Molecular variation in trichostrongylid nematodes from sheep and cattle

The application of molecular techniques such as restriction length polymorphisms, mitochondrial DNA analysis and sequence tagged microsatellite sites to determine the amount of genetic variation and population structure of trichostrongylid parasites of domestic ruminants is examined. Information already provided by these techniques indicates that the parasite populations have a large amount of genetic variation and that gene flow between populations is greater than previously expected. The implications are that parasites can rapidly respond to selection by either anthelmintics or genetically resistant hosts. Advance warning of the response to selection can be obtained, even when the trait under selection is unknown, by measuring changes in the DNA polymorphisms of the parasite populations. Resistance to the benzimidazoles in the trichostrongylids now appears to be dependent on changes at two beta-tubulin loci. The first stage in the development of benzimidazole (BZ) resistance involves selection for an allele encoding a resistant form of isotype 1 beta-tubulin. A higher level of resistance is achieved with the selection of the gene encoding isotype 2 beta-tubulin. Techniques that maintain BZs in the host for extended periods of time may select for the higher level of resistance. A DNA assay to test this hypothesis is proposed.