Updating the taxonomy of polymorphic plant taxa: six Atlantic Forest species segregated from the widely circumscribed Oxalis polymorpha Mart. ex Zucc. (Oxalidales, Oxalidaceae)

Widely circumscribed taxa are good candidates for in-depth taxonomic studies. The current taxonomy of Oxalis sect. Polymorphae includes 13 species mostly endemic to the Brazilian Atlantic Forest, in eastern Brazil. Oxalis polymorpha Mart. ex Zucc., the type-species of the section, exhibits a wide morphological variability and includes at least four recognized morphotypes. We propose the segregation of six species from this widely circumscribed O. polymorpha by combining morphological and molecular systematic data from the nuclear and plastid genomes. Thus, Oxalis decipiens Progel, previously treated under O. polymorpha, is accepted as a distinct species. The same is valid for O. polymorpha subsp. tijucana Lourteig, which is recognised at species level (O. tijucana (Lourteig) Fiaschi). Four additional new species are described based on material that were formerly placed under the wide circumscription of O. polymorpha: O. amorimii Fiaschi sp. nov., O. animarum Fiaschi sp. nov., O. idimae Fiaschi sp. nov., and O. itatingae Fiaschi sp. nov. We provide evidence that one of these species, O. itatingae, is phylogenetically closer to O. sect. Holophyllum than to other species of O. sect. Polymorphae, while the other species, excluding O. tijucana, emerged as non-sister taxa within the latter. Descriptions, illustrations, geographic distribution maps, preliminary conservation assessments, taxonomic notes, and an identification key to species of Oxalis sect. Polymorphae are provided.