The history and biogeography of adelgids and their host plants: Molecular phylogenies of the Adelgidae, hemlock adelgids and their hemlock (Tsuga) hosts

The Adelgidae are a group of insects with some of the most complex life cycles of all Metazoa. They also include some of the most destructive pest species currently threatening North American forests. Phylogenetic analyses found that the major adelgid clades correspond neatly to their association with host genera. Molecular estimation of divergence times suggests that this diversification occurred in the Late Cretaceous and Early Tertiary when the Pinaceae genera were diverging. It is not clear whether the Adelgidae and the Pinaceae co-diversified because relationships within the Pinaceae are not fully resolved. The hemlock woolly adelgid, Adelges tsugae, is an introduced pest that is causing severe mortality to hemlocks ( Tsuga spp.) in eastern North America. Phylogenetic analyses suggest that the source of A. tsugae in eastern North America was a population of adelgids associated with T. sieboldii in southern Japan. A separate lineage was associated with the other Japanese hemlock species, T. diversifolia. Hemlock adelgids in western North America appear to be endemic. Lineages from mainland China and Taiwan are clearly diverged from the other lineages. Relationships among all nine species of Tsuga were inferred using DNA sequence data from three chloroplast regions, and multiple cloned sequences from the nuclear ribosomal internal transcribed spacer region. There was strong support for a clade that includes the two western North American species, T. heterophylla and T. mertensiana, and a clade of Asian species within which the eastern North American species, T. caroliniana, is nested. The other eastern North American species, T. canadensis , was sister to the Asian clade. Tsuga chinensis from Taiwan did not group with T. chinensis from mainland China, and T. sieboldii from Ullung Island did not group with T. sieboldii from Japan, suggesting that these lineages should be treated as separate taxa. Tsuga dumosa was placed in different positions in the chloroplast and ITS trees, suggesting a hybrid origin. Comparing the phylogeny and difergence times of Tsuga with those of the adelgids that feed on them, suggests that they did not co-diversify, and that host switching between distantly related Tsuga species has occurred.