B-G gene structure, genetic variation and expression in the turkey (Meleagris gallopavo) major histocompatibility complex

Abstract

The major histocompatibility complex (MHC) is a genomic region encoding for genes with various immune functions in vertebrates. Although this region has been studied extensively in humans, mice and agriculturally important species such as cattle, horses and chickens, the organization and functional significance of the turkey MHC is relatively unknown. One set of loci located in the avian MHC, designated as B-G genes, have been identified and sequenced within the B-locus of the turkey, chicken and quail, with additional B-G loci identified in the extended MHC of the latter two species. These genes are linked to the class I and class II loci of the MHC and also show regions of extreme polymorphism (Miller, 1984). The total number, function and significance of the B-G genes are yet unknown in any bird species. In turkeys, three B-G genes were previously sequenced within the B-locus (Chaves et al., 2009a), with evidence suggesting additional functional B-G genes located past the 5' ends of the sequenced MHC region. Evidence in the chicken shows differential expression of B-G genes in various immune tissues, which suggests potential immune function. This research used 454 FLX Next Generation Sequencing technology (Roche) for sequencing a bacterial artificial chromosome (BAC) and identified two additional B-G genes located past the 5' end of the core B-locus of the turkey. These genes were annotated using in silico analysis and show organization similar to those in the chicken and quail. Using this information, sequence variation of the B-G genes was compared in different stocks of turkeys. Because each of the three groups, (commercial, heritage and wild), have been selected for different characteristics, variation within these loci was expected. This experiment found variation among loci within the three groups of turkeys. Lastly, B-G gene expression was investigated with quantitative real-time PCR using liver tissue of aflatoxin challenged birds. A low level of expression was observed for three of the four BG genes investigated, with BG5 expression invariant in all individuals across treatment groups. BG4 expression levels fluctuated within and between groups and a higher level of expression was measured in the lactobacillus + aflatoxin group. This work extended the turkey B-locus sequence past the homologous region in the chicken and marks the first examination of sequence variation and gene expression of multiple B-G genes in any species. Sequence variation and expression differences among loci support a hypothesis of distinct functions for these molecules.