Phylogeny of Isolepis (Cyperaceae) revisited: non-monophyletic nature of I. fluitans sensu lato and resurrection of I. lenticularis

Abstract

The aquatic and wetland ephemeral genus Isolepis (Cyperaceae) comprises 76 species mostly in the southern hemisphere, and especially Africa and Australasia. The latest taxonomic revision recognizes three subgenera (Fluitantes, Isolepis and Micranthae) and three sections in subgen. Isolepis. Subgen. Fluitantes, mat-forming perennial herbs typically bearing a single terminal spikelet, comprises nine species with a nearly cosmopolitan distribution except in Americas and Antarctica. Of these, I. fluitans includes infraspecific taxa from Africa–Europe and Asia–Australasia that are distinguished by the length of the involucral bract relative to the spikelet. This morphological character is also used in the key to subgen. Fluitantes that separates African–European and Asian–Australasian species. The overall morphological evidence conflicts with the species recognition of I. fluitans sensu lato and rather indicates the non-monophyly of I. fluitans, which we tested in a phylogenetic framework. Sequence data from three plastid DNA regions and nuclear ITS were analyzed using maximum parsimony, maximum likelihood, and Bayesian inference. We obtained moderately resolved phylogenies with the plastid DNA and ITS data sets. Although partially conflicting, both phylogenies rejected the monophyly of I. fluitans and instead revealed inter-continental pattern with infraspecific taxa showing close relationships with species in the subgenus within their geographic area. A revised key to species of subgenus Fluitantes is provided with the Asian–Australasian I. fluitans var. lenticularis resurrected to species rank as I. lenticularis. The phylogeny reveals a single dispersal event from Africa to Australasia, or vice versa in subgen. Fluitantes.