Host body size, not host population size, predicts genome-wide effective population size of parasites

Impact Summary Parasites, among Earth’s most diverse, threatened, and under-protected animals, play a central role in ecosystem function. The effective population size (N_e) of an organism has a profound impact on evolutionary processes, such as the relative contributions of selection and genetic drift to genomic change. Population size is also one of the most important parameters in conservation biology. For free-living organisms, it is expected that N_e is proportional to the total number of individuals in a population. However, for parasites, populations are sometimes so extremely subdivided that high levels of inbreeding may distort these relationships. In this study, we used whole-genome sequence data from dove parasites and phylogenetic comparative methods to investigate the relationship between parasite effective population size (N_e) and host population size and body size. Our results revealed a positive relationship between parasite effective population size (N_e) and host body size, but not host population size. These results suggest inbreeding may be a major factor in parasite infrapopulations, and have important implications for conservation.