The basis for haplotype complexity in VCBPs, an immune-type receptor in amphioxus

Area of Science:

• * Evolutionary immunology
• * Invertebrate genomics
• * Molecular evolution

Background:

• * Adaptive immunity in vertebrates relies on somatic gene variation for diverse antigen recognition.
• * Invertebrates lack adaptive immunity but possess alternative self-protection mechanisms.
• * Protochordate amphioxus uses variable region-containing chitin-binding proteins (VCBPs) for innate immune-type functions.

Purpose of the Study:

• * To investigate the genetic diversity and evolutionary mechanisms of VCBP genes in Branchiostoma floridae.
• * To understand the role of VCBP gene family in innate immunity and immune receptor diversification in invertebrates.

Main Methods:

• * Analysis of genomic DNA and complementary DNA sequences from diverse amphioxus populations.
• * Examination of VCBP gene alleles, haplotypes, and sequence complexity.
• * Comparative analysis of VCBP gene polymorphism with overall genome-wide variation.

Main Results:

• * VCBP genes (VCBPs 1-5) in amphioxus exhibit exceptionally high levels of polymorphism, exceeding general genome-wide variation.
• * VCBP2 and VCBP5 genes possess a highly polymorphic exon 2 within their N-terminal immunoglobulin domain.
• * Sequence complexity differences within and across VCBP2/5 haplotypes arise from genetic exchange, copy number variation, and mutation, forming distinct allelic lineages.

Conclusions:

• * Amphioxus VCBP gene family demonstrates significant allelic and haplotype diversity, indicative of immune-type function.
• * Mechanisms for selective hyperdiversification of immune-type receptors within a multigene family were present early in cephalochordate evolution.
• * These findings provide insights into the evolution of immune diversity in the absence of adaptive immunity.