Phylogeny and biogeography of Sagittaria (Alismataceae) revisited: evidence for cryptic diversity and colonization out of South America.

Sagittaria is a genus of ca. 40 species in the aquatic plant family Alismataceae with a nearly global distribution, and a center of diversity in the New World. Two thirds of the known species are native to the Americas, while only a few species are distributed in Africa, Asia and Europe. A previous biogeographic analysis of the genus suggested an African origin for the genus with subsequent dispersal to North America and then to East Asia. Here we expanded the taxon sampling with a focus on the New World taxa and applied species delimitation and biogeographic analyses to revise the knowledge of the phylogeny and evolution of the genus. We obtained largely similar topologies from the chloroplast DNA and nuclear DNA (ITS) data sets. The 74 accessions sampled for our analyses were delimited into 29 species and several cryptic taxa were revealed in widely distributed species. Biogeographic analysis supported basal diversification in South America and subsequent colonization to North America and Asia.

Molecular phylogenetics of an aquatic plant lineage, Potamogetonaceae.

Like most aquatic plants, the pondweeds (Potamogetonaceae) are among the most phenotypically reduced and plastic of all angiosperms. As such, hypotheses of structural homology present difficulties for morphological phylogenetic reconstruction. We used non-coding nuclear and plastid DNA data to address Potamogetonaceae relationships and accompanying issues in character evolution and biogeography. Genera currently assigned to Potamogetonaceae, plus Zannichellia, formed a strongly supported monophyletic group. Potamogeton and Stuckenia (Potamogeton subg. Coleogeton) were both resolved as monophyletic. Within Potamogeton proper, two major clades followed the traditional split between broad- and narrow-leaved species, with the latter condition optimized as basal. Heterophylly (submerged plus floating leaves) has evolved several times, and the ancestral distribution for Potamogeton appears to be Northern Hemispheric. Our phylogenetic results have provided a useful genetic framework from which to interpret morphological, cytological and biogeographical evolution.