GENETICS OF CAROTENOIDS IN TROPICAL-ADAPTED PROVITAMIN A MAIZE (ZEA MAYS L.)
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Department of Agricultural Science
ABSTRACT
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| Vitamin A is a group of fat-soluble vitamins essential for life and health. It plays vital role in vision, epithelial tissue protection, bone and brain development, and immune system functions. β-carotene (βC), β-cryptoxanthin (βCX) and α-carotene (αC) are vitamin A precursors found in plants and are referred to as provitamin A (PVA). Maize contains considerable levels of provitamin A making it an important crop to combat vitamin A deficiency through biofortification. The genetic diversity among 31 tropical-adapted maize inbred lines developed for medium to high PVA were determined using molecular markers. Genomic DNAs were extracted from the leaves and genotyped using 14 simple sequence repeat (SSR) markers. A total of 33 alleles with an average of 2.36 alleles per locus were compared among the lines. The gene diversity (D), revealed by all the SSR markers, ranged from 0.22 to 0.60 with a mean of 0.45 indicating moderate genetic diversity among the lines. Cluster and Principal Coordinate analyses based on genetic distances from the SSR data separated the lines into two main groups with 4 clusters and several subclusters consistent with their pedigrees. Nine lines were selected from the different clusters for inheritance study. The genomic DNAs of the selected lines were genotyped using five molecular markers for the two major functional genes in carotenoids biosynthesis, lycE and crtRB1. The results showed different profiles of the nine lines possessing three to five alleles of lycE and crtRB1genes that favour increased provitamin A active carotenoids (βC, βCX and PVA) in maize endosperm and indicated potential of these markers for marker assisted selection. The nine selected lines were crossed using half diallel mating design to develop 36 crosses. The crosses were evaluated in the field in two locations (Samaru and Kerawa). The first two plants in each plot were self pollinated to produce seeds for ix carotenoids analysis. The crosses were assessed for variability, general combining ability (GCA), specific combining ability (SCA), heritability and correlations for agronomic traits and carotenoid contents. Analysis of variance showed that the mean squares for location were highly significant (p<0.01) for the agronomic traits and the carotenoids with exception of Zeaxanthin (ZEA) and βCX. The crosses showed highly significant differences for days to 50% anthesis (DA), plant height (PH), grain yield (GY), Lutein (LUT) and ZEA at Samaru and across the locations. The mean square for GCA and SCA effects were significant for DA, PH, GY, LUT and ZEA across locations, suggesting both additive and non-additive gene effects. The GCA:SCA mean squares ratios for DA, PH, LUT, ZEA and βCX were higher than unity (>1) indicating the preponderance of additive gene effect but lower than unity (<1) indicating preponderance of non-additive gene effect for GY, βC, αC and total provitamin A. The best performing crosses for GY across locations were for E26/E6 (7.33 tons/ha) and E6/E9 (7.16 tons/ha). Lines E6, E7, E8 and E9 were the common parents in the best crosses. ZEA made up 49.5% of total carotenoids across locations making it the highest carotenoids in tropical-adapted maize. Lines with high positive GCA effects for DA, PH and GY were E8, E9, E3 and E37 while lines E6, E9, E28 and E7 had high positive GCA for βC and PVA contents. Significant positive SCA effects for αC, βC and PVA across locations were observed in the crosses E3/E16 and E8/E6. Although many of the other crosses had high positive SCA effects for the PVA carotenoids, the effects were not significant. For grain yield, 53% of the crosses had positive SCA effects. Heritability (H2) was high for grain yield (H2>60%), moderate for DA, LUT and ZEA (30-60%), and low for PH, βCX, βC, αC and PVA (H2<30%). The phenotypic correlations between GY and the carotenoids were significantly positive with βC, βCX and x αC while genotypic correlations were negative and non-significant, indicating masking of environmental effects. GY is not genotypically significantly correlated with DA, PH, αC, βC, βCX, PVA, ZEA and LUT, suggesting that GY and provitamin A carotenoids can be improved concurrently. Hybrid development and recurrent selection are, therefore, recommended for the development of maize varieties with high GY, PVA, βC and βCX for the tropical sub-saharan Africa. Direct selection for yield should also be practice while applying independent culling selection method for DA, PH, ZEA and LUT because of their negative indirect contributions on PVA. |
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