MOLECULAR CHARACTERIZATION AND HETEROSIS OF EXTRA-EARLY MATURING ORANGE MAIZE INBRED LINES UNDER CONTRASTING ENVIRONMENTS
Striga hermonthica and drought are major threats to maize (Zea mays L.) productivity and productiopn in savannas of sub-Saharan Africa (SSA), a sub-region known to be plagued with vitamin A deficiency (VAD). Incorporating both drought tolerance and Striga resistance into high yielding orange maize cultivars for Striga endemic and drought-prone areas of the savanna agro-ecologies of SSA will increase acceptability of orange cultivars by farmers and aid in alleviating VAD and food shortage in the sub-region. The specific objectives of this research were to study (i) the performance of extra-early maturing maize inbred lines in tolerance to Striga and drought, (ii) the molecular diversity in extra-early maturing orange maize inbred lines using SNP-based DArTseq markers, (iii) the combining abilities of extra-early maturing orange maize inbred lines and heterosis for tolerance to Striga and drought, (iv) the performance and stability of extra-early maturing hybrids in tolerance to Striga and drought, (v) the combining abilities of extra-early maturing orange maize inbred lines and heterosis for carotenoids in maize kernels. One hundred and eighty inbred lines comprising 152 selected orange inbreds and 28 yellow lines from other sources were evaluated under Striga infestation, managed drought stress, and optimal environments. Thirty-four (34) out of 180 inbreds evaluated and 32 out of 152 orange inbreds combined Striga resistance and drought tolerant, using base indices for selection. Twenty-four (24) of the 34 selected based on the indices were also selected by the multivariate BLUPs across all environments. The genetic purity and diversity among the 152 orange inbreds were assessed using 4620 polymorphic SNPs. The results revealed that 92% of the inbreds were considered pure with heterozygosity < 5% while the remaining 8% had heterozygosity ranging from 5.1 to 20.2%. Roger’s genetic distance for about 71% of the pairs of lines fell between 0.2001 and 0.2500. Ninety-two percent of the pairs of inbreds also showed relative kinship values ranging from 0.300 to 0.500. The population structure analysis using STRUCTURE and neighbour-joining clustering assigned 71% of the inbreds in 4 distinct groups. Fifteen inbreds selected among the 152 evaluated plus TZdEEI 7 and TZdEEI 12 were used to generate 136 diallel single cross hybrids which were evaluated together with four experimental hybrid checks under Striga-infested, drought stress, and optimal environments at three locations in Nigeria in 2016 and 2017 (11 environments). General and specific combining ability components of the genetic variance were significantly different from zero for grain yield and most of the traits. Additive and non-additive genetic effects were both important with a predominance of the latter in controlling most of the measured traits including grain yield under Striga-infested, drought stress, and across test environments. However, additive genetic effects were found to be the primary type of gene action for the staygreen characteristic and Striga resistance indicator traits, suggesting that selection for these traits could easily be done based on predictions of GCA alone. Using base indices, 26% of the hybrids combined Striga resistance with drought tolerance. Stability assessment of the top 26 hybrids across test environments based on their genetic value indicated that TZEEIOR 12 x TZEEIOR 196 was the most stable, combining resistance to Striga and tolerance to drought with grain yield of 3885 kg ha-1 and 5411 kg ha-1 across environments and under optimal conditions, respectively. Hayman diallel analysis revealed the presence of more dominant than recessive genes in the parents with the ratio of dominant to recessive genes being greater than 2 for β-carotene (2.36). Also, at the loci exhibiting dominance, recessive alleles were mostly positives for β-cryptoxanthin, lutein, and to some extent for β-carotene while dominant genes expressed both effects for the rest of the traits. In conclusion, dominance genetic effects were the primary type of gene action in the inheritance of most carotenoid traits, in the set of parental inbreds used in the present study.