Population and Immunocompetent Genetic Variation: A Field-Based Study

Abstract

In order to establish an evolutionary context, this thesis examines the phylogeography of the common frog (Rana temporaria) throughout Europe, with a particular focus on the British Isles. Mitochondrial DNA indicates divergent, yet overlapping Eastern and Western European haplotypes, and that Britain was colonized via Western Europe, likely from an Iberian glacial refugium. Ireland appears to have been colonized both via this route, and from a refugium in the South West of Ireland. Data from nuclear microsatellite markers indicate that although the mtDNA haplotypes in England and Eastern Ireland are indistinguishable, there is vastly lower genetic diversity in Ireland.Moving to a finer scale, common frogs in Britain are examined for differences inpopulation genetic patterns that may have been caused by disease outbreaks (Ranavirus). Populations that have experienced recurrent mortalities for approximately five generations are compared to those with no history of disease. Populations that suffer recurrent disease are shown to have experienced approximately 80% declines in the numbers of frogs ten years after the disease first arrived. Yet surprisingly, microsatellite markers show no signal of reduced genetic variability; it is possible that immigration has maintained allelic richness in infected populations. An excess of homozygosity, and a reduction of relatedness within infected ponds implies that assortative mating may be occurring, and population genetic simulations confirm that assortative mating can produce these patterns. A novel Major Histocompatibility Complex (a region directly involved in the immune response to viruses) marker was developed, and haplotype frequencies are shown to differ between infected and uninfected populations. Infected populations show reduced genetic diversity and more uniform allelic frequencies; implying directional selection for more resistant genotypes. These findings indicate that Ranavirus may be affecting the population dynamics of the common frog, and that this species appears to be adapting to the presence of the disease.