Inheritance of avermectin resistance in Haemonchus contortus
2000
Le Jambre, L.F. | Gill, J.H. | Lenane, I.J. | Baker, P.
A larval development assay was used to compare the responses of the Chiswick Avermectin Resistant (CAVRS) isolate of Haemonchus contortus, an avermectin-susceptible isolate (VRSG) and their crosses to avermectins. The F1 and F2 generations of reciprocal crosses between CAVRS and VRSG were denoted as CAVRS malesxVRSG females = CXV, and VRSG malesxCAVRS females = VXC. The levels of avermectin resistance in the developing larvae of the F1 of both CXV and VXC were indistinguishable from that in the avermectin-resistant parent, indicating that the resistance trait is completely dominant. Avermectin dose-response curves for the CXV F1 did not show a 50% mortality rate at low concentrations, indicating that avermectin resistance is not sex-linked. This conclusion was confirmed when adult male worms of the F1 of the CXV mating were found to have survived treatment of the host with 200 micrograms kg-1 ivermectin. This dose rate (200 micrograms kg-1 ivermectin) caused a 50% reduction in the number of adult males in the F1 from both CXV and VXC crosses, but only a non-significant reduction in the number of adult females in the F1. Dose-response curves obtained for the F2 generations in the larval development assay indicated the presence of 25% of avermectin-susceptible individuals, suggesting that a single major gene largely controls the avermectin-resistance trait. This genetic analysis of avermectin resistance in an Australian H. contortus isolate indicates that the expression of the gene for avermectin resistance is an autosomal, complete dominant in the larvae, however, in adults its expression is sex-influenced, with males having a lower resistance to avermectin than females.
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