The genetics of pathogen resistance in vertebrates

Zebrafish embryo at the somitogenesis stage. How does the innate immune system tackle pathogens?

Differential resistance to pathogens provides a strong selective force in evolution. In vertebrates, both the adaptive immune system and natural immunity defend the organism against pathogens. The biology of the adaptive immune system is well understood, and natural immunity is beginning to be widely studied, but the contributions of these defence systems to selection in the wild have not been fully investigated, although epidemiological surveys in man have contributed important insights. We propose to screen for polymorphisms associated with differential sensitivity to pathogens, using the zebrafish Danio rerio as experimental organism. We plan to follow two parallel approaches. We will use laboratory strains to screen for resistance to a model pathogen. In parallel, we will conduct a survey of wild populations and establish a collecction that will be tested for resistance to a panel of pathogens. The first experiment will allow us to establish reliable screening methods in a well defined laboratory system and provide reference data and candidate loci against which the results from the wild populations can be compared. A long term goal will be the comparison of resistance loci identified in zebrafish with those in other species with well-studied polymorphisms, such as guppy or medaka.
The pathogen to be used initially is Mycobacterium marinum, which causes tuberculosis in fish. It has very low pathogenecity for humans and inbred fish lines have been observed to show differential sensitivity to infection. Moreover, the physiology of infection is being studied, and GFP-marked stocks exist that can be used to analyse infections at the cellular level. We therefore plan to use M. marinum as the pathogen for a pilot genetic screen of mutagenized zebrafish laboratory strains. We plan to screen for dominant alleles affecting susceptibility in an F1 screen as well as for recessive traints in an F2 screen. Loci contributing to pathogen sensitivity in mutagenized progeny will be determined by modern QTL mapping methods. An RFLP and SNP map for the zebrafish has been established.

Figure: live zebrafish embryo injected with fluorescent E.coli (courtesy of D. Sieger).