Changing Concepts in Host-Pathogen Genetics

Since the rediscovery of Mendel's work at the tum of the century, there have been a number of changes in the concepts of genetics of host-parasite interactions. The kinds of data have not changed greatly but different interpretations have been given to the data. As expected, the changes in the interpretations have paralleled, to a certain degree, the progress made in genetics. In this paper I want to present my view of the principal concepts held during the past 75 years, and my reasons for deducing that these were the principal concepts. These views were deduced from an examination of the types of experiments performed. When concepts of host-parasite interac­ tions changed, there was a change in the types of experiments performed. In some instances it is possible to cite a single publication that illustrates a major change in these concepts. Other changes have obviously evolved quite gradually. For the latter, I have cited one or more papers that, in my judgment, best illustrate a concept, or the change in concepts. I restrict this discussion to genetic variability within a host species and within a pathogen species. No attempt is made to discuss, for example, why an oak tree is resistant to fuccinia graminis f. sp. tritici, normally a pathogen of wheat. The oak tree cannot be crossed with wheat to determine the number of gene differences related to resistance and susceptibility, or the kinds of gene actions, and therefore the differences cannot be manipu­ lated by present genetic procedures. (It might be possible in not too long a time to transfer a gene, by recombinant DNA techniques, from an oak tree to a wheat plant, and to experimentally analyze such differences.)