Impacts of parental genomic divergence in non-syntenic regions on cotton heterosis

Introduction: Heterosis has revolutionized crop breeding, enhancing global agricultural production. However, the mechanisms underlying heterosis remain obscure. Xiangzamian 2# (XZM2), a super hybrid upland cotton (Gossypium hirsutum L.) characterized by high-yield heterosis, has been developed and extensively planted in China.

Objectives: We conducted a systematic analysis of CRI12 and J8891, two parents of XZM2. We aimed to reveal the precise genetic information and the role of non-syntenic divergence in shaping heterosis, laying a foundation for advancing understanding of heterosis.

Methods: We de novo assembled high-quality genomes of CRI12 and J8891, and further uncovered abundant genetic variations and non-syntenic regions between the parents. Whole-genome comparison, association analysis, transcriptomic analysis and relative identity-by-descent (rIBD) estimation were conducted to identify structural variations (SVs) and introgressions within non-syntenic blocks and to analyze their impacts on promoting heterosis.

Results: Parental genetic divergence increased in non-syntenic regions. Furthermore, these regions, accounting for only 16.71% of the total genome, contained more loci with significantly higher heterotic effects, far exceeding the syntenic background. SVs covered 97.26% of non-syntenic sequences and caused widespread gene expression differences in these regions, driving dynamic complementation of gene expression in the hybrid. A set of SVs were responsible for trait improvement and had positive effects on heterosis, contributing larger heritability than short variations. We characterized numerous parental-specific introgressions from G. barbadense. Specifically, a functional introgression segment within non-syntenic blocks introduced an elite haplotype, which significantly increased lint yield and enhanced heterosis.

Conclusion: Our study clarified non-syntenic regions to harbor more loci with higher heterotic effects, revealed their importance in promoting heterosis and supported the crucial role of genetic complementation in heterosis. SVs and introgressions were identified as key factors responsible for non-syntenic divergence between the parents. They had important effects on gene expression and trait improvement, positively contributing to heterosis.