Genetic structure of the introduced heaven tree (Ailanthus altissima) in Japan: evidence for two distinct origins with limited admixture

The heaven tree, Ailanthus altissima (Mill.) Swingle, was intentionally introduced from China to many temperate areas globally, where it has become successfully established. Here, we investigated the patterns of genetic differentiation between introduced Japanese A. altissima populations using both plastid and nuclear microsatellite markers. Three hundred and eighty-eight trees were sampled from six planted and 12 naturalized populations. Two distinct chloroplastic haplotypes were observed, which allowed us to separate two groups of trees, H1 and H3, containing 128 and 260 trees, respectively. Nine nuclear genetic microsatellites revealed 17.7 alleles per locus on average. These data did not indicate significant isolation-by-distance. Analysis of molecular variance showed the absence of genetic differentiation between the planted and naturalized populations. In contrast, the two groups defined according to the chloroplastic haplotypes (H1 and H3) were differentiated. STRUCTURE analysis showed that, within each haplotype group, most trees were strictly assigned to one cluster, with the exception of a few (ca. 30) trees. Our results indicate that (1) geographical genetic structure was weak, with high genetic variation among populations; (2) a congruent signal was revealed by chloroplastic and nuclear markers, indicating two distinct provenances; and (3) admixture between the two lineages has occurred, but remains limited. Several other invasive plant species have already been reported to exhibit similar trends; hence, future management of A. altissima might be required in Japan.