Rebuttal to Greuter & Rankin‐Rodríguez's (2831) proposal to conserve Exostema against Coutarea (Rubiaceae) and to their expanded circumscription of Exostema

Abstract

Paudyal & al. performed the most comprehensive phylogenetic study of the tribe Chiococceae (Rubiaceae) using two nuclear and two plastid datasets, and resolved four well‐supported main clades. Within Clade B, Paudyal & al. recognized the genera Exostema, Hintonia, Coutareopsis, Motleyothamnus, Coutarea, Adolphoduckea, and Solenandra. Greuter & Rankin‐Rodríguez interpreted the internal clades of Clade B differently, and instead treated the entire Clade B as the broadly expanded, morphologically diverse genus Exostema. In addition, because the genus Coutarea is positioned within Clade B and its name has nomenclatural priority over Exostema, Greuter & Rankin‐Rodríguez presented a formal proposal (proposal 2831) to conserve the name Exostema against Coutarea. We strongly disagree with Greuter & Rankin‐Rodríguez naming the entire Clade B of Paudyal & al. as a widely circumscribed Exostema, and with their proposal to conserve the name Exostema against Coutarea for the following reasons: (1) their wide delimitation of Exostema and their proposal to reject Coutarea vs. Exostema are and will cause wide disruption in nomenclatural stability of traditional and current usage of generic and specific names within the Chiococceae; (2) their broad synonymisation under Exostema s.l. entails the lumping of six genera (Coutarea, Adolphoduckea, Coutareopsis, Motleyothamnus, Hintonia, Solenandra) into a broadly distributed, highly polymorphic genus, decreasing the value of diagnostic information for each distinct monophyletic taxon and reducing the systematic and morphological information of the species; (3) each of the seven “Clade B” genera of Paudyal & al. corresponds to a well‐resolved clade with a unique set of morphological characters; (4) Paudyal & al.'s genera of Clade B have been accepted by numerous Rubiaceae specialists and managers of specialized websites; (5) the broad expansion of Exostema proposed by Greuter & Rankin‐Rodríguez does not fulfill the principle of maximizing the ease of identification of the c. 40 species included in such a highly polymorphic genus, increasing the difficulty of species identification by the botanical community. In conclusion, we advise the Nomenclature Committee to reject Greuter & Rankin‐Rodríguez's proposal to conserve the generic name Exostema against Coutarea.