Insights into karyotype evolution and flower color variation from the genome assembly of wallflower ( <i>Erysimum cheiri</i> )

Wallflower (Erysimum cheiri) belongs to the monogeneric Erysimeae tribe of the mustard family (Brassicaceae). It is widely cultivated as an ornamental garden plant and appreciated for its diverse flower colors. However, the absence of a high-quality genome has hampered research on wallflower genome evolution and the mechanisms underlying variations in flower color. Here, we assembled a nearly gap-free telomere-to-telomere genome of E. cheiri. The assembled genome enabled the reconstruction of genome evolution in the genus Erysimum (274 species), tracing the changes from the ancestral n = 8 genome (in E. cheiranthoides) to the derived genomes with seven (in E. nevadense) and six (in E. cheiri) chromosome pairs. While the reduction from n = 8 to n = 7 was mediated by a nested chromosome fusion accompanied by inversions, the further decrease to n = 6 in E. cheiri resulted from an end-to-end translocation involving the other two non-homologous chromosomes. Compared with other Brassicaceae species, E. cheiri showed a notable expansion of gene families related to secondary metabolite biosynthesis. Its flower color variation was primarily determined by the biosynthesis and accumulation of carotenoids and flavonoids. We mapped the metabolic pathways for carotenoids and flavonoids, identifying the hub genes regulating their biosynthesis. This research lays an important foundation for understanding the chromosomal and genome evolution of the Erysimeae tribe and paves the way for future investigations into genetic studies and breeding applications of E. cheiri.