The salmonid MHC class I: more ancient loci uncovered

Area of Science:

• Immunogenetics
• Evolutionary Biology
• Aquatic Genomics

Background:

• The salmonid major histocompatibility complex (MHC) class I UBA gene exhibits exceptional sequence diversity.
• Previous understanding lacked insights into the origin of deep allelic lineages, possibly due to interlocus exon shuffling or ancient locus convergence.

Purpose of the Study:

• To identify and characterize additional MHC class I loci in salmon.
• To investigate the evolutionary history and functional potential of these newly discovered loci.

Main Methods:

• Sequence analysis of salmonid genomes.
• Comparative analysis of exon lineages across MHC class I loci.
• Evaluation of gene expression, conserved residues, and polymorphism for functional classification.

Main Results:

• Eight novel MHC class I loci (UCA, UDA, UEA, UFA, UGA, UHA, ULA, and ZE) were identified in salmon.
• Six loci share exon 2 and 3 lineages with UBA, with three being previously undescribed.
• Evidence suggests exon shuffling and duplications occurred before or during salmonid tetraploidization (50-100 Ma).
• UBA is the sole confirmed class Ia gene, while ZE, UCA, and ULA show partial class Ia characteristics.

Conclusions:

• The salmonid MHC class I region is more complex than previously recognized, with extensive locus duplication and exon sharing.
• Identified loci represent varying stages of evolution, with some potentially degrading into class Ib genes.
• These findings provide crucial insights into the evolution of immune gene diversity in teleosts.