Maize is susceptible to biotic and abiotic stresses. The most important abiotic stresses in Africa are drought and low soil fertility. Aflatoxin contamination is a potential problem in areas facing drought and low soil fertility. Three studies were conducted to evaluate maize germplasm for tolerance to stress. In the first study, fifteen maize inbred lines crossed in a diallel were evaluated under drought, low N stress, and well-watered conditions at six locations in three countries to estimate general (GCA) and specific combining ability (SCA), investigate genotype x environment interaction, and estimate genetic diversity and its relationship with grain yield and heterosis. GCA effects were not significant for grain yield across environments. Lines with good GCA effect for grain yield were P501 and CML258 across stresses. Lines CML339, CML341, and SPLC7-F had good GCA effects for anthesis silking interval across stresses. Additive genetic effects were more important for grain yield under drought and well-watered conditions. Heterosis estimates were highest in stress environments. Clustering based on genetic distance calculated using marker data from AFLP, RFLP, and SSRs grouped lines according to origin. Genetic distance was positively correlated with grain yield and specific combining ability. In the second study, synthetic hybrids were evaluated at seven locations in three countries to estimate GCA and SCA effects under low N stress and optimal conditions and investigate genotype x environment interaction. GCA effects were significant for all traits across low N stress and optimal conditions. The highest yielding synthetic hybrids involved synthetics developed from stress tolerant lines. Synthetics 99SADVIA-# and SYNA00F2 had good GCA for grain yield across low N stress conditions. Heterosis was highly correlated with grain yield. Optimal environments explained more variation than stress environments. The third study evaluated the agronomic performance and aflatoxin accumulation of single and three-way cross white maize hybrids at five locations in Texas. Inbreds CML343, Tx601W, and Tx110 showed positive GCA effects for grain yield. Significant GCA effects for reduced aflatoxin concentration were observed in lines CML269, CML270, and CML78 across locations. Differences in performance between single and three-way crosses hybrids were dependent mostly on the inbred lines.

Maize is susceptible to biotic and abiotic stresses. The most important abiotic stresses in Africa are drought and low soil fertility. Aflatoxin contamination is a potential problem in areas facing drought and low soil fertility. Three studies were conducted to evaluate maize germplasm for tolerance to stress. In the first study, fifteen maize inbred lines crossed in a diallel were evaluated under drought, low N stress, and well-watered conditions at six locations in three countries to estimate general (GCA) and specific combining ability (SCA), investigate genotype x environment interaction, and estimate genetic diversity and its relationship with grain yield and heterosis. GCA effects were not significant for grain yield across environments. Lines with good GCA effect for grain yield were P501 and CML258 across stresses. Lines CML339, CML341, and SPLC7-F had good GCA effects for anthesis silking interval across stresses. Additive genetic effects were more important for grain yield under drought and well-watered conditions. Heterosis estimates were highest in stress environments. Clustering based on genetic distance calculated using marker data from AFLP, RFLP, and SSRs grouped lines according to origin. Genetic distance was positively correlated with grain yield and specific combining ability. In the second study, synthetic hybrids were evaluated at seven locations in three countries to estimate GCA and SCA effects under low N stress and optimal conditions and investigate genotype x environment interaction. GCA effects were significant for all traits across low N stress and optimal conditions. The highest yielding synthetic hybrids involved synthetics developed from stress tolerant lines. Synthetics 99SADVIA-# and SYNA00F2 had good GCA for grain yield across low N stress conditions. Heterosis was highly correlated with grain yield. Optimal environments explained more variation than stress environments. The third study evaluated the agronomic performance and aflatoxin accumulation of single and three-way cross white maize hybrids at five locations in Texas. Inbreds CML343, Tx601W, and Tx110 showed positive GCA effects for grain yield. Significant GCA effects for reduced aflatoxin concentration were observed in lines CML269, CML270, and CML78 across locations. Differences in performance between single and three-way crosses hybrids were dependent mostly on the inbred lines.

A obtenção de híbridos de milho está relacionada com o aumento de produtividade dessa cultura. Para isso, normalmente são realizados cruzamentos entre linhagens de diferentes grupos heteróticos, que são determinados pelos melhoristas de forma que seja maximizada a divergência entre eles. A escolha dos genitores a serem cruzados pode ser facilitada pelo uso de cruzamentos dialélicos. Os modelos usados para a análise dos dialélicos permitem a estimação de parâmetros úteis para a seleção dos genitores e o estabelecimento de grupos heteróticos, sendo que os efeitos genéticos normalmente podem ser considerados como aleatórios. A forma tradicional de análise inclui tais efeitos na matriz de incidência dos efeitos fixos e emprega o método dos mínimos quadrados ordinários, o que impossibilita a análise usando a metodologia dos modelos mistos. Os objetivos deste trabalho foram comparar os resultados das análises dialélicas obtidas considerando o modelo fixo e o modelo misto e avaliar a eficiência do melhor preditor linear não viesado (BLUP) para predição de cruzamentos não realizados entre linhagens de milho, utilizando informações de marcadores moleculares RFLP para a estimação da matriz de parentesco. Foram considerados dados de 80 híbridos interpopulacionais e 20 testemunhas comerciais, avaliados em um látice 10 x 10 em três locais. Esses híbridos foram obtidos pelo cruzamento entre 8 linhagens do grupo heterótico BR-105 e 10 do grupo BR-106, as quais foram genotipadas com marcador RFLP. A análise dialélica foi realizada segundo a metodologia que considera o modelo genético como fixo e usa o método dos mínimos quadrados ordinários, e também usando a metodologia de modelos mistos, assumindo que a capacidade geral de combinação (CGC) dos dois grupos heteróticos e a capacidade específica de combinação (CEC) representam efeitos aleatórios. Além disso, foi avaliada a predição de híbridos simples não realizados, com base na matriz de covariâncias genéticas entre o híbrido não realizado e os híbridos preditores, usando as informações do marcador RFLP. Como todos os híbridos foram obtidos, foi simulada a retirada de cada um deles do conjunto, sendo sua performance predita a partir dos 79 restantes. Os resultados mostraram que, na comparação entre as análises dialélicas obtidas com as duas metodologias, embora os valores da CGC das linhagens da população BR-106 e da CEC tenham baixa correlação entre as duas metodologias (r=-0,11 e r=-0,06, respectivamente), a classificação dos híbridos mais produtivos sofre poucas alterações (r=0,99) e não causa dificuldades na seleção. A eficiência do BLUP para predição de cruzamentos não realizados entre linhagens de milho, utilizando informações de marcadores moleculares para a estimação da matriz de parentesco, mostrou a existência de moderada correlação entre os valores dos BLUP’s da produção de grãos e os valores preditos a partir dos híbridos observados restantes (r=0,35), o que indica que essa metodologia tem capacidade em predizer a estimativa de valores não observados, porém com alguma imprecisão. Os mesmos resultados foram observados entre as capacidades específicas de combinação observadas e as preditas (r=0,30). Conclui-se que a metodologia de modelos mistos pode ser usada, desde que com ressalvas, dada as correlações moderadas. / The obtainment of maize single crosses is related with increasing in productivity. To do so, inbred lines derived of different heterotic groups are crossed. These groups are established in such a way that the genetic divergences among them are maximized. Using diallel crosses can facilitate the choice of the inbred lines to be crossed. The models used to perform these analyses allow estimating genetic parameters that are useful in the selection of parental lines and in determining the heterotic groups. In these models, generally, the genetic effects are considered as random. However, these analyses are usually performed considering these effects in the matrix of fixed effects, and obtaining the parameter estimates according to the ordinary least squares method, making impossible using mixed linear models theory. The aims of this research were to compare the results of diallel analyses obtained by using fixed and mixed models, and evaluate the efficiency of the best linear unbiased predictor (BLUP) in predicting non-realized single crosses. Eighty interpopulation hybrids and twenty commercial checks were evaluated in a 10 x 10 lattice design with two replications located in three environments. These single crosses were obtained by crossing eight and ten inbred lines from BR-105 and BR-106 heterotic groups, respectively. These eighteen lines were genotyped by using RFLP molecular markers and then the coefficient of parentage among them was estimated. The genetic parameters general (GCA) and specific (SCA) combining abilities were estimated through diallel analyses, considering the linear model as fixed and using the ordinary least squares as the estimation method. Besides, GCA and SCA were predicted using BLUP and assuming the genetic effects as random. Besides, the prediction of non-realized single crosses was evaluated. To do so, each one of the 80 realized hybrids was predicted based on information of the others 79 single crosses and using the genetic variance-covariance matrix estimated using RFLP information. According to the results, it was found poor correlation between estimatives and predictions obtained considering the model as fixed or mixed, for GCA within BR-106 population (r=-0.11) and for SCA (r=-0.06). However, the ranking of the single crosses for productivity did not change (r=0.99) and did not make selection process more difficult. The efficiency of BLUP in predicting unrealized single crosses was moderated since the correlation between observed and predicted values was r=0.35, indicating some imprecision in these predictions. Similar results were obtained when comparing observed and predicted SCAs (r=0.30). It was possible to conclude that BLUP can be useful in diallel analyses and in prediction of single crosses, but some caution have to be account since the correlations presented moderate values.

A obtenção de híbridos de milho está relacionada com o aumento de produtividade dessa cultura. Para isso, normalmente são realizados cruzamentos entre linhagens de diferentes grupos heteróticos, que são determinados pelos melhoristas de forma que seja maximizada a divergência entre eles. A escolha dos genitores a serem cruzados pode ser facilitada pelo uso de cruzamentos dialélicos. Os modelos usados para a análise dos dialélicos permitem a estimação de parâmetros úteis para a seleção dos genitores e o estabelecimento de grupos heteróticos, sendo que os efeitos genéticos normalmente podem ser considerados como aleatórios. A forma tradicional de análise inclui tais efeitos na matriz de incidência dos efeitos fixos e emprega o método dos mínimos quadrados ordinários, o que impossibilita a análise usando a metodologia dos modelos mistos. Os objetivos deste trabalho foram comparar os resultados das análises dialélicas obtidas considerando o modelo fixo e o modelo misto e avaliar a eficiência do melhor preditor linear não viesado (BLUP) para predição de cruzamentos não realizados entre linhagens de milho, utilizando informações de marcadores moleculares RFLP para a estimação da matriz de parentesco. Foram considerados dados de 80 híbridos interpopulacionais e 20 testemunhas comerciais, avaliados em um látice 10 x 10 em três locais. Esses híbridos foram obtidos pelo cruzamento entre 8 linhagens do grupo heterótico BR-105 e 10 do grupo BR-106, as quais foram genotipadas com marcador RFLP. A análise dialélica foi realizada segundo a metodologia que considera o modelo genético como fixo e usa o método dos mínimos quadrados ordinários, e também usando a metodologia de modelos mistos, assumindo que a capacidade geral de combinação (CGC) dos dois grupos heteróticos e a capacidade específica de combinação (CEC) representam efeitos aleatórios. Além disso, foi avaliada a predição de híbridos simples não realizados, com base na matriz de covariâncias genéticas entre o híbrido não realizado e os híbridos preditores, usando as informações do marcador RFLP. Como todos os híbridos foram obtidos, foi simulada a retirada de cada um deles do conjunto, sendo sua performance predita a partir dos 79 restantes. Os resultados mostraram que, na comparação entre as análises dialélicas obtidas com as duas metodologias, embora os valores da CGC das linhagens da população BR-106 e da CEC tenham baixa correlação entre as duas metodologias (r=-0,11 e r=-0,06, respectivamente), a classificação dos híbridos mais produtivos sofre poucas alterações (r=0,99) e não causa dificuldades na seleção. A eficiência do BLUP para predição de cruzamentos não realizados entre linhagens de milho, utilizando informações de marcadores moleculares para a estimação da matriz de parentesco, mostrou a existência de moderada correlação entre os valores dos BLUP’s da produção de grãos e os valores preditos a partir dos híbridos observados restantes (r=0,35), o que indica que essa metodologia tem capacidade em predizer a estimativa de valores não observados, porém com alguma imprecisão. Os mesmos resultados foram observados entre as capacidades específicas de combinação observadas e as preditas (r=0,30). Conclui-se que a metodologia de modelos mistos pode ser usada, desde que com ressalvas, dada as correlações moderadas. / The obtainment of maize single crosses is related with increasing in productivity. To do so, inbred lines derived of different heterotic groups are crossed. These groups are established in such a way that the genetic divergences among them are maximized. Using diallel crosses can facilitate the choice of the inbred lines to be crossed. The models used to perform these analyses allow estimating genetic parameters that are useful in the selection of parental lines and in determining the heterotic groups. In these models, generally, the genetic effects are considered as random. However, these analyses are usually performed considering these effects in the matrix of fixed effects, and obtaining the parameter estimates according to the ordinary least squares method, making impossible using mixed linear models theory. The aims of this research were to compare the results of diallel analyses obtained by using fixed and mixed models, and evaluate the efficiency of the best linear unbiased predictor (BLUP) in predicting non-realized single crosses. Eighty interpopulation hybrids and twenty commercial checks were evaluated in a 10 x 10 lattice design with two replications located in three environments. These single crosses were obtained by crossing eight and ten inbred lines from BR-105 and BR-106 heterotic groups, respectively. These eighteen lines were genotyped by using RFLP molecular markers and then the coefficient of parentage among them was estimated. The genetic parameters general (GCA) and specific (SCA) combining abilities were estimated through diallel analyses, considering the linear model as fixed and using the ordinary least squares as the estimation method. Besides, GCA and SCA were predicted using BLUP and assuming the genetic effects as random. Besides, the prediction of non-realized single crosses was evaluated. To do so, each one of the 80 realized hybrids was predicted based on information of the others 79 single crosses and using the genetic variance-covariance matrix estimated using RFLP information. According to the results, it was found poor correlation between estimatives and predictions obtained considering the model as fixed or mixed, for GCA within BR-106 population (r=-0.11) and for SCA (r=-0.06). However, the ranking of the single crosses for productivity did not change (r=0.99) and did not make selection process more difficult. The efficiency of BLUP in predicting unrealized single crosses was moderated since the correlation between observed and predicted values was r=0.35, indicating some imprecision in these predictions. Similar results were obtained when comparing observed and predicted SCAs (r=0.30). It was possible to conclude that BLUP can be useful in diallel analyses and in prediction of single crosses, but some caution have to be account since the correlations presented moderate values.