Original Article

Four species within the supercolonial ants of the Tapinoma nigerrimum complex revealed by integrative taxonomy (Hymenoptera: Formicidae)

Seifert, B., D'Eustacchio, D., Kaufmann, B., Centorame, M., Lorite, P. & Modica, M.V.


Abstract: The West and Central Mediterranean ants known for 50 years under the name Tapinoma nigerrimum (Nylander, 1856) have attracted attention because of their efficient chemical weapons, impressive supercolonies and potential to limit the spreading of the Argentine Ant Linepithema humile (Mayr, 1868). The paper shows that the T. nigerrimum complex consists of at least four clearly separable species which differ in morphology of all castes, colony demography, geographic distribution, invasive potential and mtDNA data. Species delimitation by means of Nest Centroid Clustering, considering 20 quantitative phenetic characters in 159 nest samples, resolved four coincident clusters in both female and male castes which are classified as T. nigerrimum, T. magnum Mayr, 1861, T. ibericum Santschi, 1925, and T. darioi sp.n. The exploratory data analyses NC-Ward clustering and NC-k-means clustering showed a mean disagreement from the final species hypothesis between 0 and 2.7% in workers on the nest sample level, whereas the classification error of a linear discriminant analysis was 4.2% in 533 worker individuals. The four phenetic clusters were basically confirmed by analysis of the Coi segment of mtDNA with the smallest mean K2p genetic distance of 1.8% observed in T. darioi sp.n. against T. magnum, and the largest one of 4.0% in T. nigerrimum against T. ibericum. These data suggest a species divergence between late Pliocene and early Pleistocene (3.3 - 1.5 Ma). The mtDNA haplotypes of nine phenotypically ideal T. darioi sp.n. supercolonies, found at three sites in southern France, and Italy were placed within the T. magnum cluster. Among four alternative scenarios discussed for these mismatches, hybridization events in the younger evolutionary history with subsequent unidirectional genomic purging of nuDNA was proposed to be the most likely explanation. Tapinoma nigerrimum is monodomous to moderately polydomous with aggression between neighbouring colonies, whereas T. magnum, T. ibericum, and T. darioi sp.n. are supercolonial with a potential to become invasive pest ants through introduction by human commerce. For Europe north of 48° N, T. magnum could establish populations in nine cities in Germany, Belgium, and the Netherlands, whereas T. ibericum is known so far from one site in South England only, and T. darioi sp.n. from one city in the Netherlands. The differential zoogeography and biology of the four species and ways of species delimitation are outlined and discussed. Tapinoma darioi sp.n. is described as new.