Stable and Unstable Protection Mechanisms

Any mechanism that protects a host against a parasite is either within or beyond the capacity for micro-evolutionary change of the parasite. There is no recognised term for this phenomenon and, in the present work, the two categories of protection mechanism are labelled 'unstable' and 'stable' respectively. (However, it must be clearly recognised that these terms are also used in a wider context, and with a less restricted meaning, throughout this book, e.g., "ecological diversity confers stability").

In other words, a mechanism that is within the capacity for micro-evolutionary change of the parasite is described as 'unstable', because it is liable to stop functioning when a new strain of the parasite appears. An unstable mechanism is thus temporary in its effects. These unstable mechanisms include both vertical resistance and many crop protection chemicals. There is every reason to believe that the single-gene resistances of genetically engineered, transgenic plants will also be unstable (see 10.6.2).

Conversely, a stable mechanism is beyond the capacity for micro-evolutionary change of the parasite, and its effectiveness is permanent. Stable mechanisms include both horizontal resistance and various crop protection chemicals.

The corresponding nouns are 'stability' and 'instability'.

Unstable mechanisms exhibit a differential interaction (variable ranking) between the protection differentials and the parasite differentials. Stable protection mechanisms exhibit a constant ranking (see 6.1). The various categories of differential interaction (Robinson, 1987) indicate the mechanisms of instability, which range from a simple dietary preference in insects to inter-specific host hybridisation. These categories are:

  • The Person/Habgood differential interaction, (the vertical subsystem).
  • The false differential interaction.
  • The simple change differential interaction.
  • The toxin sensitivity differential interaction.
  • The environmental differential interaction (site-specificity).
  • The qualitative polyphyletic differential interaction.
  • The quantitative polyphyletic differential interaction.
  • The hybridising parasite differential interaction.
  • The hybridising host differential interaction.
  • The immunity differential interaction.

These differential interactions are beyond the scope of the present book and specialists requiring greater detail should consult the original. Generally, the less the genetic difference between host differentials, the greater the instability. For example, a host resistance based on dietary preferences in an insect is likely to be unstable, even though there is no gene-forgene relationship.

Some comment concerning Vanderplank's (1963) terms 'vertical' and 'horizontal' may be useful. 'Vertical' means that a gene-for-gene relationship is present, while 'horizontal' means that there is no gene-for-gene relationship. These terms can be used to describe host resistance, parasitic ability in the parasite, populations of both host and parasite, subsystems of a pathosystem, and so on. But they are not synonymous with 'unstable' and 'stable' respectively. Vertical resistance is usually (but not invariably, see 5.7) unstable, but not all unstable resistances are vertical. Equally, horizontal resistance is stable, but not all stable resistances are horizontal (see 5.7).

Vanderplank's terms have never been popular, probably because abstract terms are somewhat unfashionable. Their use should perhaps be restricted to technical discussion concerning the presence or absence of a gene-for-gene relationship. It is to be hoped that the terms 'stable' and unstable' become common usage in the wider context of protection mechanisms, if only to refute the popular belief that all protection mechanisms are unstable.

Finally, it should be noted that the term 'unstable' describes the protection mechanism when, in fact, the mechanism itself remains unchanged, and it is the parasite that undergoes micro-evolutionary change. Ideally, this term should describe the parasite, but no suitable words seem to exist.

The following examples illustrate the importance of this phenomenon of stability and instability.

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