Inter-haplotype inversions and repeat expansion in the sexually deceptive orchid Chiloglottis trapeziformis
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Chiloglottis trapeziformis is a sexually deceptive Australian orchid that provides a valuable system for studying orchid genome evolution, structural variation, and the molecular basis of specialized pollination. However, high-quality nuclear genome resources remain scarce for most orchids, particularly Australia’s diverse terrestrial lineages. To address this gap, we integrated PacBio HiFi, Oxford Nanopore ultra-long reads, and Hi-C chromatin-contact data to generate the first chromosome-scale, haplotype-resolved nuclear genome assembly for an Australian terrestrial orchid, Chiloglottis trapeziformis . Hi-C guided scaffolding resolved two haplotypes into 20 chromosomes each, consistent with the reported karyotype and genome size (2n=40, haplotype sizes of 1.58 Gb and 1.91 Gb). Genome completeness was high for both haplotypes, recovering 95.1% and 95.5% complete BUSCO genes for haplotype 1 and haplotype 2, respectively. De novo repeat annotation revealed a repeat-rich genome (85.79– 88.25% repetitive sequence), dominated by LTR retrotransposons. Evidence-guided annotation identified 16,287 and 16,548 protein-coding genes in Haplotype 1 and Haplotype 2, respectively. Phylogenetically informed comparisons placed C. trapeziformis as sister to Anoectochilus roxburghii among sampled Orchidoideae and showed broad gene-order conservation. Comparing the two haplotypes for structural variation, we identified large inter-haplotype inversions containing functionally annotated genes with detectable RNA expression, with focal examples further supported by local Hi-C contact patterns and breakpoint-level inspection. Inversion-overlapping genes did not show elevated dS relative to collinear background. This assembly and annotation resource provides a foundation for population and conservation genomics, structural and comparative analyses, and genome-enabled hypothesis testing of molecular traits underlying sexual deception in orchids.
Significance statement
Sexually deceptive orchids use remarkable chemical, visual, and tactile mimicry to attract specific pollinators, but the genome resources needed to understand how these complex traits evolved remain scarce. By generating a chromosome-scale, haplotype-resolved genome for Chiloglottis trapeziformis , we provide the first genomic framework for the large and unique Australasian tribe Diurideae. We show extensive structural variation between haplotypes in an otherwise highly collinear genome, with several large inversions potentially impacting expressed genes. The absence of elevated coding divergence in these regions highlights structural variation as a potentially underappreciated driver of orchid genome evolution. This resource fills a major gap for Australian orchids and provides a foundation for linking genome structure with orchid diversification, conservation, and the evolution of sexual deception.
