Impact of purging, genetic drift and inbreeding depression on populations of reduced size

Resumo

Threatened populations in nature can be driven to extinction by genetic and non-genetic factors unless conservation actions are taken. Genetic factors include inbreeding, which is responsible for inbreeding depression, the reduction in the mean population fitness due to the expression of deleterious recessive alleles hidden in heterozygotes (the inbreeding load), and genetic drift, responsible for the loss of genetic diversity. Inbreeding depression can be mitigated by genetic purging, caused by an increased efficiency of natural selection against deleterious mutations promoted by inbreeding. In the present thesis, divided into five chapters, we aimed to evaluate the joint consequences of genetic purging, inbreeding depression, and drift on small populations. Chapter I is aimed at evaluating whether the theoretical Inbreeding-Purging model can accurately predict the expected change in fitness of small populations subjected to several breeding methods of interest in conservation practice. Predictions were fairly close to the observed changes in fitness, allowing the expected outcome of the breeding methods to be foreseen. Chapter II reviews the theoretical evidence, accompanied by new simulation results, of the consequences of genetic rescue programs on the inbreeding load and the extinction risk of rescued populations in the medium to long term. Increased extinction risk was observed under some extreme situations where purging was more efficient before than after migration, while a continuous stable connection between populations reduced such a risk. Chapter III focuses on the long-term efficiency of purging to eliminate the initial inbreeding load. After more than 100 generations (or a few tens in some cases) of moderate to large sized populations of Drosophila melanogaster, the inbreeding load was virtually exhausted, leaving the populations with little or no inbreeding depression. In Chapter IV, the consequences of purging were evaluated in relation to the minimum effective size necessary for a population to be viable in the long-term. The results suggest that purging may substantially reduce the minimum size of a viable population, but this value strongly depends on the species’ reproductive rate. In Chapter V, attention was placed on the robustness of the estimation of the rate of inbreeding depression using molecular markers. Molecular measures of inbreeding provided estimates of inbreeding depression for a large population of Drosophila melanogaster close to that expected from a simple inbreeding design, even though a small number of individuals belonging to a single generation were used for the estimation. Finally, a discussion was carried out on the impact of the results on conservation management.