Genetic Variation of Chicken Growth Differentiation Factor-9 Gene and Association With Egg Characteristics: A...

Chicken layers are valued for their ability to produce eggs, an essential component of the human diet. Egg characteristics are influenced by genetic factors such as the growth differentiation factor-9 (GDF-9) gene, which plays a vital role in follicular development and ovulation. Variations in the GDF-9 gene can lead to ovulation dysfunction, infertility, and reduced egg quality and production. This systematic review explored the genetic variation of the chicken GDF-9 gene and its association with egg characteristics. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, databases including ScienceDirect, Google Scholar, PubMed, and Web of Science were used to search for articles using combinations of keywords: "chickens," "GDF-9 gene," "genetic variation," and "egg characteristics." A total of 106 articles were retrieved, and five met the inclusion criteria. The reviewed studies identified several single nucleotide polymorphisms (SNPs) within the GDF-9 gene-namely g.171515C>T, g.171530A>G, g.171529G>A, g.171554C>T, g.171577A>G, g.171600C>T, g.171541A>G, g.171560T>C, and g.171589C>T. Although no identical SNPs were shared among all breeds, these variants were associated with key egg traits such as age at first egg, egg number, and egg weight. The findings suggest that GDF-9 gene polymorphisms hold strong potential for use in marker-assisted and genomic selection to enhance reproductive performance and egg-laying efficiency in chickens. However, the current findings are primarily based on conventional genetic association studies, and higher resolution transcriptomic approaches, such as single-cell RNA sequencing, are needed to functionally validate these associations and clarify the molecular mechanisms underlying GDF-9 regulation in reproductive traits. Further research using genetically diverse populations and incorporating gene expression analysis is recommended to validate these associations and strengthen the application of GDF-9 in poultry breeding.