GENETIC-VARIATION IN PARASITIC NEMATODES AND ITS IMPLICATIONS

An absolute pre-requisite for a genetic response to a selective pressu re is genetic variation within the population under selection. Helmint h populations are clearly able to respond to selective pressures and m ust, therefore, be genetically heterogeneous. While not quite tautolog ical, this is at best indirect evidence for the existence of genetic v ariation but there are few examples of well documented helminth phenot ypic variation with a proven genetic basis. Isozyme analysis has provi ded more direct evidence for variation but attempts to link this Varia tion to responses to selection or to identify the forces maintaining t hat variation have been largely unsuccessful. Thus there is a clear ne ed for new techniques. The recent application of PCR and direct sequen cing technology to the study of helminth genetics has allowed the geno types of individual worms to be determined and the first direct measur ements of allele frequencies to be made in this group of organisms. In addition, the application of genetic and molecular data from Caenorha bditis elegans is a potentially rich source of new markers. These tech niques do not require that the genetic basis of the phenotype in quest ion be known since a large number of loci can be examined and selectio n detected through changes in the frequency of anonymous linked marker loci. Phenotypes with complex genetic bases can, therefore, be analys ed. I have applied these techniques to the study of anthelmintic resis tance genetics and others have applied them to the genetics of inhibit ed development in Ostertagia. Other phenotypes that are of great inter est are the potential for selection of resistance to vaccination and t he use of genetically resistant hosts. The ease with which helminths h ave countered all classes of anthelmintics and the apparently high lev els of polymorphism in helminth populations suggest that immunological control methods may also prove to be vulnerable to the adaptive capab ilities of the parasite. Evidence from a mouse-helminth model system h as already provided evidence that worms can meet the challenge.