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 Gbp in length with a scaffold N50 of 262.27 Mbp 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 BUSCO 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 BUSCO content of the assembly, over 1 Gbp 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.