Genetic Homeostasis

The concept of genetic homeostasis was first formulated by Lerner (1954) and it concerns the ability of a population to maintain an advantageous genetic constitution. This, of course, is another example of self-organisation (see 2.4). It is clear that the vertical subsystem must have genetic homeostatic mechanisms that ensure three things:

The presence of n/2 vertical genes in all the host and parasite individuals at all times. Any swing away from n/2 represents a loss of effectiveness in the system of locking, and genetic homeostasis will restore that effectiveness.

An equal frequency of all the n/2 vertical resistances and vertical parasitic abilities. Any swing away from this equal frequency represents a loss of effectiveness in the system of locking, and genetic homeostasis will restore that equal frequency.

A random distribution of all the combinations of n/2 genes. Any swing away from this random distribution represents a loss of effectiveness in the system of locking, and genetic homeostasis will restore that random distribution.

It is not difficult to postulate mechanisms for such a genetic homeostasis, the most simple being density dependent selection. For example, a particular n/2 vertical pathodeme might occur with a low frequency. This would give it a survival advantage, and it would be matched infrequently, because the matching vertical pathotype would be rare. This survival advantage would lead to an increased reproduction, and this vertical pathodeme would then become common. It would now have a survival disadvantage, and it would be matched frequently, because the matching vertical pathotype would also be common. This survival disadvantage would lead to a reduced reproduction, which would restore the vertical subsystem balance. Similar arguments can be made for the parasite population.

This density dependent selection would doubtless permit continuing minor fluctuations in the frequency of vertical pathodemes and vertical pathotypes. These fluctuations are what engineers call 'hunting', as seen in the automatic steering of a ship (see 1.7). This might also mean that individuals of either population could occur with (n + 1) or (n - 1) vertical genes. However, this need not seriously affect the overall stability of the vertical subsystem.

Other mechanisms can be postulated, and they may be even more effective. However, this is a matter that has apparently never been investigated, and we have little factual knowledge about the functioning of a gene-for-gene relationship in a wild plant pathosystem.

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