Interspecific hybridisations in natural populations of ants by example of a regional fauna (Hymenoptera, Formicidae)

The identification of hybrids by high-precision stereomicroscopy and chorol ogical data is described in detail by example of the ant species Formica br uni and pressilabris. In a synopsis of heterogenous data, the overall hybri d frequency in the ant fauna of Central Europe is estimated. 17 of the 164 ant species of this region are demonstrated and further 2 species strongly suspected to hybridise. The low-frequency hybridisers, showing overall hybr id ratios < 3%, were native elements of the Central European fauna before t he onset of human cultivation and experienced direct interspecific contact for longer periods of their natural history. They developed more effective mechanisms of reproductive isolation beginning at the prezygotic level. Ext remely high (12-31%) local hybridisation ratios occurred in species that in vaded the area after anthropogenic changes in landscape structure. The segr egated distribution of invaders and autochthonous species in the precultura l period apparently did not impose the need to evolve more effective mechan isms for reproductive isolation. Prezygotic mechanisms in particular are de ficient. In local situations, 19% of Lasius jensi matings and 44% of Leptot horax albipennis matings leading to successful nest foundations were mating s with heterospecific partners. Signs for a dissolution of interspecific ph enotypic differences are not detectable in most of the species. The dispers al of hybrid genotypes is apparently inhibited. Factors that probably stabi lise the genomic integrity of hybridizing parent species are: (a) inability to produce hybrid queens tin L. jensi x umbratus), (b) aneuploidy of F-1 f emales with inability to perform a balanced meiosis tin hybrids of L. albip ennis with 3 other species), and (c) selection against hybrids in the epige netic environment of alleles (Leptothorax nylanderi x slavonicus). Extreme ratios of heterospecific matings would mean a dangerous drain of genetic ma terial. A mechanism to reduce these losses is postulated consisting of a 'c leptogamy' (theft of heterospecific sperm) by queens that missed a conspeci fic male, an establishment of a functioning colony of F-1 hybrid workers, a depression of the rearing or fertility of hybrid queens, and a maximum pro duction of own sons by the queen. This mechanism could explain the persiste nce of rare species (Lasius jensi or Leptothorax albipennis) in an environm ent of more populous heterospecific hybridisation partners.