Evolution of immunoglobulin VH pseudogenes in chickens

Area of Science:

• Immunogenetics
• Evolutionary Biology
• Comparative Genomics

Background:

• Chickens possess a single functional heavy chain variable region gene (VH1) for immunoglobulin diversity.
• Immunoglobulin diversity in chickens is generated through gene conversion of VH1 by numerous pseudogenes (psi VH).

Purpose of the Study:

• To investigate the evolutionary history of the chicken immunoglobulin heavy chain variable region gene system.
• To analyze the evolutionary pressures acting on functional VH1 and pseudogene (psi VH) sequences.
• To explore the potential fusion and duplication events in the evolution of V and D genes.

Main Methods:

• Statistical analyses of chicken VH1 and psi VH genes.
• Comparative analysis with functional VH genes from other higher vertebrates.
• Dot matrix analysis of psi VH genes and genomic diversity (D) genes.

Main Results:

• Chicken VH genes are closely related, originating recently from one of three major vertebrate VH gene groups.
• Nonsynonymous substitution rates slightly exceed synonymous rates in complementarity-determining regions (CDRs), indicating diversity-enhancing selection on pseudogenes.
• Higher substitution rates in CDRs compared to framework regions (FRs) suggest positive selection interacting with meiotic gene conversion.
• psi VH genes exhibit D-gene-like sequences at their 3' ends, implying a fusion of V and D genes followed by duplication.

Conclusions:

• The chicken immunoglobulin heavy chain variable region gene system evolved from a common ancestral gene, with recent diversification.
• Selection pressures, particularly in CDRs, have influenced both functional genes and pseudogenes, driving diversity.
• Evidence suggests a significant evolutionary event involving the fusion of V and D genes, followed by multiplication via gene duplication.