One of the most prevalent species characterizing the dune slack plant community is Parnassia palustris. This species naturally occurs in a dune slack metapopulation structure, but this system is further fragmented, resulting in isolated metapopulations. We tested if the genetic isolation of this species affected pollination limitation and consequently seed set and reproduction. Genetic isolation of populations may result in genetic differentiation between metapopulations and lowered genetic diversity within the metapopulation. Gene input from outside the metapopulation can hence be expected to enhance cross compatibility and seed viability. With a pollination experiment, I investigated whether genetic rescue with pollen from another metapopulation enhanced seed set. Outside-metapopulation crosses lead to a significantly higher seed set than within-metapopulation crosses, and this effect was more pronounced in small populations. This means pollen from outside the metapopulation was more compatible than pollen from within the metapopulation, due to a lowered genetic diversity within the metapopulation. One can assume that the loss of genetic diversity at the level of the metapopulation not only affected loci related to cross compatibility. Reconnection of metapopulations by stepping stones may be necessary to prevent further genetic erosion and assure the viability of the Parnassia populations in the study area over the long term.