Geographic spread, genetics and functional characteristics of ryanodine receptor based target-site resistance to diamide insecticides in diamondback moth, Plutella xylostella

Abstract

Anthranilic diamides and flubendiamide belong to a new chemical class of insecticides acting as conformation sensitive activators of the insect ryanodine receptor (RyR). These compounds control a diverse range of different herbivorous insects including diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), a notorious global pest on cruciferous crops, which recently developed resistance due to target-site mutations located in the trans-membrane domain of the Plutella RyR. In the present study we further investigated the genetics and functional implications of a RyR G4946E target-site mutation we recently identified in a Philippine diamondback moth strain (Sudlon). Strain Sudlon is homozygous for the G4946E mutation and has been maintained under laboratory conditions without selection pressure for almost four years, and still exhibit stable resistance ratios of >2000-fold to all commercial diamides. Its F1 progeny resulting from reciprocal crosses with a susceptible strain (BCS-S) revealed no maternal effects and a diamide susceptible phenotype, suggesting an autosomally almost recessive mode of inheritance. Subsequent back-crosses indicate a near monogenic nature of the diamide resistance in strain Sudlon. Radioligand binding studies with Plutella thoracic microsomal membrane preparations provided direct evidence for the dramatic functional implications of the RyR G4946E mutation on both diamide specific binding and its concentration dependent modulation of [3H]ryanodine binding. Computational modelling based on a cryo-EM structure of rabbit RyR1 suggests that Plutella G4946E is located in trans-membrane helix S4 close to S4–S5 linker domain supposed to be involved in the modulation of the voltage sensor, and another recently described mutation, I4790M in helix S2 approx. 13 Å opposite of G4946E. Genotyping by pyrosequencing revealed the presence of the RyR G4946E mutation in larvae collected in 2013/14 in regions of ten different countries where diamide insecticides largely failed to control diamondback moth populations. Thus, our study highlights the global importance of the G4946E RyR target-site mutation, which as a mechanism on its own, confers high-level resistance to diamide insecticides in diamondback moth.