Different evolutionary histories of the two classical class I genes BF1 and BF2 illustrate drift and selection within the stable MHC haplotypes of chickens.
Compared with the MHC of typical mammals, the chicken MHC (BF/BL region) of the B12 haplotype is smaller, simpler, and rearranged, with two classical class I genes of which only one is highly expressed. In this study, we describe the development of long-distance PCR to amplify some or all of each class I gene separately, allowing us to make the following points. First, six other haplotypes have the same genomic organization as B12, with a poorly expressed (minor) BF1 gene between DMB2 and TAP2 and a well-expressed (major) BF2 gene between TAP2 and C4. Second, the expression of the BF1 gene is crippled in three different ways in these haplotypes: enhancer A deletion (B12, B19), enhancer A divergence and transcription start site deletion (B2, B4, B21), and insertion/rearrangement leading to pseudogenes (B14, B15). Third, the three kinds of alterations in the BF1 gene correspond to dendrograms of the BF1 and poorly expressed class II B (BLB1) genes reflecting mostly neutral changes, while the dendrograms of the BF2 and well-expressed class II (BLB2) genes each have completely different topologies reflecting selection. The common pattern for the poorly expressed genes reflects the fact the BF/BL region undergoes little recombination and allows us to propose a pattern of descent for these chicken MHC haplotypes from a common ancestor. Taken together, these data explain how stable MHC haplotypes predominantly express a single class I molecule, which in turn leads to striking associations of the chicken MHC with resistance to infectious pathogens and response to vaccines.
Shaw
Iain
I
Institute for Animal Health, Compton, Berkshire, United Kingdom.
Powell
Timothy J
TJ
Marston
Denise A
DA
Baker
Ken
K
van Hateren
Andrew
A
Riegert
Patricia
P
Wiles
Michael V
MV
Milne
Sarah
S
Beck
Stephan
S
Kaufman
Jim
J
D000818:Animals; D001483:Base Sequence; D002645:Chickens / Q000235:genetics* / Q000276:immunology; D019143:Evolution, Molecular; D015870:Gene Expression; D005805:Genes, MHC Class I; D040961:Genetic Drift; D006239:Haplotypes; D015395:Histocompatibility Antigens Class I / Q000145:classification* / Q000235:genetics*; D008969:Molecular Sequence Data; D009154:Mutation; D010802:Phylogeny; D011401:Promoter Regions, Genetic
D015395:Histocompatibility Antigens Class I