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 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.