Amplified fragment length polymorphism analysis of genetic diversity of Haemonchus contortus during selection for drug resistance

For the first time we used amplified fragment length polymorphism oil indiv idual nematode parasites to analyse the genetic diversity between and withi n isolates during consecutive stages of increased benzimidazole resistance and of increased levamisole resistance of Haemonchus contortus. The genetic diversity of the H. contortus genome turned out to be unusually high, with in and between the isolates. The difference between individuals of an isola te could be as high as between individuals of two different mammalian speci es that do not interbreed. During benzimidazole selection the genetic const itution of the population was changed, but did not lead to a decrease in th e genetic diversity. The selection for levamisole resistance resulted in a limited reduction of the genetic diversity only after the first selection s tep. The extensive genetic diversity apparently has allowed a fast and flex ible response of H. contortus to drug selection as shown by the appearance of drug resistant isolates. This selection however has little or no effect oil the extent of the genetic diversity of these resistant isolates. Implic ations for more sustainable control methods are discussed. (C) 2001 Austral ian Society for Parasitology Inc. Published by Elsevier Science Ltd. All ri ghts reserved.