Population Structure and Genetic Diversity Analyses Provide New Insight into the Endemic Species Aster spathulifolius Maxim. and Its Evolutionary History.

Aster spathulifolius , an ecologically significant plant species native to the coastal regions of Korea and Japan, remains understudied in terms of its genetic structure and evolutionary history. In this study, we employed four chloroplast markers and the nuclear ITS region from 15 populations of A . spathulifolius from both Korea and Japan, including their islands, to unravel the spatial genetic structure, differentiation, gene flow, phylogenetic, and biogeographical relationships. Analysis based on multiple methods identified a low level of genetic diversity, genetic differentiation and gene flow among A . spathulifolius populations. Network analysis and principal coordinates analysis showed that 15 populations could be divided into two groups: mainland and island. Furthermore, UPGMA, neighbor-net, maximum-likelihood and Bayesian inference-based phylogenetic tree confirmed that these populations formed two distinct clades. Therefore, the island populations might be treated as A . spathulifolius populations rather than A . oharai populations. Divergence time analysis estimated the divergence of A. spathulifolius lineages approximately 23.09 million years ago, while ancestral area reconstruction analysis suggested Korea as the potential origin, conflicting with alternative scenarios. These findings contribute to a comprehensive understanding of the evolutionary history, genetic structure, and adaptive strategies of A. spathulifolius in coastal environments. Our study challenges previous assumptions and underscores the necessity for further population studies to elucidate the intricate dynamics of this distinctive plant species.