MHC class I evolution; from Northern pike to salmonids

Area of Science:

• Immunogenetics
• Evolutionary Biology
• Comparative Genomics

Background:

• Salmonids are crucial farmed and wild species facing evolving environmental and pathogenic pressures.
• Understanding their immune system, particularly Major Histocompatibility Complex (MHC) genes, is essential for health and management.
• Classical MHC class I (MHCI) genes are vital for adaptive immunity, with salmonids possessing duplicate regions due to a whole genome duplication event.

Purpose of the Study:

• To compare the gene organization and evolution of MHC class I gene sequences in salmonids and Northern Pike.
• To investigate the impact of whole genome duplication events on MHCI gene evolution in salmonids.
• To understand the diversification of MHCI lineages, including U, Z, and L, in salmonids.

Main Methods:

• Comparative analysis of MHC class I gene sequences between salmonids and Northern Pike.
• Phylogenetic analysis to trace the evolutionary history of MHCI gene duplications.
• Examination of gene duplication patterns across different salmonid lineages (Ia, Ib, Z, U, L).

Main Results:

• Salmonid MHCI gene duplicates (Ia, Ib, Z lineages) are not orthologous to Northern Pike sequences, indicating unique salmonid-specific duplications.
• Both salmonids and Northern Pike show expanded U and Z lineage genes, but these duplications occurred independently.
• Massive species-specific gene duplications in the L lineage occurred after the divergence of Oncorhynchus and Salmo.

Conclusions:

• Salmonid MHCI evolution is characterized by unique gene duplications, distinct from basal relatives like Northern Pike.
• The MHCIa and MHCIb regions in salmonids originated after their split from pike, predating the Oncorhynchus and Salmo divergence.
• The extensive diversification of L lineage genes in salmonids suggests a broad range of functional roles and adaptations in their immune response.