Rhopalosiphum padi is an important global wheat pest. The pyrethroid insecticide bifenthrin is widely used in the control R. padi. We explored the resistance potential, cross-resistance, adaptive costs, and resistance mechanism of R. padi to bifenthrin using a bifenthrin-resistant strain (Rp-BIF) established in laboratory. The Rp-BIF strain developed extremely high resistance against bifenthrin (1033.036-fold). Cross-resistance analyses showed that the Rp-BIF strain had an extremely high level of cross-resistance to deltamethrin (974.483-fold), moderate levels of cross-resistance to chlorfenapyr (34.051-fold), isoprocarb (27.415-fold), imidacloprid (14.819-fold), and thiamethoxam (11.228-fold), whereas negative cross-resistance was observed to chlorpyrifos (0.379-fold). The enzymatic activity results suggested that P450 played an important role in bifenthrin resistance. A super-kdr mutation (M918L) of voltage-gated sodium channel (VGSC) was found in the bifenthrin-resistant individuals. When compared with the susceptible strain (Rp-SS), the Rp-BIF strain was significantly inferior in multiple life table parameters, exhibiting a relative fitness of 0.69. Our toxicological and biochemical studies indicated that multiple mechanisms of resistance might be involved in the resistance trait. Our results provide insight into the bifenthrin resistance of R. padi and can contribute to improve management of bifenthrin-resistant R. padi in the field.
Hippocampal place cells of freely moving animals display ‘theta phase precession’, whereby spikes are fired at successively earlier phases of the 6-10 Hz local field potential (LFP) theta rhythm. In some cases, this phase precession is interleaved with periods of ‘phase procession’, in which spikes are fired at successively later phases of the theta rhythm. Here we propose a continuous attractor neural network (CANN) with firing rate adaptation to understand both kinds of phase shift in place cell firing. We show that firing rate adaptation induces intrinsic mobility of the location represented by the ‘bump’ of population activity, which competes with the extrinsic mobility arising from external sensory inputs. The interplay between these two factors causes the activity bump to oscillate around the external input, resembling the forward and backward sweeps of decoded position during locomotion. Analysis of single cell dynamics reveals that these forward and backward sweeps naturally account for theta phase precession and procession of individual neurons, respectively. Furthermore, by tuning the adaptation strength, we explain the difference between bimodal and unimodal cells, with the former having interleaved phase precession and procession, and the latter having only the predominant phase precession. Our model reproduces other experimental findings, including the constant cycling of theta sweeps along different arms in a T-maze environment, the speed modulation of place cells’ firing frequency, and the continued phase shift after transient silencing of the hippocampus. Our results show how the intrinsic dynamics of a population of place cells affords both types of theta phase shift. We hope that they will aid an understanding of the neural mechanisms supporting theta phase coding in the brain.
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