The Japanese prickly ash, Zanthoxylum ailanthoides Siebold & Zucc. (Rutaceae), is one of the most common pioneer tree species that grow in disturbed areas, such as canopy gaps, in Japanese warm-temperate evergreen oak forests. The strong genetic structure of its current population might suggest long-term isolation of subpopulations by demographic events, in addition to ecological features of this species. Analyses of genome-wide nucleotide variation using model-based approaches are necessary to achieve a good understanding of the demographic history of a species. In this study, we analyzed the nucleotide variation in natural populations of Z. ailanthoides using a computer program that applied the isolation-with-migration model to quantitatively infer the demographic history. Nucleotide variations at 10 or 26 nuclear loci in six populations from a wide range of the species distribution were analyzed. The maximum likelihood estimate of the divergence time between the current populations in the two hypothetical refugia of Z. ailanthoides was approximately 24,000 years.Unexpectedly, the estimated size of the ancestral population was larger than the sizes of the two current populations. The results suggested a relatively recent divergence of these two populations and rapid formation of strong genetic structure among subpopulations. The large ancestral population may indicate a more complex demographic history during or after the last glacial period than the simple isolation-withmigration model implies.
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