Haplotype-resolved T2T genome assemblies and pangenome graph of pear reveal diverse patterns of allele-specific expression and genomic basis of fruit quality traits

Hybrid crops often exhibit increased yield and greater resilience, yet the genomic mechanism(s) underlying hybrid vigor or heterosis remain unclear, hindering our ability to predict the expression of phenotypic traits in hybrid breeding. Here, we generated haplotype-resolved T2T genome assemblies of two pear hybrid varieties 'Yuluxiangli' (YLX) and 'Hongxiangsu' (HXS) that share the same maternal parent, but differ in their paternal parents. We then used these assemblies to explore genome-scale landscape of allele-specific expression and create a pangenome graph for pear. Allele specific expression (ASE) was observed for close to 6000 genes in both hybrid cultivars. A subset of ASEGs related to fruit quality including sugar, organic acid and cuticular wax were identified, suggesting their important contributions to heterosis. Specifically, Ma1, a gene regulating fruit acidity, was absent in the paternal haplotypes of HXS and YLX. Further, a pangenome graph was built based on our assemblies and eight published pear genomes. Resequencing data for 139 cultivated pear genotypes (including 97 genotypes sequenced here) were subsequently aligned to the pangenome graph, revealing numerous SV hotspots and selective sweeps during pear diversification. As predicted, the Ma1 allele was found to be absent in varieties with low organic acid content, an association that was functionally validated by Ma1 over-expression in pear fruit and calli. Overall, the results unraveled contributions of allele-specific expression to heterosis involving fruit quality and provided a robust pangenome reference for high resolution allele discovery and association mapping.