Molecular Targets for Coevolutionary Interactions Between Pacific Oyster Larvae and Their Sympatric .

Bacteria of the genus are the most predominant infectious agents threatening marine wildlife and aquaculture. Due to the large genetic diversity of these pathogens, the molecular determinants of virulence are only poorly understood. Furthermore, studies tend to ignore co-evolutionary interactions between different host populations and their locally encountered communities. Here, we explore the molecular targets of such co-evolutionary interactions by analyzing the genomes of nine strains from the clade showing opposite virulence patterns towards two populations of Pacific oysters introduced into European Wadden Sea. By contrasting phylogeny to their host specific virulence patterns, we could identify two core genome genes (OG1907 and OG 3159) that determine the genotype by genotype (G × G) interactions between oyster larvae and their sympatric communities. Both genes show positive selection between locations targeting only few amino acid positions. Deletion of each gene led to a loss of the host specific virulence patterns while complementation with OG3159 alleles from both locations could recreate the wild type phenotypes matching the origin of the allele. This indicates that both genes can act as a genetic switch for -oyster coevolution demonstrating that local adaptation in distinct lineages can rely on only few genes independent of larger pathogenicity islands or plasmids.