Multiple parasites are driving major histocompatibility complex polymorphism in the wild.

Area of Science:

• Evolutionary biology
• Immunogenetics
• Ecology

Background:

• Parasite-mediated selection can lead to co-evolutionary arms races between hosts and parasites.
• Simultaneous infections may promote genetic diversity in host resistance genes at population and individual levels.

Purpose of the Study:

• To investigate the influence of parasite diversity on the evolution of Major Histocompatibility Complex (MHC) genes in three-spined sticklebacks.
• To determine if MHC diversity correlates with parasite load within individual fish.

Main Methods:

• Analysis of MHC class IIB gene diversity in eight natural populations of three-spined sticklebacks.
• Assessment of parasite diversity and abundance across different habitat types (lakes, rivers, estuaries).
• Correlation analysis between MHC diversity, parasite diversity, and general genetic diversity (microsatellites).

Main Results:

• MHC class IIB and parasite diversity varied significantly across habitats, with lowest diversity in rivers.
• Parasite diversity positively influenced MHC class IIB variation, while general genetic diversity did not.
• Individual fish with intermediate MHC allele numbers exhibited lower parasite loads.

Conclusions:

• Parasite pressure contributes to the diversification of immune genes in natural populations.
• Intermediate MHC diversity is associated with optimal immunocompetence, supporting an 'allele counting' strategy.
• Mate choice in female sticklebacks may favor partners that enhance offspring immunocompetence through MHC diversity.