Fighting to persist: Genetic rescue increases long-term fitness despite elevated genetic load

Genetic rescue (GR) can restore fitness in small genetically depauperate populations, however, long-term effectiveness, impact of rescuer quality and introduced deleterious load on population persistence remains uncertain. Using a male-dimorphic mite we tested whether a condition-dependent sexually selected trait influences GR. Genome-wide diversity and genetic load increased similarly for both morphs, while population fitness trajectories differed, suggesting that genetic quality is associated with the trait. Overall, GR sustained fitness benefits over 20 generations. The benefits persisted under thermal stress lowering extinction risk regardless of the rescuer’s morph and introduced load indicating their minimal importance. These results provide experimental evidence that maximising genome-wide variation via GR can yield lasting benefits, even when many deleterious mutations are introduced to populations facing environmental change.