Characterization and Evaluation of Heterotic Traits in a Multi-parent Advanced Generation Inter-Cross (MAGIC) Maize Population

Fusarium verticilloides is the fungal pathogen in maize (Zea mays L.) that cause ear rot and leads to substantial reduction of yield and grain quality worldwide. We developed an innovative recombinant intercross (RIX) population in maize by crossing pairs of multi-parental maize recombinant inbred lines (RILs) to evaluate the heterotic loci of agronomic and Fusarium ear rot (FER) resistance in a heterozygous background. Field phenotyping was performed on 400 RIX genotypes and considered 11 agronomic traits in 2018 and 2019. The FER resistance was evaluated using digital scoring method on artificially inoculated ears to consistently measure the spread of disease from the point of infection to the entire ear. Phenotypic data showed a normal distribution with broad range of phenotypic diversity in different traits, thanks to the high level of allelic diversity available in parental genome. The broad-sense heritability (H2) for agronomic traits was high for all traits (H2=0.48 to 0.89) illustrating a strong genetic basis. However, the disease score data showed a moderate magnitude of H2 (2018=0.40, 2019=0.33) indicating FER resistance is highly responsive to environment. The mid-parent heterosis and best-parent heterosis measures showed different magnitudes for different traits, with higher level of heterosis in production (mean MPH=232.74%, mean BPH=164.79%) and lower in flowering (mean MPH=4.56%, mean BPH=6.23%), suggesting effects from partial dominance to over-dominance. A genome-wide association study (GWAS) detected 55 significant (p≤0.05) loci associated with phenotypic values across all chromosomes, with allelic effect ranging from 0.28 to 10.90 indicating the contribution of various sized QTL. We observed several pleiotropic QTL at 17.94 Mb (days to tasseling, days to silking, ear height) and 124.71 Mb (ear height and plant height) positions of chromosome 8 and 4.7 Mb (ear diameter, cob diameter) position of chromosome 3, indicating that these loci may contain valuable genes for multiple traits. The disease score data showed 13.04% and 19.7% of RIXs as resistant in 2018 and 2019, respectively. Similarly, our GWAS model has detected two putative QTL for Fusarium resistance on chromosome 5 at 185.68 Mb and chromosome 8 between 159.95 Mb to 160.73 Mb, respectively. Overall, our findings confirm the usefulness of the RIX population to decipher heterotic loci in maize and support utilizing this resource in future to accelerate crop improvement.