The genus Wolffia was surveyed electrophoretically at 14 allozyme loci
. A total of 133 clones representing 10 of the 11 recognized species w
as examined. Genetic identities among most pairs of species are zero,
with non-zero values ranging from 0.14 to 0.40. Wolffia angusta and th
e newly described W. neglecta show the highest similarity, and the for
mer species has an identity of 0.14 with W. australiana. The next high
est similarity (0.34) occurs between W. globosa of Southeast Asia and
W. cylindracea of southern Africa, which until recently, had generally
been viewed as members of the same species. Other species showing som
e common alleles are members of a complex involving W. arrhiza, W. col
umbiana, W. cylindracea, and W. globosa. Within W. arrhiza, plants fro
m South Africa and Europe are easily distinguished electrophoretically
because each contains unique alleles at two loci. Strains from other
parts of Africa vary at these loci and are not totally distinct from e
ither the plants from South Africa or from Europe. Species of Wolffia
are much more divergent at allozyme loci than the majority of congener
s of flowering plants. This suggests that the species are quite old an
d that the difficulties in distinguishing taxa morphologically are the
result of reduction rather than lack of divergence due to recent spec
iation. Because of the lack of shared alleles between the majority of
species pairs in Wolffia, enzyme electrophoresis provides limited reso
lution of species relationships in the genus.