Molecular Biology and Evolution, Vol 12, 94-102, Copyright © 1995 by Society for Molecular Biology and Evolution
T Ota and M Nei
In chickens, there is a single functional gene (VH1) coding for the heavy
chain variable region of immunoglobulins, and immunoglobulin diversity is
generated by gene conversion of the VH1 gene by many variable region
pseudogenes (psi VH's) that exist on the 5' side of the VH1 gene. To
understand the evolution of this unique genetic system, we conducted
statistical analyses of VH1 and psi VH genes together with functional VH
genes from other higher vertebrate species. The results indicate, first,
that chicken VH genes are all closely related to one another and were
derived relatively recently from an ancestral gene belonging to one of the
three major groups of VH genes in higher vertebrates. Second, the rate of
nonsynonymous substitution is slightly higher than that of synonymous
substitution in the complementarity- determining regions (CDRs), which
suggests that diversity-enhancing selection has operated in the CDRs even
for pseudogenes. However, both the rates of synonymous and nonsynonymous
substitution are higher in the CDRs than in the framework regions (FRs),
apparently because of an interaction between positive selection and meiotic
gene conversion in the CDRs. Third, a dot matrix analysis of the psi VH
genes and genomic diversity (D) genes has indicated that the 3' end of psi
VH genes is attached by D-gene-like sequences, and this region of psi VH
genes has high similarity with D gene sequences. This suggests that V and D
genes were fused at some point of evolutionary time and this fused element
multiplied by gene duplication. Finally, two alternative hypotheses of
explaining the evolution of the chicken VH gene system are presented.
ORIGINAL ARTICLE
Evolution of immunoglobulin VH pseudogenes in chickens
Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park 16802.
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