Extensive genetic polymorphism of four plasma alpha-protease inhibitors in pigs and evidence for tight linkage between the structural loci of these inhibitors.

Two-dimensional horizontal gel electrophoresis of pig plasma samples (under non-denaturing conditions) using Immobiline pH gradient gels 4.0-6.0 for the first dimension separation, resulted in clear resolution of the variants of four different alpha-protease inhibitors (protease inhibitor -1 and -2, PI1 and PI2; post-albumin -1A and -1B, PO1A and PO1B). All these variants were readily visualized by general protein staining. About 900 families each of Swedish Landrace (SL) and Yorkshire (SY) breeds were studied. The extensive inheritance data, including the recombinants encountered, indicated that each of these four inhibitors is controlled by a separate, autosomal locus and that the four loci are tightly linked (spread over a distance of 1-1.5 cM) with the order as Pi1-Po1A-Po1B-Pi2. The alleles observed were two of Pi1, 14 of Po1A, 11 of Po1B and 8 of Pi2. About 40 haplotypes were observed in each of the two breeds. The allele frequencies at Po1A, Po1B and Pi2 loci were remarkably different in the two breeds; the alleles at these three loci showed a very strong linkage disequilibrium (0.8-1.0). The females showed much higher recombination frequencies than the males in the Po1A-Pi2 interval, suggesting that gene conversion-like events may be occurring at these loci. This linkage in pigs and similar ones comprising some plasma alpha-protease inhibitor genes in humans and in rodents, reported recently in the literature, indicate evolutionary conservation of a homologous linkage group in these species.