Kimball, R.T., D.J. Crawford, D.H. Les, and E. Landolt. 2003. Out of Africa: molecular phylogenetics and biogeography of Wolffiella (Lemnaceae). Biological Journal of the Linnean Society 79: 565-576.

The monophyletic genus Wolffiella (Lemnaceae) comprises 10 species divided taxonomically among three sections. Relative to other genera of Lemnaceae, Wolffiella has a restricted range, with species distributed in warm temperate to tropical areas of Africa and the Americas, with only one species occurring in both areas. Sequence data from coding (rbcL and matK) and noncoding (trnK and rpl16 introns) regions of cpDNA were analyzed phylogenetically to resolve relationships within Wolffiella, and these results were compared to earlier allozyme and morphological studies. Allozymes, cpDNA, and morphology all supported the recognition of three sections. Relationships among species were similar in most respects between the allozyme and cpDNA trees, as well as among the different plastid partitions. In Wolffiella, both nonsynonymous and synonymous substitutions were greater in matK than rbcL, as observed in other taxa. The synonymous substitution rate in matK was similar to the substitution rate of the noncoding regions. All partitions, including coding regions, exhibited some homoplasy. Biogeographic reconstructions from a combination of cpDNA partitions indicated that Wolffiella originated in Africa with early movement to and radiation in the Americas. The one species found in both Africa and the Americas, W. welwitschii, likely originated in the Americas and subsequently dispersed to Africa. Using the SOWH test, the cpDNA data could reject two alternative biogeographic hypotheses suggested from analyses of morphological and allozyme data. The present distribution of Wolffiella can be explained by two major dispersal events and this contrasts with the more complex species distributions in other duckweed genera. Limited dispersal in Wolffiella relative to other duckweed genera may be due to more recent origins of species, lower dispersability, and poorer colonizing ability.