A study of the impact of population bottlenecks on the genetics and morphology of reindeer (Rangifer tarandus tarandus) on the island of South Georgia

Abstract

Numerous studies have shown that population bottlenecks result in loss of genetic diversity and as a consequence of this, it is commonly inferred that there is a loss of evolutionary potential. It is rare that circumstances are such that there should be well documented details of the founder event, such as the size and date of the bottleneck, that there should be access to subsequent demographic information and to suitable samples from both the post-bottleneck and the source populations. It is even less common for this information to be available for two separate bottlenecks that occurred in parallel in a largely unmanaged population of large mammals. The importation by whalers of two separate groups of reindeer (Rangifer tarandus tarandus) onto the island of South Georgia in the early part of the twentieth century provided precisely those circumstances. There are accurate historical records of each founder events with details of dates and numbers of reindeer. The inaccessibility and geography of the island ensures that the South Georgia reindeer herds have been isolated from immigrants and separated from each other. The aim of this study was to test hypotheses about the impact of population bottlenecks on phenotype and genetic diversity and this was achieved by making genetic and morphometric comparisons of both post- bottleneck populations with the source population in Norway. Genetic diversity was primarily measured by allele numbers and heterozygosity based on data from thirteen microsatellite loci. Morphometric comparisons included measures of developmental stability, notably fluctuating asymmetry (FA), as well as phenotypic variation and body size. Each of the post-bottleneck populations showed significant genetic differentiation from the pre-bottleneck population and showed decreased levels of heterozygosity and allelic diversity. The data was used to validate commonly used 'bottleneck signatures' and considerable variability was found in the accurate detection of the known bottlenecks by the different detection methods. Both the post-bottleneck populations showed increased FA and morphometric variation compared to the pre-bottleneck population in some of the measured traits. Both post-bottleneck populations had smaller overall skull size than the pre-bottleneck population though it is discussed that this may be due as much to a plastic environmental response as to a consequence of the genetic bottleneck. Within each population the relationship between measures of genomic diversity and indirect measures of fitness were investigated on an individual basis. Although the results were of low significance, weak associations were found to support the hypotheses that developmental stability is correlated with measures of genomic diversity even at the level of the individual.