Format: ORAL

Koji Takayama1

1 Department of Botany, Kyoto University, JAPAN

Pantropical plants with sea-drifted seeds (PPSS) expanded globally through long-distance seed dispersal by sea. These plants, representing individual species or closely related species, thrive globally in tropical littoral areas. Notable examples include Ipomoea pes-caprae subsp. brasiliensis (Convolvulaceae), Canavalia rosea (Leguminosae), and Hibiscus tiliaceus (Malvaceae). Closely related species are found in marginal areas of PPSS distribution, such as Ipomoea pes-caprae subsp. per-caprae (northern Indian Oceanic region), Canavalia subgenus Maunaloa (Hawaiian Islands), and Hibiscus hamabo (temperate East Asia) and H. glaber (Bonin Islands). Despite PPSSs extensive distribution, the species cohesion breaks down in peripatric areas due to limited gene flow, fostering differentiation through alternative allele fixation, local adaptation, and genetic drift. However, the spatiotemporal processes of peripatric speciation in PPSS remain unclear. To address this gap, we conducted phylogenomic analyses using plastome and genome-wide nuclear SNP data of H. tiliaceus and its East Asian relatives H. hamabo and H. glaber. Our dated phylogeny suggests that global expansion began around four million years ago, and species diversification began approximately one million years ago. Plastome phylogeny revealed that H. hamabo haplotypes were fully nested within H. tiliaceus haplotypes, supporting a peripatric speciation process involving migration to temperate zones. In contrast, H. glaber haplotypes showed diversification at the basal position in the plastome phylogeny. Genome-wide nuclear SNP phylogenies highlight distinct genetic segregations among species and geographical regions. Hibiscus hamabo exhibited low genetic diversity, whereas H. glaber displayed relatively high genetic diversity in both plastome and genome-wide SNPs. This study provides insights into the divergent genetic backgrounds of H. hamabo and H. glaber, both of which are undergoing peripatric speciation in East Asia. Discussions will focus on the factors contributing to these distinct genetic patterns in these peripatrically derived species.