From terrestrial to aquatic habitats and back again: molecular insights into the evolution and phylogeny of Callitriche (Plantaginaceae)

Callitriche (Plantaginaceae) is a cosmopolitan plant genus in shallow lakes and river margins, wetlands, ditches and temporary pools. Its species exhibit considerable diversity in growth habit, aquatic (submerged), amphibious and terrestrial, but the polarity of growth habit evolution to or from terrestrial habitats is yet to be addressed. The origin of its cosmopolitan distribution, especially to or from Australasia, remains uncertain. We assessed evolution of growth habit and inferred biogeographic history in a phylogenetic framework using DNA sequence data from plastid DNA and the nuclear internal transcribed spacer region. These were separately analysed using maximum parsimony, maximum likelihood and Bayesian inference. The multilocus data sets, excluding two species exhibiting conflicting placement, were then used for coalescent-based species-tree analysis and the resulting species tree was used for ancestral state reconstruction and biogeographic inference. We obtained moderately resolved phylogenetic trees, showing that two terrestrial species, one from Australasia and the other from temperate and tropical Asia, are the first diverging species and the remaining terrestrial species are scattered across the trees. The ancestral state reconstruction supports a terrestrial origin of the genus and multiple reverse transitions to terrestrial conditions. The biogeographic results suggest a Northern Hemisphere origin with multiple dispersal events to Australasia or an out-of-Australasia scenario and rather recent, multiple dispersal events back to Australasia.