Combining complete chloroplast genome sequences with target loci data and morphology to resolve species limits in Triplostegia (Caprifoliaceae).

Species represent the most basic unit of taxonomy. As such, species delimitation represents a crucial issue for biodiversity conservation. Taxonomic practices were revolutionized in the last three decades due to the increasing availability of molecular phylogenetic data. The genus Triplostegia (Caprifoliaceae) traditionally consists of two species, T. glandulifera and T. grandiflora, distinguishable mainly based on quantitative morphological features. In this study, we sequenced nine chloroplast loci (i.e., accD, psbK-psbI, rbcL-accD, rpoB-trnC, rps16-trnQ, trnE-trnT, trnF-ndhJ, trnH-psbA, trnS-trnG) and one nuclear locus (ITS) of 16 individuals of Triplostegia representing the entire distribution range of both species recognized. Furthermore, we also obtained whole chloroplast sequences for 11 of the 16 individuals for which silica gel-dried leaves were available. Our phylogenetic analyses integrating chloroplast genome sequences and multiple loci data revealed that Triplostegia includes four main clades that largely match geography. Neither T. grandiflora nor T. glandulifera was recovered as monophyletic and no diagnosable differences in leaf, flower, and pollen traits were detected between the two species, indicating the need for a revised species circumscription within Triplostegia. Our study highlights the importance of combining data from different sources while defining species limits.