International Journal of Molecular Zoology, 2025, Vol.15, No.1, 20-28 http://animalscipublisher.com/index.php/ijmz 22 3 Molecular Mechanisms of Pigmentation in Birds 3.1 Melanin biosynthesis and key genes Melanin is the main pigment that determines the color of domestic goose feathers, and its generation and distribution processes are regulated by many key genes. The research of Wen et al. (2021) found that the KITgene plays an important role in the formation of white feathers and gray feathers. In Chinese domestic geese, there is an 18-base deletion mutation that is closely related to white feathers. Ren et al. (2021) and Wen et al. (2023) found that, in addition to KIT, genes such as KITLG, MITF, TYRO3, AP3B1 and TYRP1 are also involved in the synthesis of melanin and the regulation of feather color. TYRP1 has a particularly significant relationship with the feather color of goslings, and it is expressed more prominently in female goslings with darker feather colors. However, ASIP was expressed more strongly in male geese with lighter colors (Xu et al., 2022; Wen et al., 2023). Mutations in the EDNRB2 and MLANA genes have also been found to be related to feather patterns and the development of melanocytes. Mutations in EDNRB2 can cause incomplete development of melanocytes in some areas, thereby forming white patches (Yang et al., 2022; Yang et al., 2024). 3.2 Regulatory elements and non-coding RNAs After conducting chromosome-level genomic sequencing on Hungarian white geese, researchers discovered a complex regulatory system. There are 286 miRNAs, such as miR-199-x, miR-143-y and miR-23-z, etc. These mirnas can regulate the function of fibroblasts in embryonic skin and also affect the development of feather sacs (Figure 2) (Zhou et al., 2024). It indicates that the interaction between miRNA and mRNA plays a very important role in the formation and color regulation of feathers. Sello et al. (2019) also discovered some differentially expressed genes and related regulatory pathways through transcriptome analysis. For example, the calcium signaling pathway and glyceride metabolism are closely related to the development of feather sacs and pigment deposition. 3.3 Comparative insights from other avian species By comparing the genomes of geese and other birds with their transcriptomes, researchers have gained a deeper understanding of the regulation and evolution of feather color. The feather colors of graylag and swan geese look very similar, but their mechanisms at the molecular level are different. Yang et al. (2022) found that their feather color changes were respectively related to different mutations in the EDNRB2 and MLANA genes. Comparative studies have also discovered some families of genes related to feather sac development that exist in different birds. Some genes are conserved, while others are only found in specific species. Sello et al. (2019) found that Anser anser and Anser cygnoides also showed significant differences in the enrichment of signal pathways. The cross-species analysis by Sello et al. (2019) and Yang et al. (2022) indicates that the feather coloring mechanism of birds is complex and diverse, and the color evolution among different species may occur independently through parallel evolution. 4 Candidate Gene Studies in Goose Coloration 4.1 Early gene discovery efforts Early research on the feather color of domestic geese focused on their appearance characteristics and genetic patterns. Feather color features such as stripes on the neck, white spots on the chest, and solid or spotted patterns basically conform to Mendel's laws of inheritance, and some features show a trend of dominant inheritance. At that time, it was not very clear exactly on which chromosome many genes were located. Some studies suggested that the white feathers of Emden goose after reaching adulthood were the result of the gradually enhanced expression of a certain gene on the sex chromosome. 4.2 Mutation and polymorphism analysis Ren et al. (2021), Yang et al. (2022), and Wen et al. (2023) discovered through whole-genome resequencing and selective clearance analysis that key candidate genes such as KIT, EDNRB2, and TYRP1 play significant roles in the feather color changes of geese. Wen et al. (2021) found that there was an 18-base deletion mutation in the intron region of the KIT gene in Chinese domestic geese, which was highly correlated with the appearance of
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