Having applied the Habgood (1970) nomenclature for the host and parasite differentials to the pairs of genes in a gene-for-gene relationship, Robinson (1976) rearranged the Person (1959) differential interaction on the basis of the Habgood nomenclature. There was then a greatly increased simplicity, and he called this the Person/Habgood differential interaction (Fig. 4.4). Intriguingly, the patterns within the differential interaction are reproducible with the techniques of cellular automata (see 2.2) described by Wolfram (2002).
The importance of both the Person differential interaction, and the Person/Habgood differential interaction, is that they provide a phenotypic demonstration of a gene-for-gene relationship. They also reveal that every combination of vertical resistance genes can be matched by the parasite. In this sense, they predict new vertical gene combinations in the same way that the periodic table predicts new elements. They also indicate that the so-called pyramiding of genes will not normally enhance the durability of the vertical resistance to any significant extent. However, there is some evidence that this durability can be enhanced by mixing vertical resistance genes from several different species, as has been done, for example, with the vertical resistances of wild wheat species to Puccinia graminis tritici.
Think of this diagram as a map of a field trial. The vertical pathodemes (green) are planted in columns, down the matrix of the diagram, and in the numerical order of their Habgood names (see Fig. 4.3). The vertical pathotypes (red) are used to inoculate these pathodemes, and are applied in rows across the matrix of the diagram, and in the numerical order of their Habgood names. Each vertical pathodeme is thus tested with each vertical pathotype. A blue dot represents a matching interaction (susceptibility) and a blank represents a non-matching interaction (resistance). The importance of this diagram is that it illustrates all the important characteristics of the gene-for-gene relationship.
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