Genetic analysis, combining ability and yield stability of maize genotypes under maize streak virus prone environments.
Maize (Zea mays L.) is one of the most important food security crops in Tanzania. It is annually cultivated in an area of 2 million hectares representing 45% of the total area allocated to crop production. However, maize yields are very low in the country due to several biotic and abiotic stresses and socio-economic constraints. Among the biotic factors, maize streak disease caused by the maize streak virus (MSV) inflicts significant yield losses reaching up to 100%. Development of farmers’ preferred, high yielding and MSV resistant cultivars is the best strategy to boost maize productivity in Tanzania. Therefore, the objectives of this study were to: (1) determine farmers’ preferred traits of maize and production constraints limiting maize production in the northern areas of Tanzania, (2) determine agro-morphological diversity present among 80 local and introduced maize inbred lines under maize streak virus (MSV) prone environments of the northern zone of Tanzania, (3) assess the genetic diversity and genetic relationship among 79 maize inbred lines collected from five different origins using 30 polymorphic simple sequence repeat (SSR) markers, (4) determine combining ability and heterosis for grain yield and related traits and resistance to maize streak virus (MSV) among 10 elite maize inbred lines and their hybrid progenies, and (5) investigate the GXE interaction for grain yield and MSV resistance among newly developed maize hybrids in Tanzania using AMMI and GGE biplot methods. A participatory rural appraisal (PRA) study was conducted in 2012 at Babati, Arumeru and Hai Districts in northern Tanzania. Data were collected involving 500 farmers using structured interviews and focused group discussions (FGD). Results showed that maize was the most important crop in the study areas and ranked first among other food crops. Grain yield potential, disease resistance and drought stress tolerance were farmers preferred traits with relative importance of 71.9, 70.0 and 69.9%, respectively. Through FGD farmers identified ear rot, MSV and common rust as most important diseases affecting maize production. High costs of production inputs and low price of maize were also among the challenges to maize production in the study areas. Knowledge of the farmers’ preferences and production constraints is required by breeders to enhance the productivity of maize in the northern areas of Tanzania. Eighty maize inbred lines were evaluated using ago-morphological traits. Field experiment was established during 2011/2012 at maize streak virus (MSV) prone environment of Ngaramtoni Research Farm of Selian Agricultural Research Institute in northern Tanzania using a 10 x 8 alpha lattice design with two replications. Analyses of variance on seven quantitative traits revealed highly significant (P ≤ 0.001) variations among inbred lines. TL2012-42 and TLl2012-41 were identified as superior lines with grain yields of 3.52 and 2.46 t/ha respectively. These genotypes showed low (< 30%) level of MSV reaction suggesting their suitability for hybrid breeding to achieve high grain yield and MSV resistance. Principal component analysis revealed 68.9% of the total variation explained by four principal components. The Unweighted Pair Group Method with Arithmetic Mean (UPGMA) cluster analysis grouped the inbred lines into nine clusters consistent with their heterotic patterns. The study identified the following inbred lines: TL2012-53 and TL2012-61 from cluster II and TL2012-20, TL2012-70, and TL2012-78 from cluster IV for breeding. Genetic diversity and relationships of 79 maize inbred lines collected from five diverse sources were subjected to SSR analysis using 30 polymorphic markers. The mean numbers of observed and effective alleles were 4.70 and 2.40, respectively. The markers displayed high Shannon’s information index of 0.96 and polymorphic information content (PIC) of 0.51. The mean values of observed and expected heterozygosity among lines were 0.136 and 0.508, respectively. A dendrogram constructed based on UPGMA clustered the inbred lines into three main genetic groups with varied sub-clusters. The principal coordinate analysis (PCA) explained 20.4% of the total genetic variation detected among inbred lines and separated them into two main clusters. Analysis of molecular variance (AMOVA) showed that 72% of the total variation was attributed to differences among inbred lines across locations, 26% of the total variation was due to inbred lines within sub-populations/locations and 2% was attributed to variation between the five geographic origins of inbred lines. The study identified inbred lines such as TL2012-20, TL2012-24 and TL2012-54 (from cluster I) and TL2012-25, TL2012-21 and TL2012-12 (from cluster III) showing genetic difference for hybrid breeding to exploit heterosis. Ten selected inbred lines were crossed and 45 F1 hybrids developed using a 10x10 half diallel mating design. Parents, F1 hybrids and five standard checks were evaluated using a 6 x 10 lattice design with two replications at Ngramtoni, Inyala and Igomelo during 2012/13 and 2013/14. General combining ability (GCA) of parents, specific combining ability (SCA) of hybrids, heritability and heterosis of grain yield and related traits and MSV resistance were calculated. The mean squares of GCA and SCA effects showed significant differences for all the traits except days to 50% anthesis and silking. The SCA effect was important for all traits except for MSV, number of ears per plant and husk cover while the GCA effect was most important for resistance to MSV. Heritability estimates of traits were high associated with high GCA effects. Line TL2012-42 was a good general combiner for grain yield showing highly significant positive GCA effect of 0.695 while lines TL2012-41, TL2012-1 and TL2012-42 had significant negative GCA effects of -10.926, -10.792 and -10.748 respectively for MSV reaction. These inbred lines could be exploited in hybrid breeding to develop high yielding and MSV resistant varieties. Hybrids TL2012-38/TL2012-55 and TL2012-25/TL2012-6 had highest negative significant SCA effect of -10.892 and -19.451, respectively for MSV reactions (desirable direction). Maximum mid-parent heterosis for grain yield was recorded for hybrid TL2012-7/TL2012-38 at 138 while TL2012-25/TL2012-26 had the lowest and negative heterosis of -38.2 for MSV reaction. Crosses TL2012-7/TL2012-42 and TL2012-7/TL2012-68 had significant positive SCA effects for grain yield which can be used for direct production as single cross hybrids or developed further as three way hybrids for large scale production. Genotype by environment interaction (GXE) of grain yield and MSV resistance was investigated among newly developed maize hybrids in Tanzania. Forty five novel single cross hybrids and five standard check three-way cross hybrids were evaluated using a 5x10 alpha lattice design with two replications across six environments. The Additive Main Effects and Multiplicative Interaction (AMMI) and genotype, and genotype by environment (GGE) biplot models were used to assess the magnitude of GXE interaction of grain yield and reaction to MSV disease among test genotypes. Results from the AMMI analysis of variance revealed high (52.06%) contribution of the environmental effect on grain compared to genotypes and GXE interaction which, respectively accounted for 12.4% and 17.76% of the total variation on this trait among hybrids tested. Genotypes and GXE contributed to 12.4% and 17.76% of the total variation of hybrids of this trait, respectively. Genotypes explained 45.52% of the total variation of hybrids for MSV resistance while the contribution of environments was minimal (2.77%). Hybrid G43 was identified with relatively high mean grain yield of 6.70 t/ha with low MSV severity of 31.88% across environments. Experimental hybrids such as G10, G14 and G28 had high yield performance of 6.72, 6.00, and 6.23 t/ha, in that order across environments but with highly susceptible reaction to MSV. Conversely, hybrid G31 expressed low MSV infection but yielded the lowest at each environment. Hybrids such as G23 with low grain yields of 4.84 t/ha, G18 (5.14 t/ha), and G34 (1.94 t/ha) showed relatively low MSV infection levels which are useful genetic resources for resistance breeding. Experimental hybrids with high grain yield and MSV resistance selected in this study are good candidates for direct production or for future three-way hybrid development in Tanzania. Overall, the current study selected valuable maize inbred lines with high combining ability for grain yield and related traits and MSV resistance. Also, new experimental maize hybrids were generated for direct production or further development of three-way hybrids.