Abstract: Nypa fruticans (Wurmb), a mangrove palm species with origins dating back to the Late Cretaceous period, is a unique species for investigating long-term adaptation strategies to intertidal environments and the early evolution of palms. Here, we present a chromosome-level genome sequence and assembly for N. fruticans. We integrated the genomes of N. fruticans and other palm family members for a comparative genomic analysis, which confirmed that the common ancestor of all palms experienced a whole-genome duplication event around 89 million years ago, shaping the distinctive characteristics observed in this clade. We also inferred a low mutation rate for the N. fruticans genome, which underwent strong purifying selection and evolved slowly, thus contributing to its stability over a long evolutionary period. Moreover, ancient duplicates were preferentially retained, with critical genes having experienced positive selection, enhancing waterlogging tolerance in N. fruticans. Furthermore, we discovered that the pseudogenization of Early Methionine-labelled 1 (EM1) and EM6 in N. fruticans underly its crypto-vivipary characteristics, reflecting its intertidal adaptation. Our study provides valuable genomic insights into the evolutionary history, genome stability, and adaptive evolution of the mangrove palm. Our results also shed light on the long-term adaptation of this species and contribute to our understanding of the evolutionary dynamics in the palm family.

Key words: crypto-vivipary, genome stability, long-term adaptation, mangrove, Nypa fruticans, palm