Evolution of the porcine (Sus scrofa domestica) immunoglobulin kappa locus through germline gene conversion

Immunoglobulin (IG) gene rearrangement and expression are central to disease resistance and health maintenance in animals. The IG kappa (IGK) locus in swine (Sus scrofa domestica) contributes to approximately half of all antibody molecules, in contrast to many other Cetartiodactyla, whose members provide the majority of human dietary protein and in which kappa locus utilization is limited. The porcine IGK variable locus is 27.9 kb upstream of five IG kappa J genes (IGKJ) which are separated from a single constant gene (IGKC) by 2.8 kb. Fourteen variable genes (IGKV) were identified, of which nine are functional and two are open reading frame (ORF). Of the three pseudogenes, IGKV3-1 contains a frameshift and multiple stop codons, IGKV7-2 contains multiple stop codons, and IGKV2-5 is missing exon 2. The nine functional IGKV genes are phylogenetically related to either the human IGKV1 or IGKV2 subgroups. IGKV2 subgroup genes were found to be dominantly expressed. Polymorphisms were identified on overlapping BACs derived from the same individual such that 11 genes contain amino acid differences. The most striking allelic differences are present in IGKV2 genes, which contain as many as 16 amino acid changes between alleles, the majority of which are in complementarity determining region (CDR) 1. In addition, many IGKV2 CDR1 are shared between genes but not between alleles, suggesting extensive diversification of this locus through gene conversion.