Genetic analysis of founder bottlenecks in the rare British butterfly Plebejus argus

Fragmented populations of the silver-studded blue butterfly (Plebejus argus: Lycaenidae) provide a model system for metapopulations in which extinctions, colonizations, and artificial introductions have all been documented. The genetic consequences of population turnover were investigated using allozyme and mtDNA/RFLP markers and via studies of fluctuating asymmetry. Evidence for changes in genetic diversity was studied in samples of 40-90 individuals from one source and eight descendent populations in North Wales. The genetic data were analyzed using a novel permutation method, which is potentially applicable to analyses of other levels of biodiversity, such as species diversity or species counts. Significant reductions in the numbers of rare allozyme alleles were found in descendent populations, presumably as a result of drift during colonization bottlenecks. Similar trends were evident in the mtDNA data, but were largely non-significant. The reduced statistical power of mtDNA was probably due more to low overall numbers of alleles detected (19) compared with allozymes (40 alleles at 12 loci) than to any conflict between marker loci. Expected heterozygosity (H-e) also generally declined during colonization, but the changes were rarely significant, probably because drift-induced changes in the frequencies of common alleles may actually increase H-e in a bottle-necked population. Finally, overall gene frequency changes, measured by F-st, were often significant. The values of the effective population size (N-e) estimated for two introductions (N-e) estimated for two introductions (N-e = 71 and 29) suggested bottleneck population sizes of approximately half the number of diploid genomes (180 and 60 respectively) actually introduced. In contrast to evidence for loss of genetic diversity, there was no evidence for any increase in fluctuating asymmetry among descendent populations. Plebejus argus maintains large enough populations locally and colonizes in sufficient numbers that the deleterious effects of bottlenecking are temporary, affect only the rarest alleles, and cause no observed disturbances to developmental stability. As a result a single introduction of 90 mated females to Rhyd-y-foel in 1942 has led to the successful establishment of a healthy metapopulation now numbering about 100,000.