Altered avr-14B gene transcription patterns in ivermectin-resistant isolates of the cattle parasites, Cooperia oncophora and Ostertagia ostertagi

Abstract

Ivermectin (IVM) resistance is an emerging problem for the control of gastrointestinal nematodes of cattle such as Cooperia oncophora and Ostertagia ostertagi. Although there is still a poor understanding of the molecular basis of macrocyclic lactone (ML)-resistance, it is clear that IVM exerts its activity by binding to glutamate-gated chloride (GluCl) channels within the parasite's neuromuscular system. One of the GluCl genes (avr-14) encodes, via alternative splicing, two subunits, AVR-14A and AVR-14B; the latter is suggested to be the main target for IVM. The genomic DNA (gDNA) sequence of avr-14 in C. oncophora contains 21 exons separated by 20 introns and spans approximately 10 kb of gDNA. Intron 13 contains a sequence with high homology to a mammalian mariner transposase. The L256F polymorphism in the avr-14 gene, which was shown to be associated with IVM resistance in a UK isolate of C. oncophora, was not found in the IVM-resistant C. oncophora and O. ostertagi isolates investigated in this study. However, genetic analyses on C. oncophora indicated a loss in allelic diversity of the avr-14 gene in the resistant isolates compared with the susceptible isolate. This suggests that the avr-14 gene, or another genetically linked locus, is under selection in these Belgian C. oncophora isolates. Comparison of the full-length avr-14B coding sequence in the susceptible and resistant C. oncophora isolates did not show any polymorphisms specifically linked to IVM resistance, although a decrease in the number of avr-14B isoforms was observed in the resistant isolates compared with the susceptible one. Measuring the transcription levels of avr-14B in adult male and female C. oncophora and O. ostertagi worms showed significantly lower levels in resistant worms compared with susceptible ones. Whether the down-regulation of this IVM target actually contributes to the resistance mechanism in these worms remains unclear.