Animal Molecular Breeding, 2024, Vol.14, No.6, 354-361 http://animalscipublisher.com/index.php/amb 357 response to infections in canids. For instance, TLR2 is involved in recognizing bacterial lipoproteins, while TLR4 detects lipopolysaccharides from Gram-negative bacteria, and TLR7 is essential for recognizing viral single-stranded RNA (Loots et al., 2018; Mukherjee et al., 2019; Heni et al., 2020). Polymorphisms in these TLR genes can influence the host's susceptibility to infections. For example, a study on African wild dogs and lions revealed that TLR2 polymorphisms might affect susceptibility to canine distemper virus (CDV), with specific amino acid changes potentially altering TLR2 function and expression (Loots et al., 2018). 4.2 Case studies: disease susceptibility linked to TLR variants Several case studies have highlighted the link between TLRgene variants and disease susceptibility in canids. In a study of CDV outbreaks in South Africa, non-synonymous single nucleotide polymorphisms (SNPs) in TLR2, TLR3, TLR4, TLR7, and TLR8 were investigated. The study found a higher rate of TLR polymorphisms in African wild dogs compared to lions, with a specific TLR2 variant (Met527Thr) potentially influencing CDV susceptibility in lions (Loots et al., 2018). Another study on roe deer demonstrated that TLR2 polymorphisms are subject to pathogen-mediated selection, affecting susceptibility to infections like Toxoplasma and Chlamydia (Quéméré et al., 2021). These findings underscore the importance of TLR genetic diversity in disease resistance and susceptibility in wildlife. 4.3 Genetic markers and breeding for enhanced immunity The identification of TLR polymorphisms as genetic markers offers promising avenues for breeding programs aimed at enhancing disease resistance in canids. For example, the presence of specific TLR variants can be used to select individuals with enhanced immune responses for breeding. In dairy cattle, TLR SNPs have been identified as potential markers for breeding strategies to improve resistance to diseases such as mastitis and bovine tuberculosis (Maljković et al., 2023). Similarly, in canids, TLR polymorphisms could be utilized to develop marker-assisted selection programs to enhance disease resistance. The use of retrotransposon insertion polymorphisms (RIPs) in TLRgenes, such as the 192 bp ERV insertion in TLR6, has shown potential in increasing TLR expression and downstream immune responses, which could be applied in breeding programs for disease-resistant animals (Wang et al., 2021). 5 Case Study 5.1 Regional pathogen pressure in wild and domestic canids Pathogen pressure varies significantly across regions and species, influencing the genetic diversity of Toll-like receptors (TLRs) in canids. For instance, in South Africa, outbreaks of canine distemper virus (CDV) in lions and African wild dogs revealed different susceptibilities and TLR polymorphisms. Lions exhibited lower TLR diversity compared to African wild dogs, which showed higher rates of polymorphism within TLR loci (Loots et al., 2018). Similarly, in a study on neotropical rodents, TLR4 haplotypes varied across landscapes with different degrees of anthropogenic disturbance, affecting resistance to gastrointestinal nematodes and Hepacivirus (Heni et al., 2020). These findings underscore the importance of regional pathogen pressures in shaping TLR diversity and pathogen resistance in wild and domestic canids. 5.2 Sampling and genotyping of TLR variants Sampling and genotyping of TLR variants involve collecting tissue or blood samples from canids and analyzing the genetic sequences of TLR genes. In the study of CDV outbreaks in South Africa, researchers investigated non-synonymous single nucleotide polymorphisms (SNPs) in the coding regions of TLR2, TLR3, TLR4, TLR7, and TLR8 genes (Loots et al., 2018). Similarly, in a study on porcine TLR genes, bioinformatic prediction combined with PCR-based amplification was used to screen for retrotransposon insertion polymorphisms (RIPs) (Wang et al., 2021). These methods allow for the identification of specific TLR variants and their association with disease susceptibility. 5.3 Correlation between TLR variants and pathogen resistance The correlation between TLR variants and pathogen resistance is evident in several studies. For example, a single amino acid change (Met527Thr) within the leucine-rich repeat of TLR2 was observed in a surviving lioness
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