Genetics of resistance to bacterial and parasitic infection / edited by D. Wakelin and J.M. Blackwell.

Date:
1988
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics of resistance to bacterial and parasitic infection / edited by D. Wakelin and J.M. Blackwell. Source: Wellcome Collection.

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    8 Genetics of Resistance to Bacterial and Parasitic Infection 1.4.6. Evolutionary aspects The factors or mechanisms which underUe natural resistance occur in the absence of prior exposure to the invading parasite and are non-specific in their activity. These two characteristics sharply differentiate natural resistance from acquired resistance, although their operation in determining overall levels of resistance to infection may be closely integrated. Mechanisms of acquired resistance become operative only after the host has experienced infection and they are specific in their recognition of the organism concerned. Acquired resistance in this sense is characteristic of vertebrate animals in general and of the homoiothermic birds and mammals in particular. It is the property of the immune system and depends upon the production and interaction of complex cellular and humoral mechanisms. It is fruitful to consider the phenomena which underlie both natural and acquired resistance from an evolutionary point of view. All host-parasite associations, whatever their present form and characteristics, must have developed from initial accidental contact between the organisms concerned. No progress would have been possible unless the potential parasite was able to survive, even if for only a limited period, in the environment provided by the host. Selective advantage would not have been gained unless the association with the host resulted in some increase in reproductive potential of the parasite. It is easy in retrospect to see that, for well adapted parasitic organisms, the stability of host environments and the ready availability of nutrients do indeed provide the conditions necessary to maintain a high reproductive output; it is less easy to see how parasitism survived the early intermediate stages when reproduction must have been rather unpredictable. Nevertheless, the continuing existence of completely facultative parasites (see above) does demonstrate that the evolution of obligate parasitism could have involved periods of opportunistic host association long before any irrevocable commitment took place, allowing time for gradual adaptations to the rather different environmental conditions experienced. However the evolutionary process occurred, it would necessarily have involved adaptation for survival in the face of many potential sources of incompatibility. The end product of the process is a host-parasite relationship in which natural resistance does not operate, i.e. the parasite is unaffected by, or is able to circumvent, those aspects of host structure or physiology through which resistance is expressed against other organisms. Once such relationships (natural host-parasite relationships) are established, they remain stable until there is genetic change in either partner. The host genotype specifies the environment to which the parasite is adapted. Changes in this environment may therefore result in incompatibility again being apparent. The parasite genotype determines the adaptive features necessary for life within the host environment, and changes in these features may make survival much less certain. The converse of both situations is also possible, i.e. genotypic changes can reduce natural resistance by reducing host incompatibility or by increasing parasite adaptability. The successful adaptations of parasites to their natural hosts can be countered only by the operation of acquired resistance, i.e. by the expression of anti-