Premise of research. Inflorescence morphology played an important role in the systematics of the twinflower tribe Linnaeeae. Recent molecular studies clarify systematic relationships among taxa and indicate that several genera and subgenera are not monophyletic. Using this molecular phylogeny as a backbone, we present detailed ontogenetic analyses of inflorescences in order to further elucidate the relationships among taxa and to understand the evolutionary history of morphological characters.

Methodology. Fourteen taxa were studied using SEM and LM. Ontogenetic series were compared and character complexes described.

Pivotal results. Emphasizing the importance of the epicalyx and flower orientation we show that flower pairs are not homologous. We also found that a reduction in sepal number occurred independently in Zabelia biflora, Diabelia serrata, and Abelia uniflora. This further supports the distinction between subtribes Zabeliinae and Linnaeinae. The study of anomalous specimens provides further insight into the evolution of inflorescence architecture, and we propose a hypothesis on the inflorescence evolution that is based on a compound thyrse as a starting point. Zabelia and Diabelia show a more “primitive” inflorescence of reduced simple thyrses. Abelia inflorescences are more “derived” and restricted to the twice-compound part of the thyrses. Dipelta, Linnaea, Kolkwitzia, and Vesalea have intermediate inflorescences; some are restricted to the twice-compound part of the thyrse, but others do also present a terminal simple part. Inflorescences of the genus Heptacodium (Caprifolieae) are interpreted as an intermediate link between the tribe Linnaeeae and Caprifolieae, supporting the theory of a hybrid origin of this genus.

Conclusions. We show that the detailed (re)investigation of characters and their ontogenetic history provides important insight into the evolution of taxa and fosters our understanding of complex morphological structures such as the inflorescences in the twinflower clade Linnaeeae.