Avian IFN-γ genes:: Sequence analysis suggests probable cross-species reactivity among galliforms

Little is known about the evolution of cytokines in non-mammalian systems. To address this problem, we attempted to clone the gene for interferon-gamma (IFN-gamma) from a variety of avian species using oligonucleotide primers based on the sequence of the chicken IFN-gamma gene, The coding sequence and partial intron sequences were determined for four species, namely guinea fowl, ring-necked pheasant, Japanese quail, and turkey. To obtain sequence information on the gene extremities, a modified 5' and 3' RACE protocol was used. The sequence information showed that the coding regions of the IFN-gamma gene are highly conserved among the species studied (93.5%-96.7% and 87.8%-97.6% at the nucleotide and peptide levels, respectively) and are more conserved at the amino-terminal region (exons 1 and 2) than the carboxyl-terminal (exons 3 and 4), This high degree of overall identity at the predicted primary amino acid sequence level of the protein, including the deduced IFN-gamma receptor binding motifs, suggests that IFN-gamma may be cross-reactive among these species. Phylogenetic analysis shows that the similarity of the avian IFN-gamma sequences parallels the presumed evolutionary relationships between the species.