Phylogenetic Relationships and Morphological Evolution in the Carnivorous Genus Philcoxia (Plantaginaceae, Gratioleae)

Abstract—

Philcoxia (Plantaginaceae, Gratioleae) is a genus with seven species of small herbs endemic to Brazil. It is characterized by underground stems and petioles, peltate leaves, and zigzag racemose inflorescences, in addition to a remarkable carnivorous syndrome. The phylogenetic placement of Philcoxia within tribe Gratioleae has been uncertain since its discovery due to its unique morphology and limited data from phylogenetic studies. We tested the phylogenetic placement of Philcoxia within Gratioleae, increasing the number of species sampled within both the genus and the rest of the tribe, and sequencing four regions from cpDNA (rpl16, rps16, and trnL introns and the trnL-trnF intergenic spacer) and one from nrDNA (ITS1 spacer); we used both parsimony and Bayesian inference to reconstruct the phylogeny. Moreover, we assessed both the monophyly of the genus and interspecific relationships. Finally, we performed ancestral character state reconstructions of 10 morphological characters to infer synapomorphies for the genus and specific clades within it. Philcoxia was recovered as monophyletic, with maximum support values, as sister to Stemodia stellata. Five morphological character states were reconstructed as potential synapomorphies for the Philcoxia clade (rosetted herb habit, underground stems and petioles, floral resupination, number of stamens reduced to two, and number of fertile thecae per anther reduced to one), whereas four were reconstructed as potential synapomorphies for specific clades within the genus (peltate leaves, irregular rosettes, vegetative propagation by rhizomes, and presence of tubers). Further anatomical studies, in association with ecological data, would provide insight into the structure of the underground systems and its adaptive significance, and would further elucidate the evolution of carnivory in Philcoxia. Our study highlights the need to reassess the concepts and circumscriptions of some currently accepted genera of the Gratioleae such as Bacopa, Conobea, and Stemodia, and for the tribe Angelonieae.