GENETIC ANALYSIS OF GRAIN YIELD AND OTHER TRAITS OF EARLY MATURING PROVITAMIN A- QUALITY PROTEIN MAIZE INBRED LINES UNDER DROUGHT AND LOW SOIL NITROGEN CONDITIONS

ABSTRACT Early maturing provitamin A (PVA) quality protein maize (QPM) hybrids with combined drought and low soil Nitrogen (low-N) tolerance are needed to address malnutrition and food insecurity problems in sub-Saharan Africa (SSA). In response to this need, PVA-QPM inbred lines with drought and low-N tolerant genetic backgrounds were developed through the concerted effort of the International Institute of Tropical Agriculture Maize Improvement Program (IITA-MIP) for hybrid development and commercialization. The objectives of the present study were to (i) identify drought and/ low-N tolerant inbred lines with elevated levels of PVA and quality protein, (ii) assess the extent of genetic variability of selected early maturing drought and low-N tolerant inbreds, (iii) study the combining ability of a set of inbreds for drought and/or low-N tolerance as well as PVA accumulation (iv) classify the set of inbred lines into heterotic groups and identify the best inbred and hybrid testers across the test environments, (v) assess yield and stability of hybrids across the contrasting environments, and (vi) validate PVA functional genes in the set of inbred lines. The genetic diversity in the inbreds were examined using the unweighted pair group method with arithmetic mean (UPGMA) clustering, the model-based structure analysis and the principal component analysis. Ninety-six hybrids generated from 24 inbreds using the North Carolina Design two (NCDII) plus four checks, as well as 70 inbreds including six checks were evaluated under drought, low-N and optimal environments in Nigeria from 2016 to 2017. Nineteen selected inbreds and 54 hybrids were assayed for PVA and tryptophan contents. Provitamin A candidate genes were validated in the inbreds using allele specific markers. Days to 50% anthesis and silking, plant and ear heights, and plant and ear aspects complemented grain yield of the set of inbred lines in identifying 33 of the 70 inbred lines for the genetic studies. Ninety-five percent of the inbreds had desirable levels of tryptophan (> 0.075%) in sample in whole grain implying that the inbreds generally met the quality standards of QPM genotypes. Moderate levels of PVA were recorded for the inbreds assayed indicating the need to increase the frequency of the favourable PVA alleles in the inbred lines. TZEIORQ 55 and TZEIORQ 29 combined low-N tolerance with adequate levels of tryptophan and had PVA contents of 15.38 and 12.10 μg g-1 respectively, while nine inbreds combined drought and low-N tolerance with adequate and moderate levels of tryptophan and PVA respectively. These inbred lines could be used in hybrid combinations to produce outstanding PVA-QPM hybrids. Five genetically distinct clusters were identified for the inbreds and the grouping was largely based on the pedigree of the set of inbred lines. Additive genetic effects were more important than non-additive for grain yield and most other agronomic traits under drought, low-N, optimal and across environments. Maternal effects were not significant for grain yield and most other agronomic traits under each and across environments, as well as the carotenoids and tryptophan quantified. The DArTseq markers were more efficient than the heterotic grouping based on general combining ability of multiple traits (HGCAMT) and identified three heterotic groups. The inbred TZEIORQ 29 was the best inbred tester either as a male or female for heterotic group I, while TZEIORQ 24 was a good combiner only as a male parent for heterotic group II. TZEIORQ 2 x TZEQI 82 was identified as the best singlecross hybrid tester across environments. The GGE-biplot analysis and the drought and low-N multiple trait base index (MI) consistently identified TZEIORQ 24 x TZEIORQ 41 as the highest yielding and most stable hybrid across stress and non-stress environments while TZEIORQ 29 x TZEIORQ 43 was the best hybrid under low-N conditions, and TZEIORQ 26 x TZEIORQ 47 was outstanding for combined drought and low-N tolerance. The combining ability study of PVA carotenoids revealed preponderance of additive genetic effects over nonadditive for PVA and all measured carotenoids suggesting that superior hybrids could be produced by crossing parents with positive and significant GCA effects for PVA. All the 54 hybrids assayed had > 0.075% tryptophan per sample in whole grain and that the hybrids met iv the quality standards of QPM but none was higher than the check, “Obatanpa”. The hybrid, TZEIORQ 29 x TZEIORQ 43 which had PVA content of 9.78 μg g-1 , was among the top hybrids including TZEIORQ 29 x TZEIORQ 40, TZEIORQ 29 x TZEIORQ 24, TZEIORQ 20 x TZEIORQ 29 and TZEIORQ 6 x TZEIORQ 29 interms of PVA levels and also combined good agronomic performance under drought, low-N and optimal conditions. These hybrids should be further tested to confirm consistency of performance and commercialized in SSA to combat the “hidden hunger” due to vitamin A deficiency and protein energy malnutrition in the sub-region. The PVA allele specific marker, crtRB1-3’TE, was the most polymorphic and was highly consistent with the KASP SNP (snpZM0015). The two markers identified eight inbreds containing favourable alleles of the crtRB1 functional gene. These inbreds could serve as donor parents of favourable alleles for the crtRB1 gene. Despite the moderate to high PVA contents of TZEIORQ 29 and TZEIORQ 55, they did not have the favourable alleles of crtRB1 and LcyE genes implying that other genes were responsible for the increased levels of PVA in these inbreds. Moreover, the preponderance of additive genetic effects over nonadditive in the inheritance of PVA accumulation and the significant positive GCA-male and female effects for PVA levels of TZEIORQ 29 indicated that TZEIORQ 29 and TZEIORQ 55 could contribute favourable alleles other than those of crtRB1 and LcyE for improving PVA concentrations in hybrids and synthetics. The two inbreds would also be useful for the improvement of the early PVA-QPM inbred population for high PVA levels.