The minute, reduced plants of family Lemnaceae have presented a formidable challenge to systematic investigations. The simplified morphology of duckweeds has made it particularly difficult to reconcile their interspecific relationships. A comprehensive phylogenetic analysis of all currently recognized species of Lemnaceae has been carried out using more than 4,700 characters that include data from morphology and anatomy, flavonoids, allozymes, and DNA sequences from chloroplast genes (rbcL, matK) and introns (trnK, rpl16). All data are reasonably congruent (I_(MF) < 6%) and contributed to strong nodal support in combined analyses. Our combined data yield a single, well-resolved, maximum parsimony tree with 30/36 nodes (83%) supported by bootstrap values that exceed 90%. Subfamily Wolffioideae is a monophyletic clade with 100% bootstrap support; however, subfamily Lemnoideae represents a paraphyletic grade comprising Landoltia, Lemna, and Spirodela. Combined data analysis confirms the monophyly of Landoltia, Lemna, Spirodela, Wolffia, and Wolffiella. Phylogenetic relationships are used to evaluate and refine the classification of duckweeds.

Communicating Editor: Jeff H. Rettig