Unraveling the Phylogeny of Heptacodium and Zabelia (Caprifoliaceae): An Interdisciplinary Approach

Recent molecular investigations place Heptacodium as sister to the Lonicera clade, whereas morphology suggests a close relationship with the Linnaea clade s. l. (former tribe Linnaeeae). Zabelia has always been assumed to be closely related or even congeneric with Abelia. This study presents molecular and morphological data in an attempt to further clarify the systematic positions of Heptacodium and Zabelia as they are key genera to help us understand evolution in Dipsacales. Our molecular analyses strongly support a sister relationship between Heptacodium and the Lonicera clade. Zabelia, however, seems to be closer related to the Morina clade than to any other member of the Linnaea chide s. I. None of our phylogenetic analyses suggest a close relationship between Abelia and Zabelia. Morphologically, Zabelia's relationship with the Morina clade is only supported by the shared presence of psilate pollen grains with an endocingulum. Fruit and seed morphology strongly indicate a close relationship between Abelia, Heptacodium, and Zabelia. Even though wood anatomy of Zabelia is unique in several aspects, it does not aid in unraveling the systematic position of the genus. In the case of Heptacodium, hybridization is a plausible scenario that could explain the uncertain systematic position of the genus. A better understanding of the intergeneric relationships of the Lonicera and Linnaea s. l. clades is essential to solve this matter.