Systematics of Gratiola (Plantaginaceae)

Gratiola (Plantaginaceae tribe Gratioleae) is a genus of ca. 33 wetland-adapted herbaceous species found mostly in temperate or tropical-montane regions of the Americas, Eurasia, and Australasia. The only worldwide taxonomic treatment of Gratiola was published more than 160 years ago and only a few representatives of the genus have been included in published molecular phylogenetic studies. The overall aim of this investigation was to provide a modern systematic study of Gratiola by determining the phylogenetic placement of Gratiola within the Gratioleae using chloroplast DNA sequence data (Chapter 2); examining the phylogenetic relationships, morphological character evolution, and biogeographical patterns within Gratiola using a combination of chloroplast DNA sequence data and morphology (Chapter 3); and conducting a taxonomic study of the Gratiola neglecta species complex, a group of four closely related and predominantly eastern North American species (Chapter 4). This dissertation is the accumulation of information from three original research papers. The first paper (Chapter 2) has been accepted for publication in Systematic Botany. The second paper (Chapter 3) will be submitted to Systematic Botany. The third paper (Chapter 4) has been published in the Journal of the Botanical Research Institute of Texas. The results from this study show (1) that Gratiola sensu lato (including Amphianthus, Fonkia, Sophronanthe, and Tragiola) is monophyletic and firmly embedded in a monophyletic Gratioleae and is sister to the Old World genera Hydrotriche and Limnophila; (2) that the 33 species (including four undescribed taxa) of Gratiola group into four major clades (Diandrae, Gratiola, Nibora, Sophronanthe) that are each morphologically cohesive and largely correspond to major biogeographic regions; and (3) that the Gratiola neglecta Complex includes four species (G. neglecta, G. floridana, G. graniticola, and G. quartermaniae), two of which constitute previously undescribed species that are morphologically and ecologically distinct.