Although aroids (Araceae) and duckweeds (Lemnaceae) are widely regarded as having close systematic relationships, the relative phylogenetic positions of the families remain equivocal. While some data sets (i.e., morphology including fossils and rbcL sequences) suggest that Pistia is sister to the Lemnaceae, other data sets (i.e., chloroplast restriction sites) remove these to separate clades within the Araceae. We are testing these competing hypotheses using sequences from the trnL-F spacer region of the chloroplast genome. Included in the analysis are 21 aroid genera including Pistia, and five genera of Lemnaceae (including the segregate genus Landoltia). Aponogeton is used to root the tree. Approximately 520 aligned base pairs yield maximum parsimony trees nearly identical to those derived from restriction site data. Pistia and the Lemnaceae are placed in two separate and well supported clades, suggesting at least two independent origins of the floating aquatic growth form within the aroid clade. Within the Lemnaceae there is only partial support for the paradigm of sequential morphological reduction, given that Wolffia is sister to Wolffiella+Lemna sensu stricto. Interestingly, single rooted fronds assigned to Lemna minuscula are genetically identical to Landoltia, and these are sister to the (Wolffia (Wolffiella+Lemna)) clade. As in the results of the restriction site analysis, pantropical Pistia is placed with Colocasia and Typhonium of southeastern Asia, indicative of Old World affinities. The duckweeds show dramatically greater rates of nucleotide substitutions and insertion/deletion events relative to the other ingroup taxa. Morphological reduction within the duckweeds thus appears to correlate with accelerated chloroplast genome evolution.

Key words: Araceae, cladistic relationships, Lemnaceae, trnL-F