Objective:The fat mass and obesity-associated gene (FTO), a crucial gene that affects human obesity and metabolism, has been widely studied in mammals but remains poorly characterized in birds. We aimed to identify variant FTO transcripts in domestic avian species, and to characterize the expression and biological functions of FTO in chickens.Methods:Variant FTO transcripts and their expression in birds were investigated using RACE and real-time quantitative reverse transcriptase-PCR technology. The effects of FTO on glucose metabolism, growth and body composition were determined by fasting and various diet treatments, as well as association analysis in a F(2) resource population. The function of cFTO1 was further studied by overexpression in chick embryo fibroblast (CEF) cells.Results:Variant FTO transcripts were identified in chicken (cFTO1 to cFTO4), duck (dFTO1, dFTO2 and dFTO4) and goose (gFTO1, gFTO2 and gFTO5). In the chicken, the complete transcript (cFTO1) was predominantly expressed in the leg muscle, pituitary, hypothalamus and cerebellum. Fasting increased both cFTO1 and PGC1α gene expression in the cerebrum, liver, breast muscle and subcutaneous fat, but decreased expression in the pituitary and anterior hypothalamus. In all tested tissues in chickens, a high-glucose diet markedly increased cFTO1 and PGC1α expression. Feeding a high-fat diet increased both cFTO1 and PGC1α expression, except in the pituitary. Overexpression of cFTO1 in CEF cells significantly increased the expression of PGC1α (2.5-fold), STAT3 (2.2-fold) and HL (1.5-fold), a cluster of genes related to energy metabolism. A total of 65 single nucleotide polymorphisms (SNPs) were identified in chicken FTO, and 18 tested SNPs were significantly associated with traits of body weight, body composition and fatness.Conclusions:These data collectively indicate that FTO is related to glucose metabolism, body weight, fatness and body composition in birds, thus expanding knowledge of FTO function to non-mammalian species.