Gene flow in admixed populations and implications for the conservation of the Western honeybee, Apis mellifera

Anthropogenic activity, especially modern apiculture, has considerable impact on the natural distribution of the Western honeybee, Apis mellifera, leading to the spread, replacement and fragmentation of many subspecies. This creates demand for the conservation of some subspecies, in particular, Apis mellifera mellifera, which once was widely distributed in Western Europe and nowadays is endangered through habitat loss and fragmentation. Moreover, A. m. mellifera may be further endangered by hybridisation in populations that now occur in artificial sympatry with other subspecies. Here, we quantify and compare individual hybridisation between sympatric and allopatric honeybee populations of A. m. mellifera and A. m. carnica using microsatellite markers and a Bayesian model-based approach. We had a special focus on pure breeding populations, which are a major tool in honeybee conservation. Our results demonstrate that subspecies are still highly differentiated, but gene flow is not prevented by the current management strategies, creating urgent demand for an improved conservation management of A. m. mellifera. However, the occurrence of a high number of pure individuals might suggest that some sort of hybrid barrier acts against the complete admixture of the two subspecies.