Allozyme variation within and divergence between Lemna gibba and L. disperma: Systematic and biogeographic implications

Enzyme electrophoresis was employed to assess genetic diversity within and divergence between Lemna disperma and Lemna gibba, sister species that have often been considered conspecific because of the few technical morphological characters distinguishing them. L. gibba is distributed widely except in Australia and New Zealand, where it is replaced by L. disperma. Allozyme data were employed to examine: (1) whether species recognition is supported by genetic divergence between accessions assigned to the two taxa, (2) whether the level of diversity in the two species supports the hypothesis that L. gibba-L. disperma are related as a progenitor-derivative species pair, and (3) whether estimates of divergence times obtained from allozymes are in general agreement with those from plastid sequences. Accessions of the two species are highly divergent at allozyme loci, with a genetic identity of 0.404, and the putative derivative species (L. disperma) has only one-third the diversity of its proposed ancestor, L. gibba. Therefore, allozyme data support the continued recognition of the two species and are concordant with the hypothesis that the species are related as progenitor and derivative. The reduced morphology of L. disperma and the allozyme data indicate that this species originated via dispersal of L. gibba or of a common ancestor of the two species. Estimated divergence times from allozymes and plastid sequences vary widely, but assuming that actual divergence was within the broad range of estimates, long distance dispersal is required to explain the present distribution of the two species.