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Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
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Page 57
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
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Page 58
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
×
Page 59
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
×
Page 60
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
×
Page 61
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
×
Page 62
Suggested Citation:"Genetics." National Research Council. 1988. Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co. Washington, DC: The National Academies Press. doi: 10.17226/18563.
×
Page 63

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

7 Genetics When, in the 1970s, technical and economic obstacles dampened the widespread enthusiasm for opaque-2 maize, scientists at the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) continued working toward transforming this plant into a crop that could compete with normal maize both in the field and in the marketplace.1 Judging that the problems, although formidable, were not insurmountable, they set out to do the following: • Create opaque-2 germplasm with a hard endosperm; • Maintain the high-quality protein; • Use breeding schemes that permit concurrent improvement of both kernel hardness and general agronomic traits; • Screen the products at multiple sites to engender wide adaptability; and • Develop simple chemical analyses for both preliminary analysis and for rapid assessment of large numbers of samples. This was a daunting challenge. During its evolution over tens of thousands of years, maize has produced kernels that are high in prolamines. Transforming it into a low-prolamine form amounts to reversing the thrust of nature. The task is complicated even more because in this case the backcrossing procedure normally used to generate plant varieties is ineffective. However, from the beginning researchers observed that, although most opaque-2 kernels are soft and chalky, a few ears have odd kernels that are partially transparent. They also noticed other rare kernels with islands or bands of hard endosperm. (In fact, even in nature a few opaque-2 kernels are difficult to distinguish from normal dent or flint maize.) Because these kernels still had high tryptophan and lysine 1 Funds for this project were provided by the United Nations Development Programme (UNDP). Since 1970, a total of $7 million has gone to quality-protein maize (QPM) development. Today, UNDP is continuing, even increasing, its support for the breeding and international testing of QPM, as well as for training laboratory technicians from Third World nations in QPM analysis. 57

58 QUALITY-PROTEIN MAIZE levels, the opaque-2 gene was obviously still present. Thus, these unusual kernels were not caused by mutations of the opaque-2 gene. Instead, they arose from the combined interaction of the opaque-2 gene with other genes. GENE MODIFIERS In simple, classical genetic terms, modifiers are genes that influence the expression of a nonallelic gene or genes. They are minor genes that exert their influence chiefly by intensifying or diminishing the expression of major genes. Their effects are weak. The CIMMYT strategy to convert opaque-2 maize into quality- protein maize (QPM) has focused primarily on such genetic modifiers— those that can stimulate the opaque-2 gene to produce kernels with desirable marketplace characteristics. Initially, CIMMYT researchers targeted kernel appearance and weight as priorities. They intercrossed opaque-2 types with partially transparent, dense kernels, thereby creating diverse "source stocks" of hard-endosperm opaque-2 maize. Several cycles of recurrent selec- tion from each source stock then were employed to create genetic "families" whose kernels had both high nutritional value and desirable agronomic characteristics. After each cycle, individual kernels from hard-endosperm ears were selected from among these families. The protein quality was checked constantly, with some 25,000 samples being analyzed each year. In general, hard-endosperm traits were easier to find and accumulate in flint-type than in dent-type maizes. They were originally found mainly in yellow-flint varieties from the Caribbean, particularly in some samples from Cuba. At first, chemical analyses were carried out just on the tiny bands of hard endosperm. Using a device reminiscent of a dentist's drill, almost microscopic samples were removed and their tryptophan and lysine levels checked. Those kernels containing the quality protein were replanted. The others were rejected. As the bands of hardness steadily increased, to eventually dominate the kernels' makeup, anal- yses were made on larger and larger samples, and later, when favorable modifiers had been accumulated in abundance, analyses were some- times performed on the whole kernel. This is perhaps a more realistic process, but the ability in the early stages to remove portions so tiny that the seeds remained viable was the critical key to success. In this way, by 1974 CIMMYT had developed four basic donor stocks that were broadly adapted to typical agroclimatic zones. Sub- sequently, it began systematically improving them in international trials.

GENETICS 59 Initially, one cycle of improvement was completed each year. Some 250 families would be generated and analyzed in Mexico's winter season (November to April); then, during the following season, they would be tested in six locations in both the northern and southern hemispheres, using six local maize varieties as checks. While these trials were being conducted, the families were also grown in stress trials and analyzed in Mexico. In the stress trials, each was evaluated for its ability to withstand diseases, insects, and high- density planting. Specifically, families in the disease nursery were rated for their response to stalk rots and ear rots (under artificial inoculation); families in the insect nursery were rated for resistance to fall-army-worm (also under artificial infestation); and families were rated for yield, barrenness, and lodging (the inability to remain upright in windy or rainy conditions) when planted at high density. Based on both the international trials and the stress trials, about 40 percent of the superior families were analyzed and selected each year. In the subsequent growing season, reciprocal crosses were made between individual plants of these families. And, at harvest, 250 pairs of ears were analyzed and selected for the next cycle of evaluation. During more than 10 years of this massive and complex process, genetic modifiers that favor normal kernel appearance and density were accumulated. Once this approach had served to gather the desired modifier genes, all the resulting germplasm was merged and reorganized. This was done by putting closely related materials together in individual "gene pools" or "populations."2 By 1984, there were seven pools of QPMs for the lowland tropics, six pools for the subtropics, and seven pools for tropical highlands. In addition, there were six tropical and four subtropical populations. Pools were improved continuously, and su- perior selections from them were fed into the populations if and when needed.3 Today, QPM pools are constantly being improved and broadened by the addition of QPM families from other sources—carrying resis- tance to certain diseases, for instance. These serve as backups for the populations that are being advanced toward commercial applications. KERNEL APPEARANCE AND DENSITY Hardness (vitreousness) is distributed in the modified opaque-2 kernels in both irregular and regular patterns. In irregular types, the 2 In CIMMYT terminology, populations are more advanced than pools. They are more uniform and genetically less variable. 3 In each pool, 400-500 (half-sib) families are handled in an isolated half-sib recombination block. By 1986, most of the pools had gone through seven or more cycles of selection in a simple half-sib manner.

60 QUALITY-PROTEIN MAIZE ore 1 V Score 2 Sr Score 3 W Score 4 W Score 5 V 60 40- Score i 20- § 60- 40- 20- Tropical QPM pools Subtropical QPM pools C4 CYCLES OF SELECTION FIGURE 7.1 In converting opaque-2 maize to QPM, the main approach has been to select for greater kernel weight in segregating generations, carrying out recurrent selection, and hardening the endosperm by accumulating genetic modifiers. In addition, all ears that exhibited poor dry-matter accumulation were discarded. The advances made in eight cycles of selection are represented in the chart by changes in the frequendy of the various scores for kernel modification. A hardness rating of 1 indicates kernels that are completely hard (vitreous) and a rating of 5 indicates kernels that are completely soft (floury). During the conversion of opaque-2 to QPM, the scores have changed dramatically; the softness (4 and 5) has practically disappeared: the hardness (1 and 2) has increased to the point of complete dominance. (CIMMYT, 1985a) hard endosperm forms bands, "bridges," scattered specks, or con- centrations at the base of the kernel. It seems unlikely that this type of variation would be useful; therefore, it has received little attention. In the regular type, kernel hardness increases in a systematic way from the top of the kernel towards the base. Kernels showing this pattern are the ones that have been selected for improvement. Steady progress in accumulating modifiers that increase kernel density has been achieved in every cycle of selection. But when the breeding first began accumulating hard endosperm, the ears of many genotypes developed gaps between the kernel rows. Consequently, CIMMYT researchers later selected ears with less and less inter-row space, until ears filled with normal-sized kernels and near-normal numbers were obtained. This strategy led to the conversion of soft opaque-2 kernels into the hard QPM kernels (figure 7.1). Throughout the selection process, any kernels with a dull or chalky appearance were discarded. Accordingly,

GENETICS 61 Normal Maize QPM 100- Q LU Trial 1 EVT15A 100 Trial 2 EVT15B FIGURE 7.2 In many countries, QPM yields are indistinguishable from yields of the best normal-maize varieties. The results in the figure are from experimental stations in 33 locations in several countries. Local checks were the best yielding local variety of common maize. The figure shows that in results from two 1983 experimental trials, QPM performed as well as (and sometimes better than) its normal maize counterpart. The actual yield figures are given in table 8.2. (CIMMYT, 1985a) the dull endosperm of opaque-2 maize gradually changed to the "clear" endosperm of normal maize. The resulting QPM has a shiny, translucent appearance. YIELD Perhaps the main reason that opaque-2 maize was abandoned in the 1970s was low yields. To overcome this trait has not been easy. Yield and kernel hardness are not usually correlated, so CIMMYT was forced to select for yield and kernel characteristics separately and concurrently. Despite the complexities, steady progress has been made by recom- bining the highest yielding progenies. This has permitted the gradual accumulation of favorable genes (actually alleles) for yield. As noted, some of the recent selections have had yields similar to or better than their normal-maize counterparts (figure 7.2).

62 QUALITY-PROTEIN MAIZE TABLE 7.1 Ear-Rot Data for QPM in Experimental Variety Trials, 1983. Ear Rot Sample (percent) Test 1 (EVT 15A) (22 locations) QPM (Poza Rica 8140) 15.2 QPM (San Jeronimo (1) 8140 12.8 QPM check (Across 7940 QPM) 13.5 Normal maize check (Across 7926) 15.1 Local check mean (normal) 13.1 Test 2 (EVT 15 B) (7 locations) QPM (Tlatizapan 8141) 1.5 QPM (Across 8141) 1.4 QPM (LaPlatina7941) 2.3 Normal check (Across 7845) 2.7 Local check mean (normal) 1.2 SOURCE: CIMMYT, 1985a. One cause of low yield is that dry matter stopped accumulating in the grain approximately one week earlier than in normal-maize kernels. In the final stages of selection that trait was also overcome. MOISTURE CONTENT A problem requiring special attention was the higher-than-normal moisture content of the opaque-2 kernel (as measured at the time of harvest). It was observed that among genetically similar opaque-2 stocks a few ears lost moisture more quickly than the rest. By tagging plants that silked at the same time and then harvesting the crop 3-5 days earlier than usual, the fastest drying ears could be identified by measuring moisture content daily. The physical causes of quicker drying are thought to be: a kernel "skin" (pericarp) that is thinner and more permeable, better compres- sion of the starch against the pericarp, less husk cover, and perhaps hydrophilic compounds in the grain. However, so far, there are no strong data to determine which, if any, of these possibilities predom- inates. FUNGAL INFECTIONS The higher incidence of fungal ear rots in opaque-2 maize results, at least in part, from its soft kernels and higher moisture content. Hardening the kernels automatically reduced the incidence of ear rot. For most areas, the current QPM strains have shown adequate

GENETICS 63 resistance to ear rots. Nevertheless, for hot and humid climates the incidence can be above normal (table 7.1). To reduce this, QPM varieties with tight husks that cover the entire ear are now being developed. This provides additional protection for such fungus-prone regions. EARLY MATURITY CIMMYT researchers use the number of days to silking as a measure of early maturation in a breeding pool. In selecting for quick maturity, they mark early-flowering plants, and after 70 percent have flowered, they detassel all plants in the male rows to eliminate any possibility of pollination by late-flowering types. This has helped to develop QPM stocks having earlier maturity. Today, the "early QPM pools" mature in 90 days from the time of planting, as do the early normal-maize types. STABILIZING THE IMPROVEMENTS Several breeders working on QPM materials have noticed that genetic modifiers can be unstable under different environmental conditions. For instance, QPM seed from the same source may result in different- looking kernels when grown in different locations. Even isogenic QPM materials grown at a single location can show inconsistent proportions of soft and hard kernels. Experience, however, shows that it is possible to stabilize modifiers by carrying out six or more cycles of selection for stability as well as by "shuttle-breeding," in which the plants are grown successively at two different sites and only those giving high yields at both are retained. After this, the materials attain a fairly high frequency of modifiers, and most appear to be stable over many environments. Today, the variation at the ear level has been reduced to a minimum, but variability within the ears persists. It, too, is diminishing with every successive generation. GENETIC INSIGHTS CIMMYT has demonstrated that significant progress in overcoming undesirable effects of otherwise desirable single genes can be made through the use of gene modifiers. The extent to which this is applicable to other species and other problems is as yet unknown. Nevertheless, the QPM experience has been a success and could become a model for the future.

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Quality-Protein Maize: Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation Board on Science and Technology for International Development National Research Council, in Cooperation With the Board on Agriculture National Research Co Get This Book
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