for tryptophan content in the endosperm over a little sample (40 kernels) taken at random. This preliminary analysis let us establish that those ears were segregating for tryptophan content. The resulting average value was 0.6±0.2g tryp/100g prot. with a range between 0.3-0.9g tryp/100g prot. According to the fact assumed it could be deduced that the genotype a/a/a conditions normal tryp content (approx. 0.3g tryp/100g prot.), and the genotype A/A/A conditions approximately 0.9g tryp/100g prot.

Although the available data have been very few, they properly fit the expected segregation 1:1:1:1, as a quarter of the kernels have 0.3g tryp/100g prot., another quarter have 0.9g tryp/100g prot. and the rest are distributed in intermediate classes. Actually we obtained progenies from those ears segregating for tryptophan and we found as expected, homogeneous individuals for high tryptophan content.

Recently, several ears of OUSD progenies were analyzed for protein, lysine and tryptophan content, and in some of them the storage proteins of the endosperm were fractionated according to Landry-Moureaux (1970). Results are shown in Table 2. The results indicate that high quality protein OUSD is the consequence of zein repression.

High quality protein individuals will be crossed with normal lines, and then their progeny will be analyzed, to make a more critical test which will let us support or modify the stated hypothesis.

Figure 1.  Geneology of OUSD.

Table 1.  Protein and tryptophan content of OU, OUSD, and its receprocal crosses.

Table 2.  Endosperm protein pattern of selected OUSD progenies.

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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