Phylogenetic placement of the enigmatic angiosperm Hydrostachys

Hydrostachys, an enigmatic aquatic plant genus from Madagascar and southern to central Africa, has been suggested to be related to Lamiales based on embryology, whereas rbcL sequences have indicated a relationship with Hydrangeaceae (Cornales). We investigated these hypotheses by combining rbcL with atpB and ndhF sequences and increased taxon sampling in Hydrangeaceae compared to previous analyses. Rates and patterns of molecular evolution vary across ndhF with the 3 ' portion giving a different result than the 5 ' portion. The highly divergent sequences of Hydrostachys make phylogenetic placement difficult, but maximum parsimony and maximum likelihood analyses of combined data sets agree in placing Hydrostachys within Hydrangeaceae, as do separate analyses of rbcL, atpB, and the 5 ' portion of ndhF. Few morphological and embryological characters support this relationship.