Genotypic variability and combining ability of quality protein maize inbred lines under stress and optimal conditions
Gissa, Dagne Wegary
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Maize is the dominant staple crop in most regions of Africa. The nutritional value of maize protein, however, is deficient in the essential amino acids such as lysine and tryptophan. This study was conducted to (1) investigate the level of variability among elite QPM inbred lines, (2) analyze combining ability of the inbred lines and G x E interaction of the resulting hybrids under stress and optimal conditions, (3) study the associations of parental genetic distances with F1 performance, heterosis and SCA of the hybrids under stress and optimal environments, and (4) investigate the effects of low N stress on endosperm modification, protein quantity and quality of QPM and identify best donor parents. The genetic variability of 35 maize inbred lines (32 QPM and three normal maize line checks) was studied using morpho-agronomic traits and SSR markers. Both methods indicated genetic variability among the lines. UPGMA cluster analysis based on morphoagronomic data grouped the inbred lines into four clusters and two outliers mainly based on grain yield, days to anthesis, plant height and leaf area. SSR markers grouped the inbred lines into six clusters which were different from the morpho-agronomic clustering. SSR markers grouped the inbred lines more efficiently in accordance with pedigree relationships. A diallel analysis of 15 QPM inbred lines showed significant GCA and SCA mean squares for most traits under optimal environments. GCA effects were significant while SCA effects were not significant for most traits across sites with drought and low N stress. Additive genetic effects were important under stress, and both additive and non-additive genetic effects were important under optimal conditions. Inbred lines VL054178, VL05482, VL05561, VL05483, CML511, CML159, CML491 and VL06375, which had good GCA effects for most traits under stress and non-stress conditions can potentially be used in QPM breeding programs in Africa and similar environments worldwide. Mean MPH ranged from -9.1% for days to silking to 112.7% for grain yield and HPH ranged from -12.0% for days to silking to 89.8% for grain yield. All the crosses showed negative MPH and HPH for days to anthesis and silking, and positive MPH for plant and ear height, number of kernels per row and kernels per ear. SSR marker-based genetic distance was positively and highly significantly correlated with grain yield; and negatively and highly significantly correlated with days to anthesis and silking. The correlations of SSR marker distance with heterosis and SCA were low to be of predictive value. Morphological distances were not useful in predicting heterosis and SCA effects of hybrids. Environment affected the correlations of genetic distance with hybrid performance, heterosis and SCA, with lower values under stress conditions. AMMI stability value and linear regression models were positively correlated in ranking the stability of QPM hybrids. AMMI analysis based on inbred line means and selected hybrids clearly discriminated the genotypes on the base of adaptation patterns. Hierarchical clustering based on hybrid grain yield and inbred line means grouped the 13 environments mainly according to geographical location and prevailing growing conditions. QPM hybrids showed higher levels of tryptophan content and protein quality index than the normal maize hybrids under both low N stress and optimal conditions. Low N stress increased the frequency of soft or poorly modified grains and decreased tryptophan and protein concentration in grain and increased protein quality index. Tryptophan concentration was more stable than protein concentration across low N stress and optimal conditions. Additive gene effects were primarily responsible for variation in endosperm modification and protein quality in the QPM inbred lines. Inbred lines VL05200, VL05468, VL054178 and CML144 were the best general combiners for endosperm modification; hence can be used as o2 donor parents. Information from the genetic diversity analyses of the inbred lines can be used for effective utilization of the inbred lines in the breeding programs for the formation of heterotic populations and development of desirable varieties. The inbred lines used in this study, in general, were found to be useful sources for genetic variability for the development of new genotypes for stress tolerance and the study confirmed the possibility of achieving good performances across stress and non-stress conditions in QPM germplasm. However, more breeding efforts should be devoted to the development of QPM inbred lines with better field performance and acceptable levels of protein quality and quantity under both stress and optimal conditions. Inbred lines identified as good o2 donors can be used for the conversion of well adapted normal maize genotypes into QPM counterparts.