Molecular Biology and Evolution, Vol 16, 479-490, Copyright © 1999 by Society for Molecular Biology and Evolution
H Wittzell, A Bernot, C Auffray and R Zoorob
The major histocompatibility complex (Mhc) of the ring-necked pheasant
contains two polymorphic Mhc class II B genes. We show here, by screening
of a cDNA library and RT-PCR from RNA, that both of these loci, Phco-DAB1
and Phco-DAB2, normally are transcribed in the spleen. They differ mainly
in the 3' untranslated (UT) region, with the transcript lengths, not
including the poly(A) tails, being 1,100 nt for DAB1 and 955 nt for DAB2.
These two loci are orthologous to the B-LBI and B-LBII loci of the domestic
chicken, respectively. DAB1 and DAB2 therefore seem to have evolved from a
duplication before the split of the evolutionary lineages leading to the
pheasant and the domestic chicken ca. 20 MYA. This is the first report of
an orthologous relationship between avian Mhc genes. Yet, the third exons
of DAB1 and DAB2 were identical in all available sequences and differed at
10 positions from the exon 3 sequences of B-LBI/B-LBII. The species-
specific exon 3 suggests that DAB1 and DAB2 are subject to concerted
evolution, i.e., interlocus genetic exchange. The exon 2 sequences show
characteristic polymorphism, with hypervariable segments occurring in
different combinations in different alleles. Given the divergence in the
3'UT region, the finding of the same exon 2 sequence at both the DAB1 and
the DAB2 loci in one of the pheasant haplotypes also suggests that
interlocus genetic exchange does occur. Accordingly, the exon 2 sequences
tended to cluster irrespective of locus in the phylogenetic analyses.
Genetic exchange simultaneously involving both exon 2 and exon 3 may be
facilitated by the short length of the intervening intron (< 100 bp) in
pheasants and domestic chickens compared with, e.g., humans (about 3 kb).
ORIGINAL ARTICLE
Concerted evolution of two Mhc class II B loci in pheasants and domestic chickens
Centre National de la Recherche Scientifique, Villejuif, France. hakan.wittzell@teorekol.lu.se
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