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 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.