The phylogenetic position of river-weeds (Podostemaceae)" Insights from rbcL sequence data

The systematic position of the river-weed family Podostemaceae remains enigmatic due to taxonomic difficulties imposed by the radically altered morphology of these alga-like angiosperms. Although previous workers have placed this group phylogenetically among a wide variety of monocotyledons and dicotyledons, most contemporary authors have proposed that fiver-weeds are closely related to members of the dicotyledonous order Rosales. A diversity of opinion also exists as to whether the Hyclrostachyaceae are related to Podostemaceae. We have investigated the phylogeny of fiver-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 sequence divergence in rbcL that exists between fiver-weeds, Hydrostachyaceae 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 results in a topology where Podostemaceae comprise a basal angiosperm clade, but in which other renditions of angiosperm family relationships are depicted unreasonably. Restricting the comparison of river-weed sequences 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 molecular divergence in the river-weed clade confounds efforts to correctly ascertain their phylogenetic relationships. A tentative hypothesis from rbcL data is that the Hydrostachyaceae and Podostemaceae are sister taxa whose closest relatives are the rosid families Crassulaceae and Haloragaceae. © 1997 Elsevier Science B.V. * Corresponding author. 0304-3770/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0304-3770(96)01117-5 6 D.H. Les et aL/Aquatic Botany 57 (1997) 5-27