Duckweeds (family Lemnaceae) comprise a distinctive group of diminutive, aquatic monocotyledons which have been difficult taxonomically because of their cosmopolitan distribution, extreme reduction and miniaturization. As currently circumscribed, the Lemnaceae comprise 38 species in five genera which include the world's smallest angiosperms. Taxonomic studies in the 19th and 20th centuries by Schleiden, Hegelmaier, Thompson and Daubs began to clarify generic and species limits in the family, but did little to evaluate either intergeneric or interspecific phylogenetic relationships. Only within the last 15 years have detailed phylogenetic hypotheses been presented for Lemnaceae by Landolt. With few conspicuous morphological characters to serve as phylogenetic markers, molecular data have been applied to duckweed taxonomy since the 1960's when McClure studied flavonoids intensively in the family. Recently, the three senior authors have undertaken a major study to ascertain interspecific phylogenetic relationships by analyzing data sets encompassing a wide range of characters. We present results of phylogenetic analyses of more than 4,600 characters, including data from morphology and anatomy, flavonoids, allozymes and DNA sequences from chloroplast genes (rbcL, matK) and introns (trnK, rpl16). With exception of flavonoids, all data are reasonably congruent; yet even flavonoid data contribute to nodal support in combined analyses. Using parsimony, our data yield a single, well-resolved, maximum parsimony tree with most nodes supported by bootstrap values exceeding 90%. Only one major topological disparity exists between cpDNA based trees and those obtained from nuclear encoded genes such as allozymes (Lemna japonica exhibits characteristics of a hybrid origin, which was hypothesized previously by Landolt). Our studies support the taxonomic recognition of five monophyletic duckweed genera (Landoltia, Lemna, Spirodela, Wolffia, Wolffiella), and present specific hypotheses of interspecific relationships among all 38 known extant species. As a consequence, Lemnaceae are now among the most thoroughly understood angiosperm families from a systematic standpoint.

Key words: aquatic plants, duckweeds, Lemnaceae, molecular systematics, monocotyledons, phylogeny