One more piece to the puzzle: Diadorimia , a new monotypic genus in the Spermacoceae (Rubiaceae), endemic to the campo rupestre of Minas Gerais, southeastern Brazil

CARMO, JOÃO A.M.; REGINATO, MARCELO; FLORENTÍN, JAVIER E.; NUÑEZ FLORENTIN, MARIELA; SALAS, ROBERTO M.; SIMÕES, ANDRÉ O.

TAXON

INT ASSOC PLANT TAXONOMY

Año: 2022 vol. 71 p. 396 - 419

0040-0262

Spermacoceae is the largest predominantly herbaceous lineage in the Rubiaceae, and regarded as one of its most taxonomically complex and controversial tribe, especially regarding generic delimitation. Molecular phylogenies have established the conceptof a broadly circumscribed Spermacoceae, which includes the genera traditionally associated with the tribe, i.e., the Spermacoceclade, and the former Hedyotideae and Manettieae. A major challenge in the Spermacoce clade is related to the delimitation of Borreria and Spermacoce, which appeared intermingled with several smaller and morphologically well-defined genera in more recentphylogenies, such as the Brazilian endemic Psyllocarpus. One of its species, P. densifolius, diverges morphologically from the others,as preliminary phylogenetic analyses have shown that it is not closely related to Psyllocarpus nor any other genus in the Spermacoceclade. We conducted a broad phylogenetic analysis, including ca. 63% of the genera in the Spermacoceae, using a variety of approaches to address incongruence among markers and lack of resolution and/or low support for branches, to accurately assess its position. We used nuclear ribosomal (ITS) and plastid (rps16 intron, trnL-F intergenic spacer) DNA regions for Bayesian inference andmaximum likelihood analyses, as well as species tree inference. We identified incongruence not only between nuclear and plastidDNA but also between the plastid regions. We also showed that the topologies derived from the multispecies coalescent diverged fromthat of the concatenated analysis, even though the position of P. densifolius, as sister to the clade formed by the remaining Spermacoceae, remained stable across different datasets and inference methods. We here propose the recognition of the new genusDiadorimia to accommodate P. densifolius, which can be characterized by its cespitose habit arising from a well-developed, woodysubterranean system, triangular stipules with margins bearing four small lobes, heterostylous flowers subtended by a pair of hyalinebracteoles, double reticulate pollen grains, and capsules with longitudinal-oblique dehiscence, with the two valves forming one singlecaducous diaspore keeping their respective seed trapped inside. We provide a taxonomic treatment with a key to the heterostylousgenera of the Spermacoceae occurring in the Americas, a description of the new genus, and the new combination of its type, as wellas comments on its distribution and conservation.