Insights into the genomic divergence of maize heterotic groups in China

Diverse heterotic groups have been developed in China over several decades, but their genomic divergences have not been systematically studied after improvement. In this study, we performed Maize6H-60K array of 5,822 maize accessions and whole-genome re-sequencing of 150 inbred lines collected in China. Using multiple population structure analysis methods, we established a genetic boundary used to categorize heterotic groups and germplasm resources. We identified three chloroplast-cytoplasmic types that evolved during adaptation to diverse climatic environments in maize through phylogenetic and haplotype analyses. Comparative analyses revealed obvious genetic differences between heterotic groups and germplasm resources at both the chloroplast and nuclear genome levels, especially in the unique heterotic groups HG1 and HG2, which exhibited distinct regionality and genetic uniqueness. The divergent differentiation of heterotic groups from germplasm resources was driven by differential selection in specific genomic regions. Genome-wide selective sweep analysis identified core selected regions and candidate selected genes associated with traits between heterotic groups, highlighting that stress response- and plant defense-related genes were selected for environmental adaptation across a broad latitudinal range in China. Meanwhile, a genome-wide association study analysis provided evidence that core selected genes served as an important candidate gene pool with a potential role in genetic improvement. Gene exchanges among heterotic groups, which avoided the predominant heterotic patterns as much as possible, occurred to achieve population improvement during modern maize breeding. This study provides insights into the population differentiation and genetic characteristics of heterotic groups, which will facilitate the utilization of germplasm resources, the creation of novel maize germplasm, and the optimization of heterotic patterns during future maize breeding in China.