On merging sections and : Molecular and morphological evidence.

In this study, we expanded Acer sect. Rubra Pax to include A. sect. Hyptiocarpa Fang. Traditionally, section Rubra comprises two iconic species, Linnaeus (red maple) and Linnaeus (silver maple), of eastern North American forests as well as the rare Japanese montane species, K. Koch. Section Hyptiocarpa consists of Hasskarl and Merrill, which occur in subtropical and tropical regions of southwestern China to southeast Asia. Here, we confirm prior phylogenetic results showing the close relationship between sects. and , and we use scanning electron microscopy to demonstrate that leaves of species within these sections have similar arrangements of cuticular waxes, which account for the silvery color of their abaxial surfaces. We describe that the sections also share labile sex expression; inflorescences that range from compound racemose thyrses, to racemes or umbels and that may have undergone evolutionary reduction; and several features of their fruits, such as seed locules without keels, basal portion of wings straight, acute attachment angle between mericarps, and production of some mericarps that are seedless and partially developed at maturity. Our expansion of sect. Rubra to include sect. Hyptiocarpa better elucidates the biogeographic and evolutionary history of these species. Additionally, we show that and have intergrading morphology and are probably synonymous, but we note that further studies are required to conclude their taxonomic status.