Using molecular phylogenetic analyses to assess the timing and biogeography of aquatic plant origins

Aquatic plants are diverse and widespread, representing one of the most profound examples of convergent evolution. Although they are fairly well understood as lineages in isolation, there have been few comprehensive syntheses of all aquatic plant species worldwide. The opportunity for a broad study of aquatic plant evolution has been enabled by the accumulation of abundant molecular, morphological, and biogeographical data in recent years, as well as associated analysis methods. The current study utilizes available data to evaluate angiosperm phylogenetic relationships, focusing on lineages with one or more aquatic species, and to reconstruct the ancestral timing and biogeography of lineages that became aquatic. It was estimated that there were 604 independent origins of the aquatic habit, resulting in 3427 extant aquatic species. The frequency of aquatic origins apparently increased over several time periods, relative to the diversification rate of terrestrial angiosperms. Aquatic plant species most commonly exhibit the hyperhydrate (emergent) growth form, which characterizes the majority of extant species and likely represents a common ancestral habit for most aquatic lineages. Potential limitations of the current study are discussed, including data quality, ancestral biogeography and dating methods, and the uncertainty associated with defining aquatic plants. This study should serve as a foundation for future research into aquatic plant evolution and the development of a phylogenetically informed growth form classification for aquatic plants. • Estimated 604 independent origins of aquatic plants from terrestrial ancestors. • More than twice as many origins as had been estimated previously. • Original aquatic growth form was most often reconstructed as emergent wetland. • Some time periods and regions had higher rates of aquatic plant evolution. • Recommend using phylogenetic data to reevaluate aquatic species criteria.