Chloroplast (rbcL, trnL intron, trnL-trnF spacer) and nuclear ribosomal (ITS1, 5.8S, ITS2) DNA sequences were analyzed to identify the closest terrestrial relatives of the mangrove family Avicenniaceae. These plants have been classified within or near Verbenaceae in most synoptic treatments of angiosperms. Surprisingly, Avicenniaceae were placed as part of Acanthaceae s.l. in analyses of all data sets, using both parsimony and maximum likelihood. Within Acanthaceae s.l., our analyses consistently placed Avicennia as sister to Thunbergioideae but without strong support. Constrained maximum likelihood analyses indicated that alternative placements of Avicennia near the base of Acanthaceae s.l. were not significantly less likely than the sister group relationship with Thunbergioideae. However, placement with Verbenaceae was significantly less likely, as was placement with Pedaliaceae. Morphological evidence is reviewed in this phylogenetic context, and we suggest that articulated nodes and inflorescence structure (including flowers subtended by a bract and two bracteoles) may provide synapomorphies for Avicennia and Acanthaceae s.l. We can identify no clear morphological synapomorphies linking Avicennia to Verbenaceae. Avicennia shares a number of features with each of its putative relatives that are likely to be symplesiomorphic or are of uncertain phylogenetic status.

Communicating Editor: Aaron Liston