International Journal of Molecular Evolution and Biodiversity, 2025, Vol.15, No.1, 10-28 http://ecoevopublisher.com/index.php/ijmeb 21 In terms of social behavior, the hierarchical order (pecking order) of chickens is also a research focus. In the process of breeding docile chickens, some genes related to aggressive behavior may be indirectly selected. Comparison of the genomes of aggressive and docile breeds revealed that the EAAT3 glutamate transporter gene has unique mutations in docile chickens, which can enhance glutamate clearance and reduce neural excitability, and is speculated to be related to reduced aggression (Zhou et al., 2023). Similarly, the DRD2 (dopamine D2 receptor) gene showed an allele frequency bias in docile breeds, suggesting that the dopamine signaling pathway plays a prominent role in regulating the social behavior of chickens. In the framework of adaptive evolution, changes in neural and behavioral genes can be regarded as the adaptation of domestic chickens to new ecological niches (artificial breeding environments). Compared with wild chickens, domestic chickens do not need to deal with predation pressure and long-distance foraging, but instead are high-density confinement and stable feed supply. The results of genome comparison showed that the BDNF (brain-derived neurotrophic factor) gene, which is related to stress and neural plasticity, showed reduced genetic diversity in domestic chickens, suggesting that they may have experienced a domestication bottleneck and selection, which reduced the behavioral differences between individuals in the flock. On the other hand, the OXTR (oxytocin receptor) gene, which is related to social communication, was found to have mutations in multiple breeds of domestic chickens, which may affect the hen’s protection of chicks and gregarious behavior. It is worth noting that local chickens bred in special environments such as high-altitude Tibetan chickens have different adaptive variations in neural regulatory genes. For example, a study found that the sympathetic nerve-related gene ADRA2A of Tibetan chickens has a unique mutation, which makes its heart rate regulation more effective in a hypoxic environment (Zhang et al., 2021). This shows that behavioral and physiological adaptation genes may be interrelated and jointly shape the adaptive traits of chickens. Figure 1 Ovarian morphological and histological characteristics of high-laying- and low-laying-chickens (Adopted from Shi et al., 2024) Image Caption: (A): the liver from the high-laying group; (B, C): hematoxylin-eosin staining (H&E) and Oil Red O results of the liver of the high-laying group; (D): the liver of the low-laying group; (E, F): H&E and Oil Red O of the liver of the low-laying group; (G): the ovary of the high-laying group; (H, I): H&E of the ovary of the high-laying group (H 40 ×, I 100 ×); (J): the ovary of the low-laying group; (K, L): H&E of the ovary of the low-laying group (K 40 ×, L 100 ×) (Adopted from Shi et al., 2024)
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