Genetic rescue increases long-term fitness despite elevating putative genetic load in a male-dimorphic mite.

Genetic rescue is a potentially powerful conservation strategy to restore fitness in small genetically depauperate populations; however, long-term effectiveness, impact of rescuer traits and introduced variation on population persistence remains uncertain. Using a male-dimorphic mite, we tested whether males that differ in their expression of a condition-dependent sexually selected weapon, previously shown to shape genetic variation in populations and influence reproductive competitiveness, alters the efficacy of genetic rescue. Here we show that introductions from either male morph increased genetic diversity and putative load similarly, indicting even less sexually competitive males can still be effective rescuers. Yet, fitness benefits of rescue by male morphs differed over time, suggesting additional effects not captured by our estimates of genetic load exist. Despite populations being forced to remain small and introductions of males increasing segregating putative load, fitness benefits were sustained for 20 generations-increasing fecundity in benign conditions and reducing extinction risk under thermal stress-likely because genetic variation remained high and deleterious effects largely remaining masked. Overall, these results provide experimental evidence that maximizing genome-wide variation via genetic rescue can yield long-lasting benefits to populations facing environmental change, even when rescuers are in lower condition and many presumed deleterious mutations are introduced.