Spatiotemporal process of long-distance seed dispersal in a pantropically distributed sea hibiscus group

Long-distance dispersal (LDD) of seeds plays an essential role in the migration of plants to a new habitat and maintaining gene flow among geographically isolated populations. Pantropical plants with sea-drifted seeds, which have one of the largest distributions in all flowering plants, have achieved their global distribution by LDD. However, the spatiotemporal processes to achieve the wide distribution and the role of LDD in it have not yet been elucidated. In this study, we conducted phylogenomic analyses on the plastome, genome-wide nuclear SNP, and low-copy gene data of Hibiscus tiliaceus and its relatives. The dated phylogeny suggested that global expansion started approximately 4 million years ago (Ma), and species diversification occurred 1 Ma. Plastome phylogeny confirmed the nonmonophyly of the haplotypes in the two widely distributed coastal species, H. tiliaceus and H. pernambucensis. In contrast, genome-wide nuclear SNP phylogenies demonstrated clear genetic segregation among species and/or geographical regions. Ancestral polymorphisms in chloroplast genomes shared among widely distributed species have remained below the range of rapid expansion and speciation of marginal populations. This study demonstrated that the LDD of sea-drifted seeds contributed to the rapid expansion and pantropical distribution of sea hibiscus in the last few million years, and adaptation to local environment or isolation by regional effect after LDD promoted speciation, suppressing gene flow.