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CIMMYT has developed a wide array of hard endosperm opaque-2 (o2) maize (Zea mays L.) germplasm, through the combined use of two genetic systems involving the o2 gene for improving protein quality and the genetic modifiers of the o2 locus for improving kernel phenotype and ameliorating other o2-associated defects. The objective of this study was to determine the heterotic patterns and combining ability of tropical quality protein maize (QPM) germplasm, and to identify superior germplasm suitable for hybrid development. Ten parents (four QPM pools, five QPM populations, and experimental variety PR 7737) were used in a diallel study in eight environments. Data on grain yield, time to silk, plant height, and endosperm hardness were recorded. General combining ability (GCA) effects were highly significant for all traits; specific combining ability (SCA) effects were significant for time to silk and plant height. Genotype x environment interactions and their partitions were significant for grain yield and endosperm hardness. Pool 24 QPM was the highest-yielding parent (6.48 Mg ha to the -1 and Pool 24 QPM x Population 63 was the highest-yielding cross (6.56 Mg ha to the -1 among hard endosperm parents. PR 7737, a soft endosperm opaque-2, was low yielding as a parent, but performed better in crosses with other hard endosperm parents and showed high heterosis. High-parent heterosis for grain yield was generally low in all crosses except those involving PR 7737 as a parent. Endosperm hardness ratings were intermediate relative to the parents, suggesting polygenic control. Crosses among white endosperm parents generally performed better than crosses among yellow endosperm parents. Crosses among dents and dent x flints yielded higher than flint crosses, but flint parents and their crosses had a superior endosperm modification compared with dents. Populations 62, 63, and PR 7737 showed significant positive GCA effects for grain yield; Pool 23 QPM, Pool 25 QPM, and Populations 62, 64, and 65 had significant negative GCA effects for endosperm hardness. Populations 62 and 63 among white endosperm materials and Population 65 among yellow endosperm could be used for initiating hybrid development work. The broad genetic base provides opportunities for developing intrapopulation interline hybrids.

Information on the combining ability and heterotic patterns of CIMMYT's maize (Zea mays L.) germplasm is of great value to maize breeders worldwide, for hybrid development. The objective of this study was to determine the heterosis and combining ability of CIMMYT's subtropical quality protein maize (QPM) germplasm, and to identify suitable germplasm for hybrid development. Six QPM gene pools and four QPM populations were used to make a 10-parent diallel. Parents and the 45 crosses were tested in three environments in Mexico and three in the USA. Highly significant (P < 0.01) differences were observed among entries, parents, and crosses for grain yield, time to silk, plant height, and endosperm hardness in Mexican environments, and for grain yield, ear height, and time to silk in U.S. environments. General combining ability (GCA) effects were highly significant for all traits, while specific combining ability (SCA) effects were nonsignificant (P > 0.05). Population 68 QPM was the highest-yielding parent (6.99 Mg ha to the -1 in Mexican environments, and Pool 31 QPM was the highest yielding (4.65 Mg ha to the -1 in U.S. environments. The highest-yielding crosses were Pool 32 QPM x Population 68 QPM (7.05 Mg ha to the -1 in Mexican and Pool 29 QPM x Population 70 QPM (5.12 Mg ha to the -1 in U.S. environments. Under Mexican environments, Population 68 QPM and Pool 32 QPM among whites and Population 69 QPM and Pool 34 QPM among yellows were the best general combiners for grain yield. For endosperm hardness, the best combiners were Pool 33 QPM, Population 69 QPM, and Pool 31 QPM, all of which have flint grain type. Under temperate U.S. conditions, the two early-maturity gene pools (Pool 27 QPM and Pool 29 QPM) and Population 70 QPM were the best combiners for grain yield. For subtropical conditions, Population 68 QPM, Pool 32 QPM, Population 69 QPM, and Pool 34 QPM hold potential for hybrid development. These are hard endosperm opaque-2 (o2) parents (except for Pool 34 QPM, a partially modified parent). For temperate breeding programs in the USA, Pool 27 QPM, Pool 29 QPM, and Population 70 QPM could be used as exotic germplasm sources for introgression into U.S. Corn Belt QPM materials and for developing hard endosperm QPM hybrids.

Crossing maize (Zea mays L.) germplasm from different adaptation groups could result in better utilization of hybrid vigor. Such heterotic response is rarely exploited, due to problems in adaptation of some of these exotic materials. This study was conducted to determine the heterosis and combining ability between tropical and subtropical maize germplasm developed at CIMMYT. Six tropical and five subtropical populations were crossed in a Design-2 mating scheme; the 30 crosses and 11 parents were evaluated in six environments. Data were recorded on days to silk, plant height, and grain yield. Midparent heterosis was calculated for the crosses and general combining ability effects (GCA) estimated for grain yield. The six environments were classified into three mega-environments: tropical (TR), subtropical (ST), and transition and midaltitude (TM). Days to silk was lowest in TR environments (55.4 d), followed by ST (64.6 d) and TM (71.7 d) environments. Significant differences in grain yield were observed for entries, parents, crosses, and GCA effects under TR and ST environments. The three top-yielding crosses that also showed high heterosis were Population 43 x Population 42 (7.43 Mg ha to the -1 power), Population 32 x Population 44 (8.31 Mg ha to the -1 power), and Population 22 x Population 47 (7.97 Mg ha to the -1 power) under TR, ST, and TM environments, respectively. Population 43 among tropical germplasm and Populations 42 and 44 among subtropical germplasm showed positive GCA for yield; both are ideal for hybrid development work. Population 47, which involves temperate germplasm, was a good combiner in TM environments. The results demonstrate the considerable potential that exists for improving the yield performance of subtropical germplasm by crossing to tropical materials; for the tropics, introgressing tropical with subtropical germplasm could be used to improve plant type and maturity. The well-known Tuxpeno x ETO heterotic pattern, observed in many crosses, could be successfully exploited in many of these materials for hybrid development.

Knowledge about the combining ability and heterotic patterns among CIMMYT's maize (Zea mays L.) germplasm is essential for hybrid development work at CIMMYT, as well as at other national research programs using CIMMYT germplasm. This study was conducted to determine the heterosis and combining ability among CIMMYT's subtropical and temperate early-maturity maize germplasm. A seven-parent diallel involving two populations and five gene pools was made. The parents and 21 crosses were evaluated in 17 temperate and 5 subtropical environments during 1985-1986. Average yield across temperate environments (4.35 Mg ha to the -1) was comparable to that obtained in subtropical environments (4.59 Mg ha to the -1). Highest yield in subtropical environments was recorded by Population 48 x Pool 27 (5.42 Mg ha to the -1), with a high-parent heterosis of 9.9%. Maximum high-parent heterosis was observed in Population 46 x Pool 30 (13%), which yielded 5.17 Mg ha to the -1. Under temperate environments, the highest-yielding combinations included Population 48 crossed with Population 46 and Pools 27, 28, and 30, although maximum heterosis (10.2%) was recorded for Pool 27 x Pool 40. General combining ability (GCA) effects for yield were significant in both sets of environments, while specific combining ability (SCA) effects were significant only in temperate environments. Highly significant positive GCA effects for yield were observed with Population 48 (0.43 Mg ha to the -1) and Pool 30 (0.33 Mg ha to the -1) under temperate conditions. Pools 40 and 42 were poor general combiners in both sets of environments. Significant positive SCA effects for yield were observed with Population 46 x Population 48 and Population 46 x Pool 30 crosses in temperate environments and the Population 48 x Pool 27 cross under subtropical environments. Population 48 and Pool 30 may hold potential for use as source germplasm for both temperate and subtropical maize breeding programs.

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