The systematic position of the river-weed family Podostemaceae remains
enigmatic due to taxonomic difficulties imposed by the radically alte
red morphology of these alga-like angiosperms. Although previous worke
rs have placed this group phylogenetically among a wide variety of mon
ocotyledons and dicotyledons, most contemporary authors have proposed
that river-weeds are closely related to members of the dicotyledonous
order Rosales: A diversity of opinion also exists as to whether the Hy
drostachyaceae are related to Podostemaceae. We have investigated the
phylogeny of river-weeds by comparing DNA sequences of the chloroplast
encoded rbcL gene for eight river-weed genera together with 84 other
angiosperm and 11 non-flowering seed plant taxa. The high level of seq
uence divergence in rbcL that exists between river-weeds, Hydrostachya
ceae and other angiosperms presents systematic problems that parallel
those associated with the highly divergent morphology of these groups.
Rooting rbcL sequences with distant non-flowering plant outgroups res
ults in a topology where Podostemaceae comprise a basal angiosperm cla
de, but in which other renditions of angiosperm family relationships a
re depicted unreasonably. Restricting the comparison of river-weed seq
uences entirely with angiosperms places the group as a sister clade to
the Hydrostachyaceae as some authors had anticipated, but this result
is only weakly supported. The high level of both morphological and mo
lecular divergence in the river-weed clade confounds efforts to correc
tly ascertain their phylogenetic relationships. A tentative hypothesis
from rbcL data is that the Hydrostachyaceae and Podostemaceae are sis
ter taxa whose closest relatives are the rosid families Crassulaceae a
nd Haloragaceae. (C) 1997 Elsevier Science B.V.