Ectopical expression of bacterial collagen-like protein supports its role as adhesin in host-parasite coevolution

For a profound understanding of the mechanisms of antagonistic coevolution, it is necessary to identify the coevolving genes. The spore-forming bacterium Pasteuria ramosa and its host, the microcrustacean Daphnia , are a well-characterized paradigm for co-evolution, but the underlying genes remain largely unknown. A genome-wide association study identified a polymorphic carboxy-terminal globular domain of Pasteuria collagen-like protein 7 (Pcl7) as a candidate mediating parasite attachment and driving its coevolution with the host. Since P. ramosa cannot currently be genetically manipulated, we used Bacillus thuringiensis as a surrogate parasite to express a fusion protein of a Pcl7 carboxy-terminus from P. ramosa and the amino-terminal domain of a B. thuringiensis collagen-like protein. Mutant B. thuringiensis (Pcl7- Bt ) spores but not wild-type B. thuringiensis (WT- Bt ) spores, attached to the same site of susceptible hosts as P. ramosa . Furthermore, Pcl7- Bt spores attached readily to host genotypes that were susceptible to the P. ramosa clone that was the origin of the Pcl7 C-terminus, but only slightly to resistant host genotypes. These findings indicated that the fusion protein was properly expressed and folded and demonstrated that indeed the C-terminus of Pcl7 mediates attachment in a host genotype-specific manner. These results provide strong evidence for the involvement of a CLP in the coevolution of Daphnia and P. ramosa and opens new avenues for genetic epidemiological studies of host-parasite interactions.