We conducted a molecular phylogenetic study of the Boraginaceae and Hydrophyllaceae using the chloroplast genes rbcL and ndhF and including 90 species previously assigned to the two families. Results indicate that most of the taxa assigned to these families together form a monophyletic group, but that neither of the two families, as traditionally circumscribed, are monophyletic. Hydrolea (Hydrophyllaceae) and Pteleocarpa (Boraginaceae) do not belong in this group and are better placed elsewhere. Two major clades are identified. The first major group includes the predominantly temperate herbaceous subfamily Boraginoideae and the genus Codon (Hydrophyllaceae; S. Africa). The other major monophyletic group includes clades that correspond roughly to subfamilies Cordioideae, Ehretioideae, and Heliotropoideae of Boraginaceae and tribes Nameae and Hydrophylleae/Phacelieae of Hydrophyllaceae. Within this second clade, the predominantly woody and tropical members of Boraginaceae (Cordioideae, Ehretioideae, and Heliotropoideae) are monophyletic and these are sister to tribe Nameae, which also contains the only woody and tropical members of Hydrophyllaceae (along with other temperate species). Pholisma, representing the small parasitic plant family, Lennoaceae in our study, is sister to Tiquilia (Ehretioideae), with which it co-occurs in the deserts of North America. The primary characteristic that distinguishes Boraginaceae from Hydrophyllaceae is the reduced number of ovules (2 per carpel). The most parsimonious interpretation of this character on our tree requires two independent reductions in ovule number, once in the ancestor of the Boraginoideae (with the characteristic nutlet fruits) and once in the ancestor of the tropical, woody clade (mostly drupes).

Key words: Boraginaceae, Hydrophyllaceae, Lennoaceae, Solanales