Evolutionary analysis of ghrelin in Actinopterygii.

Ghrelin functions to stimulate appetite, promote the release of growth hormone, and regulate energy balance. Currently, research on the ghrelin is primarily focused on a single species, and there have been no systematic studies on the evolution of the ghrelin in fish. Therefore, this thesis conducts a comprehensive analysis of the ghrelin gene in 151 species of ray-finned bony fishes to reveal the universality and specificity of the ghrelin gene in the evolutionary history of fish, supplementing and perfecting the information on the ghrelin gene in Actinopterygii. The gene identification results show that the number of ghrelin genes varies among different fish species, 41 fish have lost the ghrelin gene, 98 fish having one ghrelin gene, and 12 fish having two ghrelin genes. Among the 110 fish species with the ghrelin gene, a total of 182 ghrelin gene sequences were identified, with transcript variant numbers ranging from 1 to 6, encoding 1 to 3 types of isoform proteins, and their mature peptides show a certain degree of similarity across different species. Phylogenetic analysis revealed that teleost ghrelin proteins segregate into three major evolutionary clades, with Salmoniformes orthologs comprising a distinct monophyletic cluster. The Cladistic and Chondrostei are clustered separately and then grouped with the more ancient Cypriniformes and Siluriformes species from the Neopterygii into a large group, while the other fish species from the Neopterygii form another large group. The Synteny analysis results indicate that the upstream gene of the ghrelin is CCDC174, and the downstream gene is TATDN2. The selection pressure analysis results show that there are no positive selection sites in the ghrelin gene, indicating that the ghrelin has been under strong functional constraint during the evolutionary process of fish. This study systematically investigates the evolutionary history of fish ghrelin, providing a theoretical basis for understanding the function and status of ghrelin in the feeding regulation system of fish, and deepening the recognition of its structural and functional evolution in the process of energy metabolism evolution.