The geographic structure of pathogen resistance phenotypes under balancing selection

Area of Science:

• Evolutionary biology
• Ecology
• Genetics

Background:

• The Red Queen model describes host-parasite coevolution driven by negative frequency-dependent selection.
• This model is often invoked to explain high genetic diversity and trans-species polymorphisms in disease genes.
• Previous studies showed reduced spatial differentiation at disease genes, but the geographic distribution of resistance phenotypes remained unstudied.

Purpose of the Study:

• To investigate the geographic distribution of host resistance phenotypes.
• To test predictions of the Red Queen model regarding the spatial distribution of traits under balancing selection.

Main Methods:

• Analysis of resistance phenotypes in 236 Daphnia magna populations across 4 continents.
• Testing these populations against 14 isolates of the pathogen Pasteuria ramosa.
• Examining the geographic distribution of resistance phenotypes in relation to host population structure and isolation by distance.

Main Results:

• Resistance phenotypes to Pasteuria ramosa were evenly distributed geographically across continents.
• No significant isolation by distance was observed for resistance phenotypes.
• Little variation in resistance phenotypes was found among different host phylogeographic clades.

Conclusions:

• The even geographic distribution of resistance phenotypes strongly supports the Red Queen model of antagonistic coevolution.
• Findings align with expectations for traits under balancing selection, where phenotypes are expected to be widespread.
• This study highlights the importance of studying phenotype distribution in the context of coevolutionary dynamics.