Relationships among the confounding genera Ammannia, Hionanthera, Nesaea and Rotala (Lythraceae)

The relationships and taxonomic limits of four morphologically closely similar herbaceous genera of Lythraceae have long been poorly understood. Ammannia, Hionanthera, Nesaea and Rotala are small-flowered herbs of aquatic to amphibious habitats in subtropical to tropical Africa and Asia, with a minor presence in the New World. The shared generalized vegetative and floral structures and an inadequate knowledge of features regarded as diagnostic of the genera have resulted in diverse taxonomic delineations and multiple species transfers among Ammannia, Nesaea and Rotala. In this study, vegetative, anatomical, floral, seed and pollen characters are compared, new chromosome numbers are reported for Ammannia and Nesaea, and phylogenetic relationships of the four genera are hypothesized based on datasets from nuclear rDNA internal transcribed spacer (ITS) and plastid rbcL and trnL–trnF regions. The results indicate that Rotala, together with the American genera Heimia and Didiplis, forms an early lineage of the family that is only distantly related to the other three genera. Ammannia, Hionanthera and Nesaea form a strongly supported clade in which Ammannia and Nesaea are paraphyletic and Hionanthera is sister to different species of Nesaea depending on the analysis. Total morphological and molecular evidence supports congeneric status for Ammannia, Hionanthera and Nesaea under the earliest name, Ammannia.