LAUSR

BACKGROUND Pyflubumide and cyenopyrafen are respiratory complex II (complex II) inhibitors. Previous quantitative trait locus analyses suggested associations of I260V and S56L in complex II subunit B (B-I260V) and subunit C (C-S56L) with pyflubumide and cyenopyrafen resistance, respectively, in Tetranychus urticae. However, although resistant strains had been selected separately by these acaricides, all strains were homozygous for both B-I260V and C-S56L. Hence, the effects of each mutation on resistance development remain unclear. RESULTS We established strains homozygous for B-I260V with C-S56 (B-I260V_I260V/C-S56_S56) and for C-S56L with B-I260 (B-I260_I260/C-S56L_S56L). High resistance levels (LC50 > 1000 mg L-1 ) to pyflubumide and cyenopyrafen was not conferred by B-I260V or C-S56L alone. Next, we prepared intermixed strains by crossing B-I260V_I260V/C-S56_S56 and B-I260_I260/C-S56L_S56L. Selection of the intermixed strains by either acaricide caused very high resistance levels (LC50 ≥ 10,000 mg L-1 ) to both acaricides and fixed both mutations. Allele-selected recoupling of the mutations without acaricide selection also conferred very high resistance levels to both acaricides in the intermixed strains. Unlike these, B-I260V or C-S56L alone conferred very high and high resistance levels to cyflumetofen, respectively. CONCLUSION We conclude that the effect of individual mutations characteristically varies among complex II inhibitors. Moreover, very high resistance levels to pyflubumide and cyenopyrafen is conferred by the co-occurrence of B-I260V and C-S56L mutations, which alone have limited effects on resistance level.