Effect of selection for benzimidazole resistance in Haemonchus contortus on beta-tubulin isotype 1 and isotype 2 genes.

In vitro selection for benzimidazole (BZ) resistance in the nematode Haemonchus contortus, a parasite of sheep, suggested the involvement of beta-tubilin genes. Two distinct beta-tubulin isotype classes were analyzed. Selection with stepwise increased concentrations of BZ showed for the first time that the increasing degrees of resistance correlate to a dual phase adaptation mechanism. At lower degrees of developing resistance, selection for a single beta-tubulin isotype 1 variant takes place. Subsequently, at higher degrees of drug resistance, an abrupt elimination of individuals carrying beta-tubulin isotype 2 genes in their genome is seen. To evaluate the in vitro experiments we carried out a survey of resistant field populations and found that in the most resistant population (RNO) apparently the same mechanism had happened.