The genetic consequences of long term habitat fragmentation on a self-incompatible clonal plant, Linnaea borealis L.

Self-incompatible species with restricted seed and pollen flow are considered the most vulnerable to the deleterious genetic effects of habitat fragmentation. Immediate effects of fragmentation are expected to be loss of allelic diversity and differentiation of fragments by genetic drift. Later, loss of S allele diversity may lead to restricted mate availability, increased relatedness of genotypes within patches, accelerated loss of genetic diversity and eventual loss of capacity for seed production. We studied the self-incompatible clonal shrub Linnaea borealis within the Cairngorms National Park, Scotland, whose pinewood habitat has been fragmented for an extensive period, possibly millennia. Exhaustive sampling revealed 123 patches (median length 15 m), 91% of which were further than the maximum pollen flow distance from their nearest neighbours (30 m). Using ten microsatellite markers, only 21% of the patches produced more than one multilocus genotype. Individual genotypes extended from 1 to 74 m. Bayesian clustering of the 179 multilocus genotypes revealed six clusters. One cluster occupied a geographically distinct area where seed production still occurs and showed significant genetic differentiation from (Fst = 0.164, P < 0.01) and significantly lower allelic richness (AR = 4.0 vs AR = 7.0 P < 0.01) than the remainder of the sample set. Spatial genetic structure in the total sample set indicated significant relatedness of clones within the first 1.5 km. Overall, L. borealis in Scotland seems to be experiencing extreme genetic effects of chronic population fragmentation with only 16% of patches having the capacity for seed production. Genetic rescue is being undertaken by transplanting unrelated clones from > 1.5 km distance into extant monoclonal patches.