Parasite mediated homogenizing selection at the MHC in guppies

Area of Science:

• Evolutionary ecology
• Immunogenetics
• Population genetics

Background:

• Understanding selective forces on genetic diversity is crucial in evolutionary ecology.
• Major histocompatibility complex (MHC) genes are vital for vertebrate adaptive immunity and are key targets for selection studies.
• Previous research indicated lower than expected divergence at MHC class IIB loci in wild guppy populations.

Purpose of the Study:

• To investigate the role of parasitic worms (Gyrodactylus spp.) as a selective agent on MHC class IIB genes in Trinidadian guppies.
• To determine the genetic architecture, including additive and non-additive effects, of MHC class IIB alleles in relation to parasite load.
• To explore the mechanism of homogenizing selection potentially acting on MHC genes in these populations.

Main Methods:

• Surveyed parasite (Gyrodactylus turnbulli and G. bullatarudis) infection in 10 wild guppy populations.
• Analyzed MHC class IIB gene architecture using a genetic algorithm.
• Partitioned additive and non-additive genetic effects of common and rare MHC allele types on parasite load.

Main Results:

• No evidence for non-additive genetic effects on parasite load was found.
• One specific MHC allele type (a-type) demonstrated a significant negative additive effect on parasite load.
• Individuals with more copies of the a-type allele exhibited lower infection rates by Gyrodactylus.

Conclusions:

• Parasite infection by Gyrodactylus spp. is directly linked to MHC genotype in guppies.
• The MHC a-type allele confers resistance to parasites, acting as a mechanism for homogenizing selection.
• These findings provide insight into how host-parasite interactions shape genetic diversity in functionally important immune genes.