Genetic variation and structure shaped by recent population fragmentation in the boreal conifer Thuja koraiensis: Conservation perspectives

A plant species has historically experienced repeated cycles of habitat connectivity and isolation, which have played a crucial role in increasing genetic diversity and promoting long-term survival. Therefore, understanding these dynamics is essential for developing effective conservation strategies. Thuja koraiensis is an endangered coniferous shrub inhabiting the mountain summits of the Baekdudaegan, a critical ecological axis in northeastern Asia. We used a genome dataset (242 SNPs) generated by the MIG-seq (Multiplexed ISSR Genotyping by Sequencing) method to evaluate the genetic diversity and structure of T. koraiensis populations across their entire distribution range. T. koraiensis exhibited a population history, with range expansion during glacial periods and contraction during interglacial periods. During the Last Glacial Maximum (LGM), genetic connectivity among populations was high, but post-LGM habitat fragmentation led to increasing isolation. This shift resulted in a rapid decline in effective population size and severe bottlenecks across all populations. Consequently, the genetic variation in current populations exhibits a geographically random pattern. Nevertheless, no signs of inbreeding or significant imbalances in genetic diversity were detected among populations. Therefore, we propose that conservation strategies should not solely focus on increasing genetic diversity or enhancing gene flow among populations but rather reflect the species' historical demographic dynamics and aim to conserve the unique genetic characteristics of each population.