Interplay between purging and admixture shapes genetic load in an invasive guppy population

Demographic history can shape the genetic load of populations by influencing the efficacy of selection, levels of heterozygosity, and by incorporation of new variants via gene flow. Understanding these dynamics is crucial for identifying threats to population viability and predicting evolutionary trajectories of invasive populations translocated by humans into non-native environments. We investigated these processes in Trinidadian guppies ( Poecilia reticulata ) by estimating genetic loads in multiple populations, with a particular focus on a single expanding population where translocated individuals have rapidly spread and replaced native populations. Overall, we observed the expected negative relationship between neutral genetic diversity and relative genetic load. In the translocated population, patterns differed between strongly and weakly deleterious mutations. Strongly deleterious alleles were purged at the isolated translocation site but tended to accumulate along the expansion front. In contrast, genetic load estimated based on weakly deleterious variants declined along the expansion gradient. These differing patterns can be explained by admixture with native populations (which carried fewer weakly deleterious mutations) reducing the overall genetic load of the population at the expansion front. However, admixture also increased genetic diversity and introduced new strongly deleterious alleles, which reversed the effect of purging. Together, our findings illustrated the complex interactions determining genetic load in subdivided populations, offering important insights into the evolutionary aspects of biological invasions.