Molecular characterisation of beta-tubulin genes present in benzimidazole-resistant populations of Haemonchus contortus.

A beta-tubulin isotype 1 gene, gru-1, from a benzimidazole (BZ)-resistant population of the nematode parasite, Haemonchus contortus, was cloned and sequenced. The predicted gene organisation showed 10 exons and 9 introns, one of which was H. contortus specific. Using probes and restriction sites selected from this sequence, restriction maps were constructed from and around beta-tubulin genes of 3 BZ-susceptible and 7 BZ-resistant populations. There was a reduction in beta-tubulin isotype 1 genes to usually one, in BZ-resistant populations. So, our previously reported reduction of beta-tubulin probe-reactive RFLP fragments in resistant populations correlated with the reduction of beta-tubulin isotype 1 genes. The beta-tubulin isotype 1 gene present on the apparently selected fragment and was not always the same, and the geographical origin of the resistant populations indicated independent development rather than geographical spread of the resistant populations. The beta-tubulin genes on the apparently selected fragments were transcribed and processed to mRNA using the nematode-specific trans-spliced leader (SL1). Comparison of the derived amino acid sequence of gru-1, with known sequences from a susceptible population, identified 3 mutations that could be involved in BZ resistance.