Integrative taxonomy reveals inflated biodiversity in the European Temnothorax unifasciatus complex (Hymenoptera: Formicidae)

Temnothorax unifasciatus (Latreille, 1798) is a widely distributed pan-European species from the Iberian Peninsula to the Caucasus. This taxon's relatively high morphological variability prompts the taxonomists of earlier times and today to mention the morphologically different elements at specific or subspecific ranks. This paper aims to understand the population structure and genetic diversity within this lineage via an integrative taxonomy, incorporating molecular phylogenetics, species delimitation analyses, and multivariate analyses of continuous morphometric data from across the geographic range of the T. unifasciatus complex. Phylogenetic analyses yielded incongruent trees. The genealogical diversity index (gdi) and the confirmatory analyses on morphological data found only weak, ambiguous delimitations within the unifasciatus group. The most highly supported scenario splits T. brackoi from the remaining unifasciatus group with ambiguous support (gdi = 0.56). This scenario is supported by multivariate morphometry with 100% accuracy in classification success. Instead, our results suggest complex morphological and genetic population structuring within the broad range of T. unifasciatus. Therefore, we confirm the validity of two species, T. brackoi Salata & Borowiec, 2019 and T. unifasciatus (Latreille, 1798), and propose five new junior synonymies, T. cordieri (Bondroit, 1918) syn. nov., T. tauricus (Ruzsky, 1902) syn. nov., T. berlandi (Bondroit, 1918) syn. nov., T. unifasciatus staegeri (Bondroit, 1918) syn. nov., T. tuberum ciscaucasicus (Arnol'di, 1977) syn. nov. with the latter. In order to achieve maximal taxonomic stability, we designated a lectotype for Temnothorax unifasciatus (Latreille, 1798).