Inbred lines generated from 10 maize population developed between 1979 and 2008 were used to investigate the inbreeding depression of tropical maize varieties developed at different breeding eras and evaluated in (optimal and stressed) condition. Across all the environments used for this study, estimates of inbred depression (I) for grain yield which ranged from 15.63% for optimum environment to 35.85 under stem borer infestation, showed differences in the severity of the effects of practicing inbreeding in each of the populations and the different environments. The highest values of inbreeding depression for grain yield were recorded under stem borer infestation. The effect of inbreeding was the most severe for var. DMR-LSR-W under borer infestation and least for DMR-LSR-Y in stress free environment. This is an indication that the responses of the maize populations to inbreeding as well as the rate of attaining homozygosity differed with environments. Across the four different environments under which the genotypes were evaluated, average inbreeding depression for grain yield were greater relative to other traits considered which should be expected since grain yield is a quantitatively inherited trait, governed by many genes each with minor effects.
Development of Provitamin A quality protein maize (PVA-QPM) conferring resistance to maize streak virus (MSV) would reduce the prevalence of the disease, alleviate malnutrition and increase food security in sub-Saharan Africa. This study was conducted to (i) evaluate early maturing PVA-QPM inbreds and their derived testcross hybrids for grain yield, MSV-resistant, carotenoids, and tryptophan contents, (ii) estimate the combining ability of the MSV-resistant PVA-QPM testcrosses for all traits. Five improved open-pollinated MSV resistance cultivars and five early maturing PVA-QPM inbreds were crossed in a half-diallel fashion without reciprocals. The derived forty-five F1 topcrosses, ten parental materials, and two commercial hybrids that served as controls were tested at the Lower Niger River Basin Development Authority in Oke-Oyi, Nigeria, during rain-fed of the year 2019 and 2020. The PVA-QPM inbreds from TZEIORQ 13, TZEIORQ 29, and TZEQI 82 that were crossed with MSV resistance AK-9528-DMRSR and Pop 28 SR appeared potentially useful combinations for enhancing grain yield, MSV-resistant, carotenoids, and tryptophan contents, Testcrossed TZEQI 82 × AK-9528-DMRSR POP 28 SR, TZEQI 82 × ACR. 91 SUWAN-1-SR C1 and TZEQI 82 × IK.91 TZL COMP 3-Y C1 were most outstanding for all measured grain yield and nutrient contents. Additive genetic effects were preponderant over non-additive genetic effects under artificial infection of MSV disease. These three promising MSV resistance PVA-QPM hybrids should be evaluated under on-farm multiplication trials for stability performance and adaptation to reduce malnourishment and food self-sufficiency in SSA. Also, further researches on genomic analyses of the superior testcrosses are desirable.
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