International Journal of Molecular Veterinary Research, 2025, Vol.15, No.1, 1-12 http://animalscipublisher.com/index.php/ijmvr 4 renders the rates of polymorphism high, which increases the immune system's capacity to identify and react to an extensive range of pathogens (Dearborn et al., 2022). Additionally, balanced polymorphisms, whereby several alleles are maintained in a population, have been identified in immune-related genes, elevating the degree of genetic diversity and adaptability (Rottschaeffer et al., 2014). 4.2 Gene duplication and diversification in canids Gene duplication is another important process that has been responsible for diversification of immune genes in canids. Gene duplication may lead to the creation of gene families with multiple copies, each of which may develop new functions or become specialized in a different function of immunity. For example, MHC class I and II genes are extensively duplicated and diversified, which results in an extensive array of alleles that expand immunity recognition abilities (Kiemnec-Tyburczy et al., 2012; Dearborn et al., 2022). It is not limited to MHC genes; other immune genes, such as those coded for in the Toll-like receptor (TLR) pathways, also demonstrate signs of duplication followed by functional diversification (Fornůsková et al., 2013). These duplications allow for the generation of novel immune functions and improvement of immune responses to specific pathogens. 4.3 Horizontal gene transfer and immune system evolution Horizontal gene transfer (HGT) has been discovered to be a major force in the evolution of the immune system, including in canids. HGT involves the passing of genetic material from one species to another, where it can introduce new genes and functions to a genome. This has been particularly seen in the example of microbial and viral interactions, where immune-supportive genes can be received from the external environment (Marchalonis and Schluter, 1998; Lawrence, 1999). For instance, the rapid evolution of phylogenetically specialized immune responses in vertebrates has been explained by horizontal transfers of recombination signal sequences and site-specific recombinases from bacterial sources, which facilitated the evolution of sophisticated immune gene repertoires (Marchalonis and Schluter, 1998). Such events are able to cause profound evolutionary change, providing adaptive advantage simultaneously in the face of new or rising pathogens. 5 Immune System Adaptations in Wild Canids 5.1 Immune challenges in wolves and coyotes Wolves and coyotes are subjected to a variety of immune challenges in the wild, prompted by their exposure to a variety of pathogens. Their canid immune system has developed to optimize defense against pathogens. For instance, molecular characterization of MHC class I genes in a close wolf and coyote relative, the raccoon dog, has high allelic diversity and positive selection due to pathogen pressure, suggesting similar evolutionary pressures in coyotes and wolves (Bartocillo et al., 2021). Additionally, the complex history of introgression and admixture among canids, as witnessed in the case of Himalayan and Tibetan wolves, is used to highlight the role of genetic diversity in immune responses to environmental stresses (Wang et al., 2020). 5.2 Unique immune traits in arctic and desert canids Arctic and desert canids possess unique immune features that reflect their special adaptation for survival in extreme conditions. Tibetan wolves and Himalayan wolves, for example, possess a very high frequency of EPAS1 haplotypes, which confer an adaptive advantage at high elevation by enhanced oxygen delivery and possibly an effect on immunity. Such an adaptation testifies to the role of unique genetic traits in supporting survival in adverse conditions. Similarly, desert canids may have evolved specific immune mechanisms in order to acclimatize to the environment of the desert and desert pathogens, although few direct studies on desert canids exist (Wang et al., 2020) (Figure 2). 5.3 Role of pathogen pressure in shaping immune genes Pathogen pressure represents a strong force in shaping the evolution of canid immune genes. The fact that MHC class I genes in raccoon dogs possess high allelic diversity, driven by pathogen-driven positive selection, recombination, and long-term balancing selection, is an illustration of how pathogen pressure may shape the evolution of immune genes (Bartocillo et al., 2021). This phenomenon would also hold true for other canids, wolves, and coyotes, where frequent exposure to diverse pathogens necessitates a robust and adaptable immune
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