Relationships among duckweed taxa may be difficult to resolve because their extreme reduction offers few morphological characters for analyses. Molecular data are providing refined insights into duckweed taxonomy and phylogeny. Lemna japonica is largely restricted to warm temperate regions of Eastern Asia; L. minor is widely distributed in temperate regions but is rare in Eastern Asia; and L. turionifera occurs in temperate regions of North America and Eurasia. It has been hypothesized that L. japonica is of hybrid origin with L. minor and L. turionifera as the parents; a prior allozyme study provided some support for this hypothesis. To test this hypothesis, intersimple sequence repeats (ISSRs), allozymes, and sequences from the chloroplast rpl16 intron were examined from 45 clones of the three species. Two rpl16 sequences were found; clones clearly referable to L. minor have one type and L. turionifera have another. Clones assigned to L. japonica have either of the sequences; those with well-developed turions have L. turionifera sequences and those lacking or with weakly-developed turions have L. minor sequences. A neighbor-joining tree based on ISSR banding patterns shows, with rare exceptions, clones with the same rpl16 sequences grouping together. All three species are very similar at allozyme loci; however, clones with L. turionifera rpl16 sequences are distinguishable from those with L. minor sequences. The molecular data suggest that L. japonica consists of two entities, one conspecific with L. turionifera and another similar to, if not conspecific with L. minor. There are, however, small yet consistent morphological differences between L. minor and “minor-like” clones of L. japonica, suggesting that the latter may be a recent derivative of the former.

Key words: allozymes, ISSRs, Lemna, rpl16, sequences