Differential Interaction

Vanderplank (1968) first pointed out that one of the differences between vertical resistance and horizontal resistance was that vertical resistance exhibited a variable ranking (Fig. 4.1), while horizontal resistance exhibited a constant ranking (Fig. 4.2). He named the variable ranking with its statistical term 'differential interaction'. However, it subsequently became clear that there are several categories of differential interaction, depending on the degree of genetic difference between the host and parasite differentials (Robinson 1976, 1987). The vertical subsystem is defined by a particular differential interaction that Robinson (1976, 1987) called the Person/Habgood differential interaction (Fig. 4.4). This is the differential interaction with smallest degree of genetic difference between differentials, these differences being those of only the genes of the gene-for-gene relationship.

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