High-quality protein maize with normal genotype: Results after eight generations of selection

It has been possible to obtain maize lines of the red flint type with a high protein quality. These lines, as previously reported (MNL 52:37, 1978; MNL 55:66, 1981 and MNL 56:110, 1982), do not carry the floury mutation which modifies endosperm structure.

A corn line carrying the flour -a gene and with high protein quality (HQP fl-a) was crossed with two normal lines (red flint)OU and DY (Fig. 1). Some red flint ears were discovered in S2 progeny, with a high level of endosperm tryptophan (MNL 52:37, 1978), which led us to think that the protein quality could be improved without floury-a. Subsequent tests (Magoja et al., Mendeliana 5 (2):71-80, 1982) show that these normal phenotype lines are also "normal genotype" because in their genetic background they do not carry any floury mutant. In order to accumulate genes conditioning high protein quality, S2 plants of different lineage were crossed: one of the progeny crossing with OU and another of the progeny crossing with DY. Up to the S3 generation, selection was based upon the amount of endosperm tryptophan. As of S4, selection was based on the amount of lysine (g lysine per 100 9 of endosperm protein). The general methodology was to constantly inbreed the progeny of the initial crosses and analyze protein and lysine contents in the segregating generations. In each generation, between 50 and 100 ears were analyzed for protein and lysine contents. Those ears were always obtained by selfing, and the following generation always sprung from the best ears of the former generation, for the lysine content of the endosperm protein.

The progress achieved from the S4 and S8 generations is represented in Figure 2. In Table 1 a summary of the lysine and protein levels during the last selection generation is shown. As can be seen, from S6, the improvement progress with respect to the content of lysine is not statistically important, although it should be pointed out that, nevertheless, the increase of frequency of lines with high lysine level continues. As a consequence of the selection for high lysine level in the endosperm protein, the endosperm protein level has diminished, owing to the fact that those features are inversely correlated, as shown in Table 2. Notwithstanding the protein content reduction, the average level is acceptable because it is connected to a better protein quality. Actually, some of the S8 lines obtained have a lysine content comparable to that conditioned by mutants such as opaque-2, and are stable.

It is important to point out that the selection for lysine content of the endosperm protein, and not for whole kernel, eliminates the possibility that the increase results from a greater germ proportion. In view of the fact that the germ has a good amino acid balance, the lysine levels on a whole kernel are superior to those of an endosperm. In these "normal genotype" lines the whole kernel has around 25% more lysine than the endosperm. It is also important to point out that the red flint lines have a 2.1 g average lysine per 100 g of protein in the endosperm, and the effective average increase of the lysine level has been approximately 48%. In the case of better lines obtained, which have an average of 3.6 g lysine per 100 g of endosperm protein, the increase over normal lines is about 70%. From the results obtained, it can be expected that high-quality protein lines and "normal genotype" may be used to improve the protein quality of flint corn without using floury mutants.

Figure 1.

Figure 2.

Table 1.

Table 2.

Jorge Luis Magoja and Angel Alberto Nivio

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

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