Is it all Phacelia pinnatifida? Molecular delimitations and taxonomic revision of Phacelia sect. Glandulosae in South America based on ddRADseq

Abstract

The Phacelia sect. Glandulosae, with ca. 62 taxa, is the most diverse of the 11 sections of the genus Phacelia (Hydrophyllaceae). The family is largely restricted to North and Central America, but the genus Phacelia has an amphitropical distribution with nine species reported from western South America, five of which have been assigned to P. sect. Glandulosae. These species are found in arid to semi-arid Andean habitats between 1000 and 5000 m.a.s.l. Species delimitation has long been confounded for this plant group in South America by some degree of taxonomic chaos—lack of typifications, dozens of synonyms, different names used for the same taxon between different countries—and poor representation in molecular phylogenies. Here, we try to improve our understanding by employing a ddRADseq strategy to investigate phylogenetic relationships of all South American representatives of P. sect. Glandulosae based on a total of 33 sampled specimens. Both ML and species tree inference analyses of the ddRADseq data resulted in a well resolved topology. All five species currently recognized in P. sect. Glandulosae in South America form a highly supported monophylum including P. nana, previously considered as belonging to Phacelia sect. Euglypta. However, the currently recognized morphospecies form a clade with very short branch lengths indicating coalescence processes at the level of population, except for P. artemisioides which is clearly sister to the rest. Additionally, different morphospecies and ecotypes are largely retrieved as poly- and/or paraphyletic. Based on these results and on re-evaluation of the morphological characters traditionally used to delineate species within sect. Glandulosae, we argue that these characters are of limited diagnostic value. We recommend recognizing only two South American species within this section: polymorphic P. pinnatifida and its closely related sister species P. artemisioides. A revised formal taxonomy is provided.