Animal Molecular Breeding, 2024, Vol.14, No.6, 354-361 http://animalscipublisher.com/index.php/amb 355 canid conservation and disease management. It aims to provide comprehensive insights into how these receptors contribute to disease resistance and overall immune competence. 2TLRGenes: Structure and Function 2.1 General overview of TLRgene families Toll-like receptors (TLRs) are a critical component of the innate immune system, serving as pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs) from various microbes. TLRs are evolutionarily conserved across species, including canids such as wolves, coyotes, and dogs. These receptors are integral in recognizing a wide array of microbial components, including bacterial lipoproteins, lipopolysaccharides, and viral nucleic acids, thereby initiating immune responses (Dolasia et al., 2018; Fitzgerald and Kagan, 2020; Guo et al., 2023). The TLR family is composed of multiple members, each with specific ligand recognition capabilities, such as TLR1, TLR2, TLR4, and TLR5, which are known to recognize bacterial components, and TLR3, TLR7, TLR8, and TLR9, which are involved in viral recognition (Mukherjee et al., 2019; Zhou et al., 2021). 2.2 Mechanisms of action in immune response Upon recognition of PAMPs, TLRs activate downstream signaling pathways that lead to the production of cytokines and other inflammatory mediators. This process is primarily mediated through adaptor proteins such as MyD88 (myeloid differentiation primary-response protein 88), which forms a complex known as the myddosome. This complex recruits and activates IL-1R-associated kinases (IRAKs), which are crucial for the propagation of the signal leading to the activation of transcription factors like NF-κB and the production of pro-inflammatory cytokines (Guo et al., 2023; Pereira and Gazzinelli, 2023). Additionally, TLRs can form supramolecular organizing centers (SMOCs) that ensure precise and robust signaling responses (Fitzgerald and Kagan, 2020). The activation of TLRs not only triggers immediate innate immune responses but also shapes adaptive immunity by influencing the activation and differentiation of T and B cells (Dolasia et al., 2018). 2.3 Evolutionary perspectives of TLRgenes in canids The evolutionary dynamics of TLR genes in canids reveal significant polymorphism and adaptive evolution, driven by pathogen-mediated selection. Studies have shown that TLRgenes exhibit high levels of genetic diversity, similar to the major histocompatibility complex (MHC) genes, which are crucial for adaptive immunity (Těšický et al., 2020; Quéméré et al., 2021). This diversity is maintained through balancing selection, where different alleles confer advantages against various pathogens, thus promoting heterozygosity within populations. For instance, specific TLR polymorphisms have been associated with differential susceptibility to infections, highlighting their role in the evolutionary arms race between hosts and pathogens (Mukherjee et al., 2019; Wang et al., 2021). In canids, the conservation and variation of TLRgenes suggest a complex interplay between genetic drift, selection pressures, and environmental factors, contributing to the robustness of their immune responses (Těšický et al., 2020; Guo et al., 2023). 3 Comparative Insights Across Canid Species 3.1 TLRgene variability in wolves In North American gray wolves, the TLR3 gene has been studied in relation to the KB allele, which is associated with black coat color. This allele, introduced through hybridization with dogs, underwent a selective sweep, increasing its frequency in wild wolf populations. Despite this positive selection, wolves with the KBB genotype exhibit lower fitness compared to those with the KyB genotype, suggesting pleiotropic effects of the KB allele on phenotypes beyond coat color. However, studies have shown that the K locus genotype does not predict the transcriptional response to TLR3 pathway stimulation or infection by canine distemper virus (CDV), indicating that the gene expression response does not explain the pleiotropic effects on fitness (Figure 1) (Johnston et al., 2021).
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