Combining Abilities of Quality Protein Maize Inbreds

content in maize alone can add an estimated annual gross value of $360 million per year and can go up to Development and adoption of quality protein maize (Zea mays $480 million per year if protein also is increased (JohnL.) (QPM) would increase the nutritional value of food and feed son et al., 2001). maize products. Breeding programs at the International Center for The CIMMYT has developed QPM that has imMaize and Wheat Improvement, Mexico (CIMMYT); Texas A&M University (TAMU); and University of Natal, South Africa (SA) have proved kernel quality characteristics over o2/o2 soft gedeveloped high-lysine inbreds. Information about how elite QPM notypes, by introducing modifier genes and selecting for inbreds of different origins combine and perform in hybrids will facilia hard, vitreous endosperm in o2/o2 germplasm (Vasal, tate the selection of parents and breeding strategies for hybrid devel2001). The CIMMYT QPM populations, pools, inbreds, opment. Our objectives were to estimate the general (GCA) and and hybrids adapted to subtropical and tropical environspecific combining abilities (SCA) for grain yield and secondary traits ments are widely used in the development of high-lysine among high-lysine inbreds from different sources and to identify pomaize in Brazil, China, Ghana, India, and several Latin tential heterotic relationships among them. Seven white (CML176, American countries (Vasal, 2001). The maize breeding CML181, CML184, Bo59W, Tx807, Tx811, and TxX124) and nine program at SA has developed high-lysine white (e.g., yellow QPM inbreds (CML190, CML193, Tx802, Tx814, Tx818, Bo46W and Bo59W) and yellow inbreds (e.g., Do940y Tx820, Do940y, TxX808, and TxX806) were evaluated in two separate diallel experiments in five southern U.S. environments. The QPM and Ho4664) that produce hybrids competitive in yield hybrids yielded less than commercial checks. Across environments, with normal hybrids and tolerant to diseases (Gevers GCA effects were nonsignificant for grain yield but highly significant and Lake, 1992). for agronomic and kernel-quality traits. On the basis of GCA effects, In the USA, more than 20 yr ago after the discovery of TAMU inbreds had earlier maturities, shorter plants, and less grain o2 mutant effects, breeding programs converted normal moisture content than more subtropical CIMMYT and SA inbreds. inbreds and populations to their opaque-2 soft counterThe best-yielding hybrids and highest SCA effects resulted from parts (NTR1, NTR2, BSAA-o2, B73o2, SSSS-o2) (Mertz crosses among inbreds from different programs: TxX124 CML176, et al., 1964). After this initial effort, the interest in QPM Tx811 CML181, and Bo59w CML184 among the white hybrids, or high-lysine maize decreased and it has since remained and Tx802 Do940y among the yellow hybrids. The QPM inbreds low. To our knowledge, only Crow’s Hybrid Seed Comdeveloped in different programs could represent potential heterotic groups for use in hybrid development and introgression of germplasm. pany has continuously conducted a breeding program to improve high-lysine maize. Texas A&M University has also maintained a breeding program to develop QPM inbreds and hybrids with normal seed appearance, G maize contributes 15% (representing competitive yield, and adaptation to the southern USA more than 50 million t) of the protein and 20% (Betrán et al., 2003a, 2003b, 2003c). of the calories derived from food crops in the world’s There is an increasing number of elite exotic QPM diet (National Research Council, 1988). In many develinbreds being developed outside the USA. Therefore, oping countries in Latin America, Africa, and Asia, maize characterization and selection for adaptation of these is the staple food and sometimes the only source of subtropical and tropical white and yellow QPM inbreds protein in diet, especially in weaning food for babies. and a systematic introgression into temperate germNormal maize, being deficient in amino acids lysine and plasm could enhance protein quality, increase genetic tryptophan that are essential for monogastric animals variability for quality, improve productivity, and be a and humans, is nutritionally poor with a biological value source of valuable genes for abiotic and biotic stress (BV) of 40 to 57% (Bressani, 1992). High-lysine maize resistance. Some of CIMMYT’s tropical and subtropical with homozygous embryo and endosperm for mutant germplasm with intermediate and early maturity has alleles o2 at the -zeins regulatory gene opaque-2 shows desirable kernel quality characteristics and can signifiabout 60 to 100% increase in lysine and tryptophan and cantly enhance the nutritional value of temperate maize a higher BV (80% as compared with casein). Substitutgermplasm for both food and feed purposes (Vasal, ing normal maize with high-lysine maize on an equal2001). Furthermore, QPM hybrids have been reported weight basis for growing pigs and sows can diminish the to be less susceptible to aflatoxin, a potent carcinogen use of synthetic lysine in animal feeds to maintain proper that causes losses worth millions of dollars in the southamino acid balance (Asche et al., 1985; Burgoon et al., ern USA, than current commercial hybrids (Bhatnagar 1992; Knabe et al., 1992). In the USA, doubling lysine et al., 2003). Introgression of exotic germplasm into temperate adapted maize has been widely emphasized Maize Breeding and Genetics Program, Soil and Crop Sciences Dep., Texas A&M Univ., College Station, TX 77843-2474. Received 12 Nov. Abbreviations: BV, biological value; CIMMYT, International Center 2003. *Corresponding author ([email protected]). for Maize and Wheat Improvement, Mexico; GCA, general combining ability; PH, plant height; QPM, quality protein maize; SA, University Published in Crop Sci. 44:1997–2005 (2004). © Crop Science Society of America of Natal, South Africa; SCA, specific combining ability; TADD, tangential abrasive dehulling device; TAMU, Texas A&M University. 677 S. Segoe Rd., Madison, WI 53711 USA