Allozyme Studies in Spirodela (Lemnaceae): Variation Among Conspecific Clones and Divergence Among the Species

Enzyme electrophoresis was employed to measure genetic diversity among clones of the three species of Spirodela, and to estimate allozyme divergence among the species. A total of 131 clones was examined at 16 presumptive loci. Total diversity within S. polyrrhiza is low (0.034) compared to reports for most species of flowering plants. Diversity in S. punctata is over twice as high (0.072) as in S. polyrrhiza, and S. intermedia has considerably higher diversity (0.144) than its congeners. The low diversity in S. polyrrhiza is correlated with a lower frequency of reproduction by seed compared to S. intermedia and S. punctata. Four multilocus genotypes were found among the 67 clones of S. polyrrhiza, with each largely restricted to particular geographic areas. The 21 clones of S. intermedia represented 15 multilocus arrays and 14 genotypes were found in the 43 clones of S. punctata examined. The three species are highly divergent at allozyme loci. The genetic identity between S. intermedia and S. polyrrhiza is 0.404 despite the fact that the two species are not easily separable in the basis of morphology. Spirodela punctata shares no alleles with the other two species at the 16 loci examined. This is concordant with the current taxonomic treatment for Spirodela in which the former two species are placed in one section and the latter in another.