Fitness Trade-off Associated With Spinosad Resistance And The Preliminary Regulation Mechanism In Frankliniella Occidentalis (Thysanoptera:Thripidae)

Frankliniella occidentalis?Pergande?is an important pest on agricultural crops.It feeds on leaves,stems,flower parts,and young fruits,and can also transmit many tospoviruses including tomato spotted wilt virus?TSWV?.Although diverse pest management methods exist,insecticides are the most widely used for the control of F.occidentalis.However,the overuse of insecticides and the pest's short generation time,high fecundity,haplodiploid reproduction and polyphagy have lead to the resistance appearance of F.occidentalis to many insecticides through multiple metabolic detoxification pathways.Spinosad has highly toxicity active for western flower thrips?WFT?,but for the widely use of spinosad in agricultural production,resistance of WFT had rapidly occurred in Spain,Australia,the United States and Beijing,Yunnan in China.In general,fitness cost exists when the insects become resistant to insecticide,which means resistant insects show higher fitness than sensitive insects in the presence of insecticide pressure.But when the pressure removed,the resistant species usually shows disadvantage.Therefore,the resistance of this pest species may be reduced by using other pesticides which lack of cross-resistance with spinosad or by using insecticides with different resistance mechanisms.To confirm whether there is fitness cost for the resistance of WFT to the spinosad and to expound its possible mechanism,we compared the biological parameters,such as growth,development and reproduction of the two species?i.e.the near isogenic lines of the Ivf03 and NIL-R strains of WFT?.We study the difference of feeding behavior of Ivf03 and NIL-R by using EPG technique.We also cloned the Vg gene and compared the difference in the sequence and expression of Vg mRNA of Ivf03 and NIL-R.The results can provide the theory basis for the resistance control of WFT.1.Fitness cost associated with insecticide resistance in Frankliniella occidentalisWe generated life table,measured the body size and feeding behaviors of spinosad-susceptible?Ivf03?and-resistant?NIL-R?near-isogenic lines of F.occidentalis.The results showed that it took longer time for NIL-R in pre-adults except for the first instar nymph,and the period of pre-adults had been delayed for 0.2 day.Compare to Ivf03 individuals,the longevity of females NIL-R individuals were shortened by8.3 days,while the males were shortened by 8.3 days.The oviposition period for females NIL-R individuals were only 13.8 days,which were significant shorter than females Ivf03 individuals?25 d?.Oviposition per female in NIL-R and Ivf03 strain was 94.5 and 198.0,respectively.The intrinsic rate of increase in NIL-R and Ivf03 strain was 0.19 and 0.23,separately.The relative fitness of the NIL-R strain was only 0.43.The body size of NIL-R strain was significant smaller than that of Ivf03 strain.Compared with the feeding behaviors of the two species,Ivf03 strain has a significant longher time and higher frequency in the non-ingestion,short-ingestion and long-ingestion probes than those of NIL-R strain.The above results show that WFT will have significant fitness cost in biology and feeding behaviors when it becomes resistant to spinosad,this especially reflect in the reproductive capacity.2.Vg cDNA was cloned and sequenced in F.oocidentalisThe possible sequence of Vg gene was obtained through the analysis of transcriptome,and the sequence of Vg gene in F.oocidentalis was cloned by RACE-PCR technology.The full length of it was6159 bp within the open reading frame of 6024 bp,which encoded 2007 amino acids.The 1^st to16^th amino acid sequence were predicted as signal peptide.The predicted molecular weight?Mw?is 218.8 kDa with an isoelectric point?pI?of 8.96.There are 17 N-glycosylation sites and 180 O-glycosylation sites.It also has typical conservative domains like LPD_N?21-903 BP?,DUF1943?936-1236 BP?and VWD?1659-1843 BP?.A phylogenetic tree was generated by searching the predicted amino acid sequence and finding the same gene in other insects which had a high similarity with WTF in NCBI database.The results showed that the similarity of Vg gene in Frankliniella occidentalis and Nesidiocoris tenuis was 49%.3.The expression of Vg gene was down regulated significantly in resistant F.oocidentalisUsing Tubulin gene as the reference gene,the levels of Vg mRNA expression at different stages between Ivf03 and NIL-R were compared by RT-PCR.The results showed that the expression pattern of Vg gene in two species at different developmental stages was consistent.The expression level of pre-adults were extremely low,but it were high in the female adults.The expression level was first increased and then decreased,and there were two peaks in the middle of this period.The expression reached the highest point at the time of 11 d.The expression of Vg mRNA in NIL-R was lower than that of Ivf03,especially in the 1^st,3^rd,5^th,and 11^th day,it was separately down-regulated about 70%,38%,34%and37%.It is presumed that the decrease of the reproduction of NIL-R may be caused by the down-regulation of synthesis of Vg.