Beta-tubulin complementary DNA sequence variations observed between cyathostomins from benzimidazole-susceptible and -resistant populations.

The molecular mechanism of benzimidazole (BZ) resistance in cyathostomins of horses is still unclear. Previous studies revealed that the TTC or TAC polymorphism in codon 200 of the beta-tubulin isotype 1 gene is not as strictly correlated with BZ resistance as in trichostrongyles in sheep. To identify further sites of polymorphism within the beta-tubulin gene related to BZ resistance, complete complementary DNAs (cDNAs) encoding beta-tubulin of adult worms of Cylicocyclus nassatus, Cyathostomum pateratum, Cyathostomum coronatum, Cyathostomum catinatum, Cylicostephanus longibursatus, and Cylicostephanus goldi of a BZ-resistant cyathostomin population were characterized using specific primers. The cDNA sequence of each species spans 1,429 bp, encoding a protein of 448 amino acids. The interspecific identities are 95.2-99.6% at the nucleotide and 98.7-100.0% at the peptide level. The comparison of the amino acid sequences of individuals isolated from the BZ-resistant cyathostomin population with those from individuals of Cc. nassatus, Cy. coronatum, Cy. pateratum, and Cy. catinatum of a BZ-susceptible one showed differing amino acids in 11 positions. The commonness of a phenylalanine to tyrosine mutation at position 167 in all the 6 cyathostomin species isolated from a BZ-resistant population suggests its involvement in the molecular mechanism in BZ resistance.