Host-switching events are not always the driver of speciation in social parasites: a case study in Temnothorax (Myrmoxenus) ants (Hymenoptera, Formicidae)

Host-parasite systems, including social parasites that exploit resources of the host colonies, are fascinating objects for evolutionary biologists mainly due to the dynamic and often rapid host-parasite coevolution. Host-switching events are believed to induce rapid speciation of parasitic species. The socially parasitic ant lineage Myrmoxenus, which corresponds to the monophyletic Temnothorax corsicus group, counts in total a dozen species. Most Myrmoxenus species utilize a single host species, but a few others, like Myrmoxenus ravouxi (Andre, 1896) and M. gordiagini Ruzsky, 1902, are known to use multiple host taxa. Myrmoxenus zaleskyi (Sadil, 1953) was described as a putative congener of M. ravouxi based on its distinct host selection. In this paper, we investigate the diversity of the widely distributed European lineages M. ravouxi and M. zaleskyi from multiple and complementary perspectives to understand whether the host preference exhibited by these two forms implies speciation. We integrated evidence from molecular genetics using mitochondrial CO I/CO II genes, including the tLeu-region, and multivariate analyses of morphometric data collected from workers and female sexuals (gynes). Although there is substantial regional host species specificity, results suggest that host switching did not result in phylogenetic or morphological divergence and that the central European M. zaleskyi can be considered the junior synonym of M. ravouxi. As the lineage Myrmoxenus has been the subject of considerable evolutionary research, these results are essential to achieve a more accurate picture of host-parasite systems in the future and further strengthen the justification of an integrative approach in studying similarly complex systems. We advise against describing new parasitic species based on host preference unless coupled with marked heritable phenotypic adaptations.