Chapter category: Bacterial Virulence
Pathogens and Hosts: Who Wins?
Intracellular Pathogens in Membrane Interactions and Vacuole Biogenesis
Edited by: Jean-Pierre GorvelISBN: 0-306-47833-1
» Get more information about this book at landesbioscience.com «
Chapter authors:
Jonathan C. Howard
Who wins? is certainly an evolutionary question, and to some extent also the semantic question, what do we mean by winning? The ultimate victory over microbes is Pyrrhic: we have seen the immunodeficient children in their sterile hoods. A sterile life is not a fulfilled one. So that idea of winning has to be put aside and a more biological context for the question analysed. Haldane identified a critical attribute of the host-pathogen relationship in his celebrated paper 'Disease and Evolution',7 namely the tendency of host-pathogen systems to generate polymorphism in their virulence and resistance genes. Indeed he went so far as to imply, if not to say outright, that persistent biochemical polymorphism in a host species could most plausibly all be attributed to disease resistance genes. This study preceded by only a short while Lewontin and Hubby's exploitation of starch gel electrophoresis to reveal extensive polymorphism in housekeeping enzymes of wild Drosophila15 and the development of the neutral theory of gene evolution (reviewed in ref. 14). The neutral proposition is that most mutations in a gene are nearly neutral so that their rate of penetration into finite populations is more a function of randomness than it is of their selective value. A clear consequence of neutrality is that polymorphism in multiple genes, indeed in essentially all genes, is the expected state, as nearly neutral substitutions wander at random through the population. Indeed because neutral change in allelic frequencies is slow, variations that are under intense positive directional selection enter the population at relatively great speed, carrying with them whatever neutral variants may be linked with the target of selection. Natural selection of this kind thereby purges a genetic region of neutral polymorphism. The importance of polymorphism under selection has been to a degree obscured by the neutrality revolution and it is noticeable that Kimura's book,14 which established polymorphism as an unavoidable attribute of neutrality, and paid generous tribute to Haldane's contributions to quantitative genetics, failed to refer to his Disease and Evolution paper which, to the contrary, generalised infectious disease as the most important cause of polymorphism under selection.
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