Phylogenetics and Biogeography of Apios (Fabaceae) Inferred from Sequences of Nuclear and Plastid Genes

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Premise of research. Apios is a perennial twining herbaceous plant genus with a disjunct distribution between eastern Asia and North America. Some of its species have shown great economic potential as a tuber crop and a source of useful flavonoids. However, the phylogenetics of Apios has not been studied using molecular data. Several questions remain unanswered, including the monophyly of Apios, species boundaries, interspecific relationships, the pattern and timing of the intercontinental disjunct distribution, and the evolution of tuber formation. Methodology. Phylogenetic analyses were conducted on the basis of sequences from nuclear and plastid genomes using both maximum parsimony and maximum likelihood methods. Species trees were inferred via Bayesian analysis in BEST. Divergence times were estimated using BEAST with multiple calibration points, and ancestral areas were inferred with S-DIVA. Pivotal results. Our DNA sequence data recognize the five species of Apios, including A. carnea, A. fortunei, A. delavayi, A. americana, and A. priceana. Apios species form a clade in the basal position of the phaseoloid group, and within Apios, A. carnea is sister to the remaining species. The two North American species (A. americana and A. priceana) are more closely related to each other than either is to any of the Asian species, and their ancestral populations may have migrated from Asia in the late Miocene and early Pliocene via the Bering land bridge. Well-developed tuber is a derived trait in Apios that may function as assisting vegetative reproduction. Parapatric speciation may have happened in both eastern Asia and North America, adapting to specific habitats or pollinators. Conclusions. Apios is a monophyletic lineage that diverged early in the phaseoloid group. Within Apios, the two North American species form a clade that was derived from Asian ancestral populations in the late Miocene. Unequal range of distribution of species in both eastern Asia and North America may be explained by parapatric speciation.