Järvinen, A., Seifert, B., Satokangas, I., Savolainen, R. & Vepsäläinen, K.

Isolated hybrid wood-ant population Formica aquilonia x F. lugubris in subarctic Finland

A large geographically isolated hybrid population of the mound-building wood ants Formica aquilonia and F. lugubris, living syntopically with a F. lugubris population at Mount Saana in the subarctic Finland, is documented. Samples of both populations were included to and identified with a numeric analysis of 10 morphological characters in a large data set published earlier. A principal component analysis (PCA) with nest averages placed all hybrid samples between the clusters of the parental species F. aquilonia and F. lugubris. A PCA using 16 morphological characters, including workers only from selected nests at on Mt. Saana, clustered the hybrid individuals separately from those of F. lugubris. From one individual of a hybrid nest, genome-wide nuclear DNA (SNPs) was sequenced. When analysed together with a published genomic set of five Formica rufa group species, both principal component and ADMIXTURE analyses placed the hybrid individual between F. aquilonia and F. lugubris, thus corroborating the morphological classification. The morphological analyses also supported the hybrid status of the F. aquilonia x F. lugubris of three earlier single-nest samples, one from southern and another from eastern Finland, and one from mid Norway. Palaeoclimatic and palaeobotanic data suggest that populations of F. aquilonia and F. lugubris invaded Mt. Saana during the warm period of the Atlantic, about 7000 to 6000 years BP, but became isolated probably by 3600 BP owing to declining temperatures and the subsequent disappearance of Scots pine (Pinus sylvestris). This isolation has persisted up to the present, despite local warming since about 1980: the nearest nests of the parental species F. aquilonia are currently found 35 km NW and that of F. lugubris 16 km NW from Mt. Saana. Thorough sampling of 177 wood-ant mounds on Mt. Saana revealed a large proportion of both hybrid and F. lugubris nests, but no F. aquilonia. Assuming that the primary colonisers were not themselves hybrids, the suggested evident disappearence of F. aquilonia is attributed to the post-glacial cooling of the climate and the retreat of pine forest (its main habitat in the north), and its subsequent long-lasting blockade of colonisation of F. aquilonia. While hybridisation between the two wood-ant species may be interpreted as a factor which has contributed to the local extinction of F. aquilonia, it may also help to understand how its evolutionary lineage has survived in spite of the adverse environment.

Open access, licensed under CC BY 4.0. © 2025 The Author(s).

Hymenoptera, Formicidae, hybridisation, palaeoclimate, dispersal, geographic isolation