Genomic structure and evolution of beta-defensin genes in the golden pheasant and hwamei

Defensins play a vital role in the early stage of infection before adaptive immune responses are generated. Thus far, only limited detailed genomic data for avian defensin genes have been described. Here, using bacterial artificial chromosome libraries, we found that a 95 kb and 177 kb sequences in the golden pheasant (Chrysolophus pictus, Galliformes) and hwamei (Garrulax canorus, Passeriformes) corresponds to 16 and 30 defensin genes, respectively. Fluorescence in situ hybridization assigned defensin gene clusters in the golden pheasant and hwamei to chromosomes 2q and 3q, respectively. In combination with the previous chicken (Gallus gallus, Galliformes) and zebra finch (Taeniopygia guttata, Passeriformes) defensin clusters, the comparative genomic analysis revealed that lineage-specific duplications and deletions have given rise to clearly different genomic structures. On the basis of genomic characteristics, we further found that transposable elements act as agents of evolution, causing direct and indirect copy number variations in defensin genes via duplications. Tissue expression analysis showed that the Passeriformes-specific duplicated AvBD1 and AvBD3 genes are mainly expressed in the upper and lower respiratory tract, tongue, and spleen. Our analyses indicate that the duplication-and-deletion of defensin genes conformed to the birth-and-death evolutionary process and that transposable elements induced the duplication of defensin genes. Moreover, the respiratory system-specific expression pattern of Passeriformes-specific expanded AvBD1 and AvBD3 genes suggests their important role in maintaining the singing trait of songbirds. The understanding of the genomic structure, expression, and evolution of defensin genes can provide a crucial immunological foundation to study and prevent avian diseases.