The Ig and MHC gene complexes of the chicken differ in interesting ways from their mammalian counterparts. In the case of the Ig light (\) chains,

DNA rearrangement during embryonic development joins a unique functional VL gene to a single JL sequence adjacent to the single CL gene. Light chain variability is generated principally by gene conversion (or a mechanism with similar effect), which leads to replacement of sequences in the rearranged VL gene by sequences from one or more of a series of 25 V, pseudogenes located 5' of the rearranged VL. The whole light chain gene complex has been isolated, and all the pseudogenes as well as the functional V, J and CL genes sequenced. A similar gene conversion mechanism generates heavy chain V region diversity, although in this case only 30 of an estimated 70 V pseudogenes have been sequenced (in addition to the single functional VH, D and JM regions and the unique C^and C, genes). D regions fused to the pseudogenes also contribute to variability. Recent evidence suggests that gene conversion and somatic point mutations additionally diversify the peripheral B cell repertoire in splenic germinal centers. As in mammals, the light and heavy chain gene complexes are not linked, whereas jj, and y chain genes are closely linked.

Chicken TCR a, (3, y and 8 genes have been cloned and sequenced. As in mammals, 8 and (3 genes comprise rearranged VDJC elements, whereas a and y genes result from VJC rearrangement. There are only two Va, two Vp, two V8 and probably three V7 families in chicken. The TCR a/8 locus organization is similar to that of mammals. Preliminary results indicate that both V01 and V„2 gene segments can be used to encode TCR 8 chains. The TCR repertoire is generated, in contrast to the B cell receptor repertoire, by combinatorial and junctional diversity, as in mammals.

The chicken MHC (B complex) is located on microchromosome 16, which carries the nucleolar organizer region (NOR). The B complex is smaller and simpler than mammalian MHCs. The average distance between genes in the complex is only about 10-20 kB, and typical intron sizes are about 100 bp. The class I a and class II (3 genes (two of each in the B complex of the B12 strain) are closely spaced, interspersed and G+C-rich. The single class II a gene is situated on the same microchromosome, some distance (about 5 cM) away from the MHC. In most MHC haplotypes there is a preferential expression of only one class I a and one class II (3 gene, although the number of expressed genes may be strain-depen-dent. Within the MHC, the BF/BL region containing class I a and class II (3 genes can be separated from a region encoding polymorphic B-G antigens, although the frequency of recombination between the two regions is low (about 0.05%). The B-G region contains multiple B-G genes, of which at least two are transcribed and presumably give rise by multiple splicing, processing or other mechanisms to the multiple B-G products seen in protein analyses. At least one expressed B-G-like gene is also located within the B-F/B-L region. Other genes, one coding for a protein homologous to a G protein subunit, are also present in this region. In contrast to the mammalian situation, factor B polymorphism is not linked to the MHC in chickens.

Rfp-Y, a second region bearing MHC-type (two class I a and three class II 3) genes, is also located on chromosome 16. It is separated from the B complex by a region which contains the NOR and exhibits frequent recombination. These MHC genes seem to be neither highly polymorphic nor highly expressed.

The chicken (32-microglobulin gene has been cloned and sequenced. It shows little polymorphism and is located on a large microchromosome (either 10 or 11).

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