The genus Wolffiella (Hegelm.) Hegelmaier, together with the genus Wolffia Horkel ex Schleid., comprise the subfamily Wolffioideae of the Lemnaceae (duckweeds). Prior to the treatment of Landolt in 1986, species had been transferred between the genera, and several segregate genera were recognized in this subfamily. Problems with generic delimitation were the result of the few morphological characters available in these very small and simple flowering plants, and the relative taxonomic significance assigned features. The most recent treatment of Wolffiella by Landolt recognized nine species in three sections, and he described a new species in 1992. The present phylogenetic study of Wolffiella includes more than 4,600 characters, including flavonoid chemistry, morphology, anatomy, allozymes, and DNA sequences from two chloroplast genes and three chloroplast introns. A parsimony analysis of our data provide strong support for the monophyly of Wolffiella as recognized by Landolt. The two sections Wolffiella and Stipitatae likewise enjoy 100% bootstrap support. Landolt recognized two major lineages within the genus, one consisting of sect. Wolffiella and another comprised of sect. Stipitatae and the monospecific sect. Rotundae. There is 100% bootstrap support for these two groups. Within sect. Wolffiella, the three species W. gladiata, W. lingulata, and W. olbonga are close morphologically, have high genetic identities at allozyme loci, and appear to result from recent speciation. They form a highly derived clade within the section, which is concordant with recent divergence. The three species of sect. Stipitatae and sect. Rotundae, which form one of the two major clades in the genus, show high similarities at allozyme loci, suggesting relatively recent speciation in this strictly African lineage. The phylogeny suggests that the radiation of Wolffiella was from southeastern Africa into South America, and ultimately to North America.

Key words: duckweeds, Lemnaceae, molecular systematics, phylogeny, Wolffiella