Mapping of the chicken immunoglobulin heavy-chain constant region gene locus reveals an inverted α gene upstream of a condensed υ gene

Chicken antibodies are increasingly being used as diagnostic and therapeutic tools. As only the genomic organization of the mu encoding gene was previously known, we analysed the chicken immunoglobulin Y (IgY) and IgA (upsilon and alpha chain) immunoglobulin heavy-chain constant region (IGHC) genes and the organization of the chicken IGHC locus. The alpha gene is encoded by four separate exons, whereas, surprisingly, there is no intervening DNA sequence between the CH1 and CH2 domains of the IgY heavy chain, which is thus encoded by three exons separated by two introns. DNA sequence analysis shows that the exon boundaries of the chicken IGHC genes are not consistent with published domain borders. Furthermore, differences in DNA sequence confirm the existence of IgY, IgA and IgM allotypes in chickens. Finally, our results show that the IGHC genes of chicken (IgY, IgA and IgM) are all colocated on chromosome E18C15W15, where the alpha gene is located upstream of the upsilon gene in an inverted transcriptional orientation. The distances between the mu and alpha genes and the alpha and upsilon genes are about 18 and 15 kilobases, respectively, and thus, the size of the whole chicken IGHC locus is approximately 67 kilobases.