Genetic rescue in an isolated metapopulation of a naturally fragmented plant species, Parnassia palustris.

The loss of habitat of naturally fragmented species may result in isolated metapopulations: small groups of populations that are still connected by gene flow but have become isolated from other metapopulations. Genetic isolation 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 crosscompatibility and seed viability. Simultaneously, due to genetic drift, inbreeding, and pollen limitation in the small populations, fitness is likely to increase with population size within a metapopulation. I tested these hypotheses in three isolated metapopulations of the endangered plant species Parnassia palustris L. in a study area of 15 x 0.5 km along the coast of western Belgium and northwestern France. Seed and fruit weight, measured in the 27 populations distributed over the three metapopulations, significantly differed among the metapopulations, which suggests genetic differentiation. These fitness variables also increased with population size within two metapopulations. With a pollination experiment, I investigated whether genetic rescue with pollen from another metapopulation enhanced seed set. Outside-metapopulation crosses led 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. The seed set of naturally pollinated flowers was at least equal to that of hand-pollinated flowers, which can be explained by a compensation effect of pollen quantity and donor diversity for pollen quality. One can assume that the loss of genetic diversity at the level of the metapopulation affected loci not just 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.