Dispersal, gene flow and sibship analysis of Phengaris (Maculinea) alcon: implications for conservation An Vanden Broeck, Dirk Maes , Andreas Kelager, Irma Wynhoff , Michiel F. Wallisdevries, David R. Nash, J. Gerard B. Oostermeijer & Hans Van Dyck Effective dispersal determines the long-term stability of metapopulations and plays a crucial role in the conservation and evolution of species. For butterflies, dispersal data are mainly based on mark-recapture observations which are usually poor predictors of long-distance dispersal. Detailed knowledge on effective gene flow through dispersal is often missing. During recent decades, the Alcon Blue (Phengaris (Maculinea) alcon) declined strongly throughout Europe and, therefore, attracted much attention among conservation biologists. Here, we analysed the genetic population structure and oviposition patterns of M. alcon in Belgium and the Netherlands using 12 microsatellite markers. We collected 211 wing clip-samples of adult butterflies from 8 populations in Belgium and 191 caterpillars from flower buds of the host plant Gentiana pneumonanthe in 2 locations in the Netherlands. At the regional scale, there was a clear spatial structure with an overall genetic differentiation among populations (Fst) of 0.26. At the local scale, spatial genetic structure was low or absent indicating frequent within-habitat dispersal over distances up to of 3 km. Populations in large habitat patches revealed higher levels of genetic diversity compared to small habitat patches. This emphasizes the importance of conserving large suitable habitat patches for maintaining viable populations. Egg-laying females did not move further than 100 meter between host plants and on average crossed less than 10 meter among host plants for oviposition. Parentage and sibship-relationships among caterpillars revealed as many as 18 full-sibs on neighboring host plants indicating a high level of spatial clustering of eggs from individual females.