Morphological and phylogenetic analyses reveal the taxonomic distinctiveness between Nymphaea pygmaea and N. tetragona (Nymphaeaceae) in East Asia

We aimed to clarify the taxonomic identity between Nymphaea tetragona , which has a red to purple stigmatic disk and the widest distribution range among the genus, and Nymphaea pygmaea , which has a yellow stigmatic disk and is ordinarily treated as a synonym of N. tetragona , based on detailed morphological measurements and phylogenetic analyses. Through morphological analysis, we assessed 26 quantitative characters of 117 individuals from 18 populations. Seventeen characters were significantly different between the two species; however, these characters overlapped. Principal component analysis based on 13 morphological characters revealed different trends in the morphology of the two species. In DNA analysis, one nuclear ribosomal DNA region, internal transcribed spacer (ITS) region, and one chloroplast DNA region ( rps4-trnF ) were compared. The ITS region showed differences in 18–19 substitutions and two indels between N. pygmaea and N. tetragona . Additionally, the two species were closely related within the subgenus Nymphaea , and N. tetragona and Nymphaea leibergii formed a robust monophyletic group with N. pygmaea as a sister taxon. These results indicate that N. tetragona and N. pygmaea have differentiated morphologically and genetically, thus making these two species taxonomically distinct.