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. 150-word “Importance” paragraph During host-parasite coevolution, hosts evolve to evade the damaging effect of the parasite, while parasites evolve to maximize their benefits by exploiting the host. The genes underlying this coevolution remain largely unknown. For the prime model-system for coevolutionary research, the crustacean Daphnia and the parasite Pasteuria ramosa , collagen-like proteins (CLPs) in Pasteuria were suggested to play a crucial role for host-parasite interactions. Here we report that transferring part of a CLP coding gene from the unculturable P. ramosa to Bacillus thuringiensis ( Bt ), confirmed the function of this protein as a genotype-specific adhesin to the host’s cuticle. Our finding highlights the importance of a CLP in host-parasite interactions and will enable us to explore the population genetic dynamics of coevolution in this system.