Phylogenetic relationships of the enigmatic angiosperm family Podostemaceae inferred from 185 rDNA and rbcL sequence data

The phylogenetic relationships of some angiosperm families have remained en igmatic despite broad phylogenetic analyses of rbcL sequences. One example is the aquatic family Podostemaceae, the relationships of which have long b een controversial because of major morphological modifications associated w ith their aquatic habit. Podostemaceae have variously been associated with Piperaceae, Nepenthaceae, Polygonaceae, Caryophyllaceae, Scrophulariaceae, Rosaceae, Crassulaceae, and Saxifragaceae. Two recent analyses of rbcL sequ ences suggest a possible sister-group relationship of Podostemaceae to Cras sulaceae (Saxifragales). However, the branch leading to Podostemaceae was l ong, and use of different outgroups resulted in alternative placements. We explored the phylogenetic relationships of Podostemaceae using 18S rDNA seq uences and a combined rbcL + 18S rDNA matrix representing over 250 angiospe rms. In analyses based on 18S rDNA data, Podostemaceae are not characterize d by a long branch; the family consistently appears as part of a Malpighial es clade that also includes Malpighiaceae, Turneraceae, Passifloraceae, Sal icaceae, 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 outg roup also suggest that Podostemaceae are part of a Malpighiales clade. Thes e searches swapped to completion, and the shortest trees showed enhanced re solution and increased internal support compared to those based on 18S rDNA or rbcL alone. However, when Gnetales are used as the outgroup, Podostemac eae appear with members of the nitrogen fixing clade (e.g., Elaeagnaceae, U lmaceae, Rhamnaceae, Cannabaceae, Moraceae, and Urticaceae). None of the re lationships suggested here for Podostemaceae receives strong bootstrap supp ort. Our analyses indicate that Podostemaceae are not closely allied with C rassulaceae or with other members of the Saxifragales clade; their closest relatives, although still uncertain, appear to lie elsewhere in the rosids. (C) 1999 Academic Press.