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Original Article

Light at the end of the tunnel: Integrative taxonomy delimits cryptic species in the Tetramorium caespitum complex (Hymenoptera: Formicidae)

Wagner, H.C., Arthofer, W., Seifert, B., Muster, C., Steiner, F.M. & Schlick-Steiner, B.C


Abstract: Species delimitation is fundamental for many biological studies; its importance extends from regional faunistics over behavioral research to the reconstruction of evolutionary history. However, species delimitation in the Palearctic Tetramorium caespitum species complex (formerly Tetramorium caespitum / impurum complex) has stayed ambiguous over a century. A 2006 study argued for the presence of eight Western Palearctic cryptic species but did not draw formal taxonomic conclusions due to multiple unresolved issues. Using 1428 nest samples in an in-depth integrative-taxonomy approach, we here revise the European species of the complex. Unsupervised analyses provide independent species hypotheses based on molecular and morphological disciplines. Following the unified species concept, we show the presence of ten clearly distinguishable European species differing in mitochondrial Dna, nuclear Dna, morphology of workers and males, and ecology. We explain the evolution of the observed mitochondrial-nuclear discordances by peripatric speciation, historical introgression, and recent hybridization, and that of morphological-nuclear discordances by interspecific similarity and intraspecific variability, that is, morphological crypsis. Based on the final species hypotheses, we confirm T. caespitum (Linnaeus, 1758), T. impurum (Foerster, 1850), T. indocile Santschi, 1927, T. hungaricum RÖSZLER, 1935, and T. alpestre Steiner, SCHLICK-STEINER & Seifert, 2010 as good species. We raise T. fusciclava Consani & Zangheri, 1952 stat.n., T. staerckei KRATOCHVÍL, 1944 sp.rev., and T. immigrans Santschi, 1927 stat.n. (as an introduced species also in the Americas) to species rank and synonymize T. semilaeve var. kutteri Santschi, 1927 under T. indocile and T. staerckei var. gregori KRATOCHVÍL, 1941 under T. impurum. Finally, we newly describe T. breviscapus sp.n. and T. caucasicum sp.n. We present dichotomous determination keys to workers and males and make freely available an online identification key to workers at https://webapp.uibk.ac.at/ecology/tetramorium/. Of relevance to resolving other highly complicated taxonomic problems, we highlight that no single data type was sufficient to disentangle the final species boundaries, which underlines the importance of integrating multiple data sources in taxonomy.

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