Phenotypic and genotypic analysis of benzimidazole resistance in reciprocal genetic crosses of Haemonchus contortus.

Haemonchus contortus is arguably one of the most economically important and ubiquitous parasites of livestock globally and commonly involved in cases of anthelmintic resistance. Here, we performed reciprocal genetic crosses using susceptible (MHco3(ISE)) and multiple anthelmintic resistant (MHco18(UGA2004)) H. contortus isolates. Resultant admixed populations were designated MHco3/18 or MHco18/3, where the lead isolate reflects the origin of the females. Three independent filial generations were generated for each cross, which were subjected to bioassays, molecular approaches and population genetic analyses to investigate the phenotypic and genotypic inheritance of benzimidazole (BZ) resistance at each stage. A panel of microsatellite markers confirmed the success of the genetic cross as markers from both parents were seen in the F crosses. Egg hatch tests revealed a stark difference between the two F crosses with ED estimates for MHco18/3 being 9 times greater than those for MHco3/18. Resistance factors based on ED estimates ranged from 6 to 57 fold in the filial progeny compared to MHco3(ISE) parents. Molecular analysis of the F167Y and F200Y SNP markers associated with BZ resistance were analysed by pyrosequencing and MiSeq deep amplicon sequencing, which showed that MHco3/18.F and MHco18/3.F both had similar frequencies of the F200Y resistant allele (45.3% and 44.3%, respectively), whereas for F167Y, MHco18/3.F had a two-fold greater frequency of the resistant-allele compared to MHco3/18.F (18.2% and 8.8%, respectively). Comparison between pyrosequencing and MiSeq amplicon sequencing revealed that the allele frequencies derived from both methods were concordant at codon 200 (r = 0.97), but were less comparable for codon 167 (r = 0.55). The use of controlled reciprocal genetic crosses have revealed a potential difference in BZ resistance phenotype dependent on whether the resistant allele is paternally or maternally inherited. These findings provide new insight and prompt further investigation into the inheritance of BZ resistance in H. contortus.