Chloroplast DNA phylogeography of the Holly mangrove Acanthus ilicifolius in the Indo-West Pacific

Acanthus ilicifolius is a widespread mangrove species in the Indo-West Pacific (IWP) with high ecological value. In this study, we analyzed the patterns of genetic variation in the species across its distribution range and assessed factors that may have influenced its genetic structure across the IWP using data from four chloroplast DNA fragments. Our results demonstrated high genetic diversity at the species level with little variation within populations, indicating restricted gene flow among A. ilicifolius populations. Strong genetic divergence was found between populations in the Indian Ocean and populations in the Pacific Ocean, likely due to the land barrier effect of the Malay Peninsula (or in the past, the Sundaland). Ecological niche modeling revealed no significant niche differentiation between the two groups, suggesting that ecological divergence was less responsible for the divergence of A. ilicifolius. Despite the ability for long-distance dispersal, genetic barriers identified within the Pacific Ocean suggested ocean currents to be a cryptic barrier for gene flow in this region. This study provides new insights into the historical population dynamics of mangrove species in the IWP region and is valuable for the long-term conservation of A. ilicifolius.