Matching-Allele-Models explain host specificity and high diversity of Collagen-Like Proteins in a virulent pathogen

Host specificity is a defining feature of many parasitic interactions, yet the genetic mechanisms behind this specificity remain unclear. Pasteuria ramosa, a bacterial pathogen of Daphnia magna, provides a model for studying genotype-specific infectivity. We investigate the role of collagen-like proteins (PCLs) in mediating host attachment using genome-wide association studies (GWAS) and presence/absence variation (PAV) analyses. Our results reveal high diversity and lineage-specific patterns within the PCL gene family. For three PCL triplets, we find strong associations with parasites attachment phenotypes. Nearly perfect genotype-phenotype matches for certain PCL alleles support the matching allele model (MAM), considered to be an important model of host-parasite coevolution and balancing selection. This is the first molecular evidence of MAM in a pathogen. This study highlights the role of PCL genes in infection specificity, advancing our understanding of host-pathogen coevolution and supporting the Red Queen hypothesis as a driver of genetic diversity.