IngentaConnect Biogeography of the Pistia Clade (Araceae): Based on Chloroplast ...

Abstract:

Pistia stratiotes(water lettuce) andLemna(duckweeds) are the only free-floating aquatic Araceae. The geographic origin and phylogenetic placement of these unrelated aroids present long-standing problems because of their highly modified reproductive structures and wide geographical distributions. We sampled chloroplast (trnL-trnFandrpl20-rps12spacers,trnLintron) and mitochondrial sequences (nad1b/c intron) for all genera implicated as close relatives ofPistiaby morphological, restriction site, and sequencing data, and present a hypothesis about its geographic origin based on the consensus of trees obtained from the combined data, using Bayesian, maximum likelihood, parsimony, and distance analyses. Of the 14 genera closest toPistia, onlyAlocasia,Arisaema, andTyphoniumare species-rich, and the latter two were studied previously, facilitating the choice of representatives that span the roots of these genera. Results indicate thatPistiaand the Seychelles endemicProtarum sechellarumare the basalmost branches in a grade comprising the tribes Colocasieae (Ariopsis,Steudnera,Remusatia,Alocasia,Colocasia), Arisaemateae (Arisaema,Pinellia), and Areae (Arum,Biarum,Dracunculus,Eminium,Helicodiceros,Theriophonum,Typhonium). Unexpectedly, all Areae genera are embedded inTyphonium, which throws new light on the geographic history of Areae. A Bayesian analysis of divergence times that explores the effects of multiple fossil and geological calibration points indicates that thePistialineage is 90 to 76 million years (my) old. The oldest fossils of thePistiaclade, though notPistiaitself, are 45-my-old leaves from Germany; the closest outgroup, Peltandreae (comprising a few species in Florida, the Mediterranean, and Madagascar), is known from 60-my-old leaves from Europe, Kazakhstan, North Dakota, and Tennessee. Based on the geographic ranges of close relatives,Pistialikely originated in the Tethys region, withProtarumthen surviving on the Seychelles, which became isolated from Madagascar and India in the Late Cretaceous (85 my ago).PistiaandProtarumprovide striking examples of ancient lineages that appear to have survived in unique or isolated habitats.1Present address: Systematische Botanik, Ludwig-Maximilians-Universitaet,D-80638Munich,Germany.2Present address: Department of Biology, Colorado State University,Fort Collins,Colorado80523,USA.

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Biogeography of the Pistia Clade (Araceae): Based on Chloroplast and Mitochondrial DNA Sequences and Bayesian Divergence Time Inference