Two haplotype-resolved genomes reveal important flower traits in bigleaf hydrangea (Hydrangea macrophylla) and insights into Asterid evolution.

The Hydrangea genus belongs to the Hydrangeaceae family, in the Cornales order of flowering plants, which early diverged among the Asterids, and includes several species that are commonly used ornamental plants. Of them, Hydrangea macrophylla is one of the most valuable species in the nursery trade, yet few genomic resources are available for this crop or closely related Asterid species. Two high-quality haplotype-resolved reference genomes of hydrangea cultivars 'Veitchii' and 'Endless Summer' [highest quality at 2.22 gigabase pairs (Gb), 396 contigs, N50 22.8 megabase pairs (Mb)] were assembled and scaffolded into the expected 18 pseudochromosomes. Utilizing the newly developed high-quality reference genomes along with high-quality genomes of other related flowering plants, nuclear data were found to support a single divergence point in the Asterids clade where both the Cornales and Ericales diverged from the euasterids. Genetic mapping with an F1 hybrid population demonstrated the power of linkage mapping combined with the new genomic resources to identify the gene for inflorescence shape, CYP78A5 located on chromosome 4, and a novel gene, BAM3 located on chromosome 17, for causing double flower. Resources developed in this study will not only help to accelerate hydrangea genetic improvement but also contribute to understanding the largest group of flowering plants, the Asterids.

Genome-wide association studies for inflorescence type and remontancy in Hydrangea macrophylla.

Inflorescence type and remontancy are two valuable traits in bigleaf hydrangea (Hydrangea macrophylla L.) and both are recessively inherited. Molecular marker-assisted selection (MAS) can greatly reduce the time necessary to breed cultivars with desired traits. In this study, a genome-wide association study (GWAS) using 5803 single-nucleotide polymorphisms (SNPs) was performed using a panel of 82 bigleaf hydrangea cultivars. One SNP locus (Hy_CAPS_Inflo) associated with inflorescence type was identified with general linear model (GLM) and mixed linear model (MLM) methods that explained 65.5% and 36.1% of the phenotypic variations, respectively. Twenty-three SNPs associated with remontancy were detected in GLM whereas no SNP was detected in MLM. The SNP locus (Hy_CAPS_Inflo) was converted to a cleaved amplified polymorphic sequence (CAPS) marker that showed absolute identification accuracy (100%) of inflorescence type in a validation panel consisting of eighteen H. macrophylla cultivars. The SNP was investigated in 341 F1 progenies using genotyping by sequencing (GBS) and co-segregated with inflorescence type (χ 2 = 0.12; P = 0.73). The SNP was subsequently used for breeding selection using kompetitive allele specific PCR (KASP) technology. Future directions for the use of genomics and MAS in hydrangea breeding improvement are discussed. The results presented in this study provide insights for further research on understanding genetic mechanisms behind inflorescence type and remontancy in H. macrophylla. The CAPS and KASP markers developed here will be immediately useful for applying MAS to accelerate breeding improvement in hydrangea.