Chicken immunoglobulin gamma-heavy chains: limited VH gene repertoire, combinatorial diversification by D gene segments and evolution of the heavy chain locus.

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Abstract
CDNA clones encoding the variable and constant regions of chicken immunoglobulin (Ig) gamma‐chains were obtained from spleen cDNA libraries. Southern blots of kidney DNA show that the variable region sequences of eight cDNA clones reveal the same set of bands corresponding to approximately 30 cross‐hybridizing VH genes of one subgroup. Since the VH clones were randomly selected, it is likely that the bulk of chicken H‐chains are encoded by a single VH subgroup. Nucleotide sequence determinations of two cDNA clones reveal VH, D, JH and the constant region. The VH segments are closely related to each other (83% homology) as expected for VH or the same subgroup. The JHs are 15 residues long and differ by one amino acid. The Ds differ markedly in sequence (20% homology) and size (10 and 20 residues). These findings strongly indicate multiple (at least two) D genes which by a combinatorial joining mechanism diversify the H‐chains, a mechanism which is not operative in the chicken L‐chain locus. The most notable among the chicken Igs is the so‐called 7S IgG because its H‐chain differs in many important aspects from any mammalian IgG. The sequence of the C gamma cDNA reported here resolves this issue. The chicken C gamma is 426 residues long with four CH domains (unlike mammalian C gamma which has three CH domains) and it shows 25% homology to the chicken C mu. The chicken C gamma is most related to the mammalian C epsilon in length, the presence of four CH domains and the distribution of cysteines in the CH1 and CH2 domains. We propose that the unique chicken C gamma is the ancestor of the mammalian C epsilon and C gamma subclasses, and discuss the evolution of the H‐chain locus from that of chicken with presumably three genes (mu, gamma, alpha) to the mammalian loci with 8‐10 H‐chain genes.