Genome of the Lord Howe Island Stick Insect Reveals a Highly Conserved Phasmid X Chromosome

We present a chromosome-scale genome assembly for Dryococelus australis, a critically endangered Australian phasmid. The assembly, constructed with Pacific Biosciences continuous long reads and chromatin conformation capture (Omni-C) data, is 3.42 Gb in length with a scaffold N50 of 262.27 Mb and L50 of 5. Over 99% of the assembly is contained in 17 major scaffolds, which corresponds to the species' karyotype. The assembly contains 96.3% of insect Benchmarking Unique Single Copy Ortholog genes in single copy. A custom repeat library identified 63.29% of the genome covered by repetitive elements; most were not identifiable based on similarity to sequences in existing databases. A total of 33,793 putative protein-coding genes were annotated. Despite the high contiguity and single-copy Benchmarking Unique Single Copy Ortholog content of the assembly, over 1 Gb of the flow-cytometry-estimated genome size is not represented, likely due to the large and repetitive nature of the genome. We identified the X chromosome with a coverage-based analysis and searched for homologs of genes known to be X-linked across the genus Timema. We found 59% of these genes on the putative X chromosome, indicating strong conservation of X-chromosomal content across 120 million years of phasmid evolution.