Phylogenetic relationships of the enigmatic angiosperm family Podostemaceae inferred from 18S rDNA and rbcL sequence data.

The phylogenetic relationships of some angiosperm families have remained enigmatic despite broad phylogenetic analyses of rbcL sequences. One example is the aquatic family Podostemaceae, the relationships of which have long been controversial because of major morphological modifications associated with their aquatic habit. Podostemaceae have variously been associated with Piperaceae, Nepenthaceae, Polygonaceae, Caryophyllaceae, Scrophulariaceae, Rosaceae, Crassulaceae, and Saxifragaceae. Two recent analyses of rbcL sequences suggest a possible sister-group relationship of Podostemaceae to Crassulaceae (Saxifragales). However, the branch leading to Podostemaceae was long, and use of different outgroups resulted in alternative placements. We explored the phylogenetic relationships of Podostemaceae using 18S rDNA sequences and a combined rbcL + 18S rDNA matrix representing over 250 angiosperms. In analyses based on 18S rDNA data, Podostemaceae are not characterized by a long branch; the family consistently appears as part of a Malpighiales clade that also includes Malpighiaceae, Turneraceae, Passifloraceae, Salicaceae, Euphorbiaceae, Violaceae, Linaceae, Chrysobalanaceae, Trigoniaceae, Humiriaceae, and Ochnaceae. Phylogenetic analyses based on a combined 18S rDNA + rbcL data set (223 ingroup taxa) with basal angiosperms as the outgroup also suggest that Podostemaceae are part of a Malpighiales clade. These searches swapped to completion, and the shortest trees showed enhanced resolution and increased internal support compared to those based on 18S rDNA or rbcL alone. However, when Gnetales are used as the outgroup, Podostemaceae appear with members of the nitrogen fixing clade (e.g., Elaeagnaceae, Ulmaceae, Rhamnaceae, Cannabaceae, Moraceae, and Urticaceae). None of the relationships suggested here for Podostemaceae receives strong bootstrap support. Our analyses indicate that Podostemaceae are not closely allied with Crassulaceae or with other members of the Saxifragales clade; their closest relatives, although still uncertain, appear to lie elsewhere in the rosids.