LES, DONALD H.*, DANIEL J. CRAWFORD, ELIAS LANDOLT, JOHN D. GABEL, AND REBECCA T. KIMBALL. Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269-3043; Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, Columbus, OH 43210; Geobotanisches Institut ETH, Zürichbergstrasse 38, CH-8044, Zürich, Switzerland. - Phylogenetic relationships in Lemnaceae Dumortier, the duckweed family.
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