Gapless Genome Sheds Light on Hybrid Origins of Hong Kong's Iconic Orchid Tree.

On International DNA Day, a landmark scientific achievement was announced: the complete, gapless genome sequence of Hong Kong’s beloved floral emblem, the Hong Kong Orchid Tree (Bauhinia x blakeana Dunn). Published in GigaScience, the study is the culmination of a decade-long research initiative led by scientists at the Chinese University of Hong Kong (CUHK), with significant contributions from the local community, including students and researchers.

Renowned for its striking purple, orchid-like blooms, Bauhinia x blakeana is prominently featured on Hong Kong's flag and currency. Despite its iconic status, the tree's genetic origins have long puzzled scientists. First discovered on Hong Kong Island in the 1880s by French missionary and horticulturist Jean-Marie Delavay, the species is entirely sterile and can only be propagated through cuttings. Its inability to reproduce sexually left its taxonomic classification uncertain for over a century.

Earlier morphological analyses and single-gene studies hinted that B. blakeana might be a hybrid of two other species—Bauhinia purpurea and Bauhinia variegata. However, these suggestions lacked comprehensive genomic confirmation. In 2015, a community-backed crowdfunding initiative launched the Bauhinia Genome Project to address these lingering questions. Initial efforts focused on sequencing the transcriptomes—gene expression profiles—of the suspected hybrid and its potential parents.

By 2019, the project advanced to whole-genome sequencing using a blend of cutting-edge second- and third-generation sequencing technologies. This multi-platform strategy yielded chromosome-scale draft genomes for the three species. Inspired by the 2023 milestone of the first telomere-to-telomere (T2T) human genome, researchers from the National University of Singapore collaborated to refine the Bauhinia genome assembly. Using advanced computational tools, they achieved a complete, haplotype-resolved T2T genome.

The final genome revealed 28 chromosomes—14 inherited from each parent—confirming the hybrid nature of B. blakeana. Remarkably, the study pinpointed B. purpurea as the maternal parent and B. variegata as the paternal parent, despite the two parent species having diverged over 13.4 million years ago. This singular hybridization event resulted in a genetic chimera that underpins the orchid tree’s distinct appearance and resilience.

Transcriptome analysis of flower tissues showed gene expression in B. blakeana more closely aligned with B. purpurea, the maternal lineage. It also revealed significant differences among the three species in biosynthetic and metabolic pathways, offering insight into the orchid tree’s unique floral traits.

The findings not only solve a long-standing botanical mystery but also illuminate the genetic basis of heterosis—commonly known as hybrid vigor—where hybrid organisms exhibit enhanced characteristics. This high-resolution genome provides a foundation for future studies into hybridization, plant breeding, and conservation, celebrating both scientific advancement and Hong Kong's cultural heritage.

Source:https://phys.org/news/2025-04-gapless-genome-sequence-reveals-hybrid.html

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