Phylogenetic Relationships of the Monotypic Genus Amphianthus (Plantaginaceae Tribe Gratioleae) Inferred from Chloroplast DNA Sequences

Within the past decade, Scrophulariaceae sensu lato has been shown to be polyphyletic and, as a result, is currently undergoing major systematic revision. The traditionally recognized family is now generally considered to comprise several smaller families including the newly expanded Plantaginaceae, a family of 12 tribes, 92 genera, and approximately 2000 species. Recent evidence from molecular phylogenetics supports the inclusion of the tribe Gratioleae within the Plantaginaceae. Gratioleae includes 16–40 genera, depending on generic circumscription, many of which have yet to be assessed phylogenetically. Amphianthus is a monotypic genus whose systematic affinities have long been poorly known. We included Amphianthus, 10 additional Gratioleae genera, and several outgroup genera from Plantaginaceae in a phylogenetic investigation to examine the relationships of Amphianthus. We present the most complete phylogeny of the Gratioleae to date and provide evidence from chloroplast DNA sequences of the ndhF gene and the trnS–trnG–trnG intergenic spacer and intron that unequivocally place Amphianthus within Gratiola, and discuss the morphological evidence supporting our findings. Based on this evidence, we transfer the sole species of Amphianthus (Amphianthus pusillus) to Gratiola, establishing the new name Gratiola amphiantha and placing Amphianthus in synonymy with Gratiola. Keywords—Gratiola, Gratiolaceae, ndhF, Scrophulariaceae s.l., trnS–trnG–trnG. Within the past decade, Scrophulariaceae sensu lato (s.l.) has been shown to be polyphyletic and, as a result, is currently undergoing major systematic revision (Olmstead and Reeves 1995; Olmstead et al. 2001; Beardsley and Olmstead 2002; Albach et al. 2005; Oxelman et al. 2005; Rahmanzadeh et al. 2005). Scrophulariaceae s.l. is now generally treated as several smaller families including the newly expanded Plantaginaceae (Veronicaceae sensu Olmstead et al. 2001; Olmstead et al. 2001; Fischer 2004; Albach et al. 2005). According to the circumscription of Albach et al. (2005), Plantaginaceae contains 12 tribes with 92 genera and approximately 2000 species. Olmstead et al. (2001) and more recently, Albach et al. (2005) and Oxelman et al. (2005), provided molecular evidence supporting the inclusion of tribe Gratioleae, the focus of the current study, within the Plantaginaceae. Bentham and Hooker (1876) reported 37 genera and ca. 306 species for Gratioleae. Taxonomic modifications to the tribe were subsequently made by Wettstein (1891), Ruoy (1909), Pennell (1935), and Thieret (1954, 1967). Not until the recent application of molecular phylogenetic analysis, however, were these traditional taxonomic treatments rigorously evaluated. Many genera once included within Gratioleae (Bentham and Hooker 1876; Wettstein 1891) have recently been shown to be distantly related (Beardsley and Olmstead 2002; Albach et al. 2005; Oxelman et al. 2005; Rahmanzadeh et al. 2005). With many genera now excluded from Gratioleae, the tribe is left with ca. 16–40 genera (depending on generic circumscription) and ca. 320 species (Fischer 2004; Albach et al. 2005; Estes et al. unpubl. data). Morphologically, the tribe is characterized by leaves simple, opposite or whorled, and frequently glandular-punctate; trichomes often with a pluri-cellular head; inflorescences of solitary axillary flowers or bracteate racemes; pedicels ± bibracteolate; corollas usually slightly to markedly zygomorphic, mostly tubular or campanulate, and more or less bilabiate; abaxial stamens without appendages; locules distinct and mostly with 2 anther thecae; ovules with 1–3 intermediate layers of integument; stigmas mostly distinct or 2-lobed and flattened; capsules 2–4 valved and primarily septicidally (sometimes primarily loculicidally) dehiscent; seeds small and numerous, generally simply reticulate with testa cells that have hooklike wall thickenings; and endosperm mostly terete (Wettstein 1891; Pennell 1935; Thieret 1967; Fischer 2004; Rahmanzadeh et al. 2005). The genera of Gratioleae are distributed throughout the world but are best represented in the Neotropics and closely adjacent temperate regions. Several of the New World genera are monotypic including Amphianthus Torr., Benjaminia Mart. ex Benj., Boelckea Rossow, Braunblanquetia Eskuche, Geochorda Cham. et Schlecht, Ildefonsia Gardn., Maeviella Rossow, Schizosepala G.M.Barroso, and Tetraulacium Turcz. In addition, Sophronanthe Benth. and Tragiola Small & Pennell have each been regarded as monotypic (Pennell 1935) or as members of Gratiola L. sect. Sophronanthe Benth. (Bentham 1846). Four molecular phylogenetic studies (Olmstead et al. 2001; Albach et al. 2005; Oxelman et al. 2005; Rahmanzadeh et al. 2005) have helped clarify the position of Gratioleae relative to the other major clades of Plantaginaceae. Due to limited sampling of Gratioleae genera, however, the circumscription of the tribe and relationships among its genera remain unclear. Combined, these studies included seven genera (here termed core Gratioleae): Amphianthus, Bacopa Aubl., Gratiola, Mecardonia Ruiz & Pav., Otacanthus Lindl., Scoparia L., and Stemodia L. Some additional genera, including Angelonia Humb. & Bonpl., Basistemon Turcz., Melosperma Benth., Monopera K. Barringer, Monttea Gay, and Ourisia Comm. ex Juss. (here informally referred to as the “Angelonieae clade”), have been assigned to Gratioleae by some authors (e.g. Bentham 1846, in part; Oxelman et al. 2005), but excluded from Gratioleae by others (e.g. Pennell 1920; Thieret 1954, 1967; Rossow 1985; Olmstead et al. 2001; Fischer 2004; Albach et al. 2005). All but Monopera have been included in the recent molecular studies cited above. In the three studies that have included representatives of both Gratioleae and the “Angelonieae” (Olmstead et al. 2001; Albach et al. 2005; Oxelman et al. 2005), the “Angelonieae” generally form a clade sister to Gratioleae although support for their sister relationship is low or, in some cases, nonexistent. In this paper, we do not consider the genera of the “Angelonieae” to be part of the Gratioleae, but further investigation of their relationships are underway (Estes et al. unpubl. data). Amphianthus, a monotypic genus represented by the species Amphianthus pusillus Torrey, is a rare aquatic annual enSystematic Botany (2008), 33(1): pp. 176–182 © Copyright 2008 by the American Society of Plant Taxonomists