Genome assembly of Elcysma westwoodi provides insights into host-plant cyanogenic glucoside detoxification and chemical defence gene evolution.

Elcysma westwoodi is a major pest of cherry trees (Rosaceae), which contain high levels of cyanogenic glucosides that release toxic hydrogen cyanide upon tissue damage. To investigate the genomic basis of host-plant cyanogenic glucoside detoxification and the evolution of chemical defence genes, we generated a PacBio HiFi-based genome assembly of E. westwoodi. The final assembly spans 460.62 Mb with high contiguity (contig N50 > 15 Mb) and near-complete recovery of conserved genes (99.6% BUSCO completeness). We predicted 19,502 protein coding genes, with annotation completeness reaching 95.2% BUSCO. Comparative analyses of three Zygaenidae genomes revealed strong conservation of chromosomal synteny and showed that E. westwoodi shares a more similar detoxification gene repertoire with Achelura yunnanensis, another Rosaceae feeder, than with the more divergent, non-Rosaceae feeding Zygaena filipendulae. Genome-wide analysis identified 272 detoxification-related genes, indicating substantial capacity for xenobiotic metabolism. Phylogenetic analysis supported long-term conservation of a vertically inherited, single-copy β cyanoalanine synthase (bCAS) gene involved in cyanide detoxification. In addition, structural and phylogenetic analyses revealed parallel retention of CYP405A and CYP332A in the two Rosaceae feeding species, a pattern most parsimoniously explained by the inheritance of an ancestral cyanogenic toolkit rather than by independent convergent recruitment, whereas UGT33A retained the canonical GT-B fold characteristic of UDP glycosyltransferases, indicating the evolutionary stability of phase II conjugation enzymes. Together, these findings indicate that adaptation in E. westwoodi proceeds through targeted gene-level evolution within an otherwise conserved genomic framework.