Pleistocene population differentiation in the ant Myrmica scabrinodis (Hymenoptera: Formicidae) – a taxonomic borderline case

Three taxa have been supposed in the past to be conspecific with or heterospecific from Myrmica scabrinodis Nylander 1846: M. rugulosoides Forel 1915, M. pilosiscapus Bondroit 1920 and M. martini Seifert et al. 2014. The taxonomic relations of these taxa are investigated here based on 146 nest samples with 479 worker individuals collected in the whole Palaearctic range extending from the Pyrenees and England to South Central Siberia (Baikal region). Exploratory and hypothesis-driven data analyses considering 17 morphometric characters confirmed two main clusters: a western cluster distributed from 5°W to about 22°E and an eastern cluster occurring from 6°E to 104°E. The classification error of the exploratory data analyses NC-NMDS.kmeans and PCA relative to the controlling linear discriminant function (LDA) were 1.4 % and 0.7 % respectively. At the first hand, this seemed to justify considering the two clusters as separate species. The posterior probabilities of the type series when run as wild-card runs in a LDA were 0.9366 in M. rugulosoides, 0.9999 in M. martini, 0.0284 in M. scabrinodis and 0.0081 in M. pilosiscapus – allocating the former two to western cluster and the latter two to the eastern cluster. However, strong and highly significant reduction of morphological distance between the western and eastern cluster in the sympatric zone (ranging from 6°E to 22°E) compared to the situation in the allopatric ranges indicates frequent hybridization and introgression of genes. Due to this morphological convergence in sympatry, M. rugulosoides, M. pilosiscapus and M. martini are considered as junior synonyms of M. scabrinodis. The placement of the main Pleistocene refuge centers of M. scabrinodis concluded from data of this study is congruent to the picture derived from a mtDNA haplotype analysis of Leppänen et al. (2012) in Myrmica rubra (Linnaeus, 1758): South France and the Apennine Peninsula for the western group and the Balkans, Middle Asia and SW Siberia for the eastern group. Conclusions on nearly equal glacial retreat and postglacial expansion routes of M. scabrinodis and M. rubra are also supported by the high overlap of their realized niches found in the broad-based survey of Seifert (2017) and their almost congruent actual geographic ranges.