Print version ISSN 0103-9016
Sci. agric. vol.57 n.1 Piracicaba Jan./Mar. 2000
Combining ability of maize inbred lines evaluated in three environments in Brazil
Luciano Lourenço Nass1*; Marlene Lima2; Roland Vencovsky3; Paulo Boller Gallo2
1Embrapa Recursos Genéticos e Biotecnologia, SAIN Parque Rural - CEP: 70770-900 - Brasília, DF.
2IAC, C.P. 28 - CEP: 13001-970 - Campinas, SP.
3Depto. de Genética - USP/ESALQ, C.P. 83 - CEP: 13400-970 - Piracicaba, SP.
*Corresponding author <firstname.lastname@example.org>
ABSTRACT: For tropical areas like Brazil, genotype x environment interaction is an important source of variation. Our objectives were to examine the combining abilities and their interaction with environments in ten selected maize (Zea mays L.) inbred lines in diallel crosses and to identify candidates for promising hybrid combinations. Crosses and checks were evaluated through a 7 x 7 triple lattice design at three locations during the 1984/85 season. Several agronomic traits were evaluated, but here only data on ear yield are discussed. Yield data were adjusted for stand variation (correction to 50 plants per plot) and grain moisture (15.5%). Data for ear yield were analysed using an adaptation of Griffings method IV for complete diallel crosses, and experiments were repeated in several environments. The means of the crosses over all environments ranged from 6.6 t/ha to 10.3 t/ha. The combining ability analysis of diallel data across environments showed highly significant (P<0.01) effects for environment, general combining ability (GCA), specific combining ability (SCA), and GCA x environment interaction. The SCA x environment interaction was not significant. Results showed that both GCA and SCA were important for this diallel cross. Considering the 13 hybrids that showed higher yields than the commercial hybrid controls, contributions by GCA and SCA effects were 56% and 44%, respectively. On the other hand, selecting only the best five hybrids, SCA effects were always more important than GCA effects for each environment and over all environments.
Key words: diallel analysis, combining ability, genotype x environment interaction
Capacidade de combinação de linhagens de milho avaliadas em três ambientes do Brasil
RESUMO: Em regiões tropicais a interação genótipo x ambiente é uma importante fonte de variação. Esse estudo teve por objetivos avaliar em cruzamentos dialélicos as capacidades de combinação e suas interações com ambientes e identificar as combinações híbridas mais promissoras entre dez linhagens selecionadas de milho (Zea mays L.). Os híbridos e as testemunhas foram avaliados no delineamento látice triplo 7 x 7 em três ambientes durante o ano agrícola de 1984/85. Nesse trabalho são discutidos apenas os dados de produção de grãos, os quais foram corrigidos para a variação de estande (50 plantas por parcela) e umidade de grão (15,5%). Os dados de peso de espigas foram analisados utilizando-se uma adaptação do método IV de Griffing, no qual os cruzamentos obtidos no dialélico completo são avaliados em vários ambientes. A média dos híbridos considerando todos os ambientes variou de 6,6 t/ha a 10,3 t/ha. Na análise dialélica conjunta foram detectadas diferenças altamente significativas (P<0,01) para ambientes, capacidade geral de combinação (CGC), capacidade específica de combinação (CEC) e para a interação CGC x ambientes; a interação CEC x ambientes não foi significativa. Os resultados obtidos mostraram que tanto CGC como CEC foram importantes para esse conjunto de híbridos. Para os 13 híbridos mais produtivos as contribuições dos efeitos da CGC e CEC foram 56% e 44%, respectivamente. Por outro lado, tomando-se apenas os cinco melhores híbridos, os efeitos da CEC foram sempre mais expressivos em relação aos efeitos da CGC, para cada híbrido e na média dos ambientes.
Palavras-chave: análise dialélica, capacidade de combinação, interação genótipo x ambiente
Maize is grown from 58°N latitude without interruption through the temperate, subtropical, and tropical regions of the world to 40°S latitude (Hallauer & Miranda Filho, 1988). Several types of hybrids are possible in maize, however the most commom ones used for commercial production are derived from inbred lines (Nass & Miranda Filho, 1995).
The concepts of general combining ability (GCA) and specific combining ability (SCA) defined by Sprague & Tatum (1942) have been used extensively in breeding of several economic crop species. For maize yield, they found that GCA was relatively more important than SCA for unselected inbred lines, whereas SCA was more important than GCA for previously selected lines. Rojas & Sprague (1952) compared estimates of the variances of GCA and SCA for yield and their interaction with locations and years. They stressed that the variance of SCA includes not only the non-additive deviations due to dominance and epistasis but also a considerable portion of the genotype x environment interaction. Matzinger et al. (1959) defined and interpreted GCA and SCA and their interactions with environments for the diallel mating design. The concepts of GCA and SCA became useful for characterization of inbred lines in crosses and often have been included in the description of an inbred line (Hallauer & Miranda Filho, 1988).
The International Maize and Wheat Improvement Center (CIMMYT) has used measures of GCA and SCA effects to establish heterotic patterns among its maize populations and pools (Beck et al., 1990; Crossa et al., 1990; Han et al., 1991; Vasal et al., 1992). Although both inbred and non-inbred progenitors can be used to form new heterotic groups, inbred progenitors will provide better source germplasm suitable for hybrid development. New synthetic populations developed from inbred lines, in general, have lower inbreeding depression and tend to be promising sources of new superior inbred lines (Vasal et al., 1992).
Sprague & Eberhart (1977) recommended two replications per location and three to five environments for evaluation of maize crosses, because the additive by environment interaction is usually a significant factor. Increasing the number of environments reduces the contribution of both the pooled error and the additive by environment interaction to the phenotypic variance, whereas increasing replications only reduces the pooled error contribution (Eberhart et al., 1995). Characteristics of temperate and tropical areas for maize production are quite different. Thus, Brewbaker (1985) suggested multiple-season testing in the tropics instead of multiple-location as is usual in temperate areas. According to Paterniani (1990) this alternative represents more properly the situation of farmers, reduces cost, results in greater precision of the field trials, and also simulates different years within a single year.
This study was carried out to evaluate the combining ability patterns of a diallel cross involving 10 selected inbred lines; to measure the interaction of these effects with environments; and to provide informations for selection of superior hybrid combinations.
MATERIAL AND METHODS
Seventy S2 lines derived from Suwan-DMR, a downy mildew (Peronosclerospora sorghi) resistant population, were introduced from Thailand into the maize breeding program of the Instituto Agronômico de Campinas (IAC), Brazil. After three additional generations of selfing, the most promising ten lines were intercrossed in a diallel scheme to obtain forty five single cross hybrids. The inbred lines were numbered as follows: (1) L34, (2) L42, (3) L31, (4) L29, (5) L32, (6) L40, (7) L35, (8) L43, (9) L38, and (10) L37.
The 45 single crosses, two commercial double cross hybrids (C-511 and AG-401), and two open-pollinated varieties (IAC-Taiúba and Suwan-2) were evaluated using a 7 x 7 triple lattice design at three locations during the 1984/85 growing season. These experiments were conducted by the Experimental Stations of the Instituto Agronômico de Campinas (IAC) and Coordenadoria de Assistência Técnica Integral (CATI) at São Paulo State - Brazil. Locations used were Campinas (22°S 47°W), Manduri (23°S 49°W), and Mococa (21°S 47°W).
Plots were 10m long, 1.0m apart, and with 0.40m between hills (two plants per hill), with 50 plants per plot after thinning. Several agronomic traits were evaluated, but in this report only data on ear yield are discussed. Yield data were adjusted for stand variation (correction to 50 plants per plot) and grain moisture (15.5%).
Data for ear yield were analysed using an adaptation of Griffings method IV for complete diallel crosses and experiments repeated in several environments (Ferreira et al., 1993). This methodology is based on the following mathematical model:
Yiik = m + lk + gi + gi + sii + (lg)ik +(lg)ik + (ls)iik + eiik
where Yiik is the mean over replications of the single cross (i x i) in the kth environment; m is the overall mean; lk is the kth environment effect; gi, gi, and sii are general and specific combining ability effects as described by Griffing (1956); eiik is the error term; and the remaining parameters correspond to interactions of the main effects with environments.
RESULTS AND DISCUSSION
The means of the crosses over all environments (TABLE 1) ranged from 6.553 (L34 x L29) to 10.345 t/ha (L31 x L40). The means of the commercial hybrids and the open-pollinated varieties used as checks were of 9.275 and 8.794 t/ha, respectively. Thirteen hybrids had greater overall means than hybrid checks. These results show the potential of these specific hybrid combinations for ear yield. The individual analysis of variance for ear yield showed highly significant treatment differences (P<0.01) for all environments (data not shown). Significant differences (P<0.05 and P<0.01) between environments, genotypes, and genotype x environment interactions were detected in the combined analysis (TABLE 2). Crosses showed highly significant differences but checks and groups did not. Genotypes evaluated in this study were single crosses while both of the commercial hybrid checks were double crosses.
Hallauer & Miranda Filho (1988) pointed out that external environmental factors such as weather, soil, and pests probably have a greater effect on the single crosses than another types of hybrids. Single cross hybrids usually interact more with the environment than double cross hybrids (Troyer, 1996). The results for all interactions observed in the combined analysis suggest that the hybrids did not have the same relative performance across locations. The highest mean for ear yield (9.179 t/ha) was obtained at Campinas, which also had the best experimental precision (CV = 8.8%).
The combining ability analysis of diallel data across environments showed highly significant effects (P<0.01) for environments, GCA, SCA, and GCA x environment; the SCA x environment effects were not significant (TABLE 3). Significant GCA and GCA x environment effects suggest the need of selecting different parental lines for hybrids at specific environments. Previous investigations have shown that both GCA and SCA can interact with environments (Rojas & Sprague, 1952; Matzinger et al., 1959; Paroda & Hayes, 1971; Pixley & Bjarnason, 1993). Everett et al. (1995) evaluated the optimal combining ability patterns among promising source populations for inbred line development in tropical mid-altitude zones and detected highly significant differences for GCA, SCA, and GCA x environment interaction. Our results show that GCA and SCA were important for this set of 10 inbred lines. Consequently, we have to consider the average performance of a line in hybrid combinations and the specific hybrid combinations. Gama et al. (1995) observed the same trend in a diallel involving 11 inbred lines derived from a Tuxpeño variety in Brazil.
Line 3 (L31) and line 4 (L29) had the largest positive and negative GCA effects, respectively (TABLE 4). These lines showed the same relative performance for GCA effects in 57 top-crosses using the population Amarillo Dentado as tester (M. Lima, unpublished results). Half the parental lines had positive GCA effects, indicating that on average these parents contributed to increase yield in crosses. The largest positive and negative SCA effects were observed with line 5 (L32) x line 8 (L43) and line 6 (L40) x line 7 (L35) crosses, respectively. Han et al. (1991), Vasal et al. (1992), and Gama et al. (1995) reported that, on average, crosses produced by crossing interpopulation lines have more positive SCA effects than those produced by crossing intrapopulation lines which tend to have more negative SCA effects.
Considering the 13 hybrids that showed higher yields than the hybrid checks, the contributions of the GCA and SCA effects were approximately 56% and 44%, respectively. These results showed that for the best single crosses both, GCA and SCA effects were important for ear yield. TABLE 5 shows the relative contribution of the GCA and SCA effects considering the top ten and top five superior hybrids for each and over all environments. For the top ten, GCA effects were slightly higher for all situations, except for environment 2, where the SCA effects were more important. On the other hand, selecting the best five superior hybrids for ear yield, SCA effects were always more important than GCA effects for all cases. These results show the great contribution of the SCA effects for the most promising hybrid combinations. Maize breeders breed and test to identify that unique combination of inbred lines for high grain yield; i.e., SCA is expressed although the GCA was probably more important in identifying the lines for the unique combination. Non-additive gene effects seem to be small on the average, but they may be important for specific combinations (Hallauer & Miranda Filho, 1988).
Genetically, the GCA effects obtained in one environment contain the average GCA effect and the GCA x environment interaction. Consequently, when estimates of GCA x environment interaction are near zero, the average GCA effect will approach the GCA effects obtained for each environment. Lines 3 (L31), 10 (L37), 9 (L38), and 2 (L42) showed the best average GCA effects (TABLE 6). The best estimates of GCA in Campinas and Manduri were for lines 3 (L31), 2 (L42), and 10 (L37); and for Mococa they were for lines 9 (L38), 8 (L43), and 10 (L37). These results suggest that this set of inbred lines changed its performance in crosses for ear yield in Mococa. This location showed the worst climate and soil conditions among all environments evaluated. The estimates of environmental effects (TABLE 4) reflect the adverse contribution of this environment.
The choice of the best parents based on the average GCA effects can be done if there is interest in single cross hybrids adapted to all environments. However, the GCA x environments interaction indicates that the best GCA effects were not the same over all environments. Thus, to maximize the hybrid yield potential for each environment the choice must be made with GCA effects within each environment. The most promising hybrids were line 6 (L40) x line 10 (L37), line 3 (L31) x line 6 (L40), and line 8 (L43) x line 10 (L37) for Campinas, Manduri, and Mococa, respectively. This is an important decision because tropical environmental conditions are quite different from the conditions of temperate areas. Paterniani (1990) discussed several characteristics of temperate and tropical areas for maize production such as growing conditions, types of maize plant and infrastructure, and stated that the problems facing maize cultivation in the tropics are more numerous and are of greater magnitude and more challenging than in temperate areas. For tropical regions like Brazil, Miranda Filho (1985) emphasized that apart from the great variations as determined by latitude, daylength and temperature, there is an expressive variation among locations, even if they are not far one from another, that makes genotype by environment interaction an important source of variation.
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Received September 02, 1999