Monophyly and Relationships of the Enigmatic Family Peridiscaceae

Abstract

Peridiscaccae, comprising Peridiscus, Soyauxia, and Whittonia, are an enigmatic angiosperm family of uncertain composition and placement. Although some have placed Soyauxia in other families (e.g., Flacourtiaceae, Medusandraceae), rather than in Peridiscaceae, sequence data for five genes (material of Whittonia could not be obtained) provide strong support for a clade of Soyauxia and Peridiscus. This evidence, combined with the strong morphological similarity of Peridiscus and Whittonia, support a monophyletic Peridiscaceae of three genera. Molecular analyses of a three-gene (rbcL, atpB, 18S rDNA) dataset for 569 taxa indicate that Peridiscus + Soyauxia together with Daphniphyllaceae form a clade that is sister to the rest of Saxifragales. Maximum likelihood and Bayesian analyses of Saxifragales using a five-gene (rbcL, atpB, matK, 18S rDNA, 26S rDNA) dataset place Peridiscaceae (posterior probability of 1.00) Peridiscaceae as sister to the remainder of Saxifragales, albeit without high posterior probability (pp = 0.78). Parsimony places a well-supported Peridiscaccae (100% bootstrap) as sister to Paeoniaceae within a paraphyletic Hamamelidaceae, a placement that may be due to long-branch attraction. Following removal of Paeoniaceae from the dataset, parsimony trees place Peridiscaceae as sister to the remainder of Saxifragales. Although the placement of Peridiscaccae is not well supported in any analysis, molecular data suggest that Peridiscaceae do not have as their closest relatives Saxifragaceae, Iteaceae, Pterostemonaceae, Haloragaceae, or Crassulaceae, but instead are more closely related to woody members of Saxifragales (Altingiaceae, Cercidiphyllaceae, Hamamelidaceae, and Daphniphyllaccae); several morphological features similarly suggest a relationship of Peridiscaceae to these woody families. The low support for the placement of Peridiscaceae is not surprising; previous analyses indicate that Saxifragales underwent a rapid, ancient radiation, and resolving relationships among members of the clade, particularly the basal grade of woody taxa, has been extremely difficult.