Hybridization and Immunology in Animals: A Review

Pathogens exert strong selective pressures on their hosts by threatening their fitness and survival, driving evolutionary innovations in many aspects of host biology. The innate immune system and adaptive immunity enable many host species to withstand such pressures; however, when pathogens exceed a host's defensive capacity, the host is forced to evolve novel adaptations to survive and reproduce. One understudied mechanism for generating this novel adaptation is hybridization, a process by which beneficial immune alleles may introgress into hybrid offspring and, via backcrossing, spread through parental populations. In this review, we synthesize evidence across diverse animal systems showcasing how hybridization shapes immunological traits. We highlight cases where hybrid offspring exhibit enhanced tolerance or resistance to pathogens relative to their parent species, and identify gaps in research needed to further examine hybridization as a pathway for species to persist in the face of infectious disease.