Molecular phylogeny of the cosmopolitan aquatic plant genus Limosella (Scrophulariaceae) with a particular focus on the origin of the Australasian L. curdieana

Limosella is a small aquatic genus of Scrophulariaceae of twelve species, of which one is distributed in northern circumpolar regions, two in southern circumpolar regions, two in the Americas, one endemic to Australia, and six in tropical or southern Africa or both. The Australasian L. curdieana has always been considered distinct but its close phylogenetic relationships have never been inferred. Here, we investigated the following alternative phylogenetic hypotheses based on comparative leaf morphology and habitat preferences or floral morphology: (1) L. curdieana is sister to the African L. grandiflora; or (2) it is closely related to a group of other African species and the northern circumpolar L. aquatica. We tested these hypotheses in a phylogenetic framework using DNA sequence data from four plastid DNA regions and the nuclear ITS region. These were analyzed using maximum parsimony and Bayesian inference. We obtained moderately resolved, partially conflicting phylogenies, supporting that accessions of L. grandiflora form the sister group to the rest of the genus and that L. curdieana groups with the African taxa, L. africana and L. major, and L. aquatica. Thus, the molecular evidence supports the second hypothesis. A biogeographic analysis suggests an out-of-southern Africa scenario and several dispersal events in the Southern Hemisphere. Past dispersal from southern Africa to Australasia is suggested, yet it cannot be excluded that a route via tropical Africa and temperate Asia has existed.