Notes on the genus Polycarpon (Caryophyllaceae) in South America, with the description of a new subspecies from Peru

Background and Aims: Polycarpon is a genus traditionally comprising about 16 species distributed in tropical and temperate regions. Molecular studies demonstrated its polyphyly, highlighting three supported clades: the P. coquimbense-suffruticosum aggregate (from South America), P. prostratum (tropical), and the P. tetraphyllum aggregate (mainly in the Mediterranean region). The first two clades have been excluded from Polycarpon: P. coquimbense, P. suffruticosum, and P. anomalum recently transferred to Augustea, and P. prostratum recognized under Polycarpaea. The remaining members represent a polyploid complex treated as a single species, P. tetraphyllum. Taxonomic questions remain open, especially concerning the number of taxa occurring in some geographical areas. Among them is South America, for which we here present a synopsis.
Methods: This work is based on field surveys, examination of specimens, and analysis of literature. The International Code of Nomenclature for Algae, Fungi and Plants was consulted.
Key results: Three subspecies of Polycarpon tetraphyllum are recognized in South America: subsp. apurense (from Argentina, Colombia, Paraguay, and Venezuela), subsp. tetraphyllum (alien in most of the countries), and subsp. peruvianum (endemic to Peru). The latter subspecies is newly described here and it differs from subsp. diphyllum and subsp. tetraphyllum by stipule shape, leaf arrangement and colour, arrangement of flowers in inflorescences, petal length, and stamen number. It also presents a distinct habitat: dry, rocky soils, in contrast to the sandy soils or disturbed areas where other subspecies grow. The name P. urbanianum, previously considered as a Peruvian endemic, is here demonstrated to be not a species of Polycarpon. A diagnostic key of the South American subspecies of P. tetraphyllum is proposed.
Conclusions: The results obtained remark the variability of Polycarpon tetraphyllum s.l. and highlight that the flora of South America, especially concerning the small Caryophyllaceae needs to be further investigated.