Study On The Fecundity Difference Between Fenpropathrin-resistant And Susceptible Tetranychus Cinnabarinus(Boisduval) And The Underlying Mechanism

As a worldwide pest mite, the carmine spider mite(Tetranychus cinnabarinus) is categorized to Tetranychidae, Acari. T. cinnabarinus causes damage to a variety of agricultural crops, such as eggplant, tomato, pepper, potato, cucumber, beans etc. Besides, it’s also detrimental to cotton, flowers, etc. Thus, the control of T. cinnabarinus is of great importance to economy and ecology.The control of T. cinnabarinus mainly depends on chemical approaches, and owing to frequent use of acaricides, carmine spider mites can easily become resistant to various pesticides, which lead to pest resurgence. Resistance monitoring showed that T. cinnabarinus has developed severe resistance to pyrethroids. It has been reported that when pyrethroids degraded to sublethal doses, they tended to induce hormesis effect on the target organisms with positive stimulus on their fecundity and thus resulted in an enlargement of population. Through long-term resistance screening, our laboratory has obtained a fenpropathrin-resistant strain of carmine spider mites(Fe R). We found that Fe R of the same origin has a higher population growth rate than the susceptible strain(SS). Therefore, this study investigated the underlying mechanisms for the enhanced fecundity in Fe R which may provide preferences for the treatment of pest resurgence.Vitellogenin and vitellogenin receptor are proteins that play indispensable roles in the vitellogenesis process of oviparous organisms. They are critical to the oocytes maturation as well as the ovary maturation. In theory, the vg and vgr of T. cinnabarinus play significant roles in the reproductive process.Starting with the differences of fecundity between SS and Fe R, this study was carried out with the emphasis on the content of vg and vgr between the two strains. The content and results are as follows:1. The daily fecundity of Fe R reached a peak at the three-day adult stage with average eggs of 13.00 per day. The daily fecundity of SS reached a peak at the four-day adult stage with average eggs of 9.64 per day. The daily fecundity of Fe R was always above that of SS. The total fecundity of Fe R was 173.25, while that of SS was 100.92. The total fecundity of Fe R was significantly higher than that of SS.2. The results of the content difference of vg and vgr in the 3-day adult T. cinnabarinus showed that the two proteins exhibited higher conten in Fe R than in SS, respectively. The vg in Fe R and SS were 0.1617±0.0146μg/mg and 0.0904±0.0038μg/mg respectively; while the vgr in Fe R and SS were 0.0455±0.0035μg/mg and 0.0231±0.0085μg/mg respectively.3. Two vg genes and one vgr gene have been cloned and were named as Tcvg1, Tcvg2 and Tcvgr. The length of the three genes were 5268 bp, 5253 bp and 5559 bp and the Gen Bank accession numbers were KR090058, KR090059, KT180304, respectively. The sequences of two vg genes contained typical conserved domains of Vitellogenin_N, DUF1943, VWD. The sequence of vgr gene contained typical conserved domains of LDLa、EGF_CA、LY domain and NHL repeats.4. The expression levels of Tcvg1, Tcvg2 and Tcvgr in both SS and Fe R showed similar tendencies during the developmental stages examined in this study. The three genes expressed very little before the mites reached adult stage. Then their expression exhibited a great increase, peaking at 3A stage, after which the expression levels dropped at 10 A stage. The expression levels of the three genes between SS and Fe R were examined at 0A and 3A stages. The results showed that the expression levels of the three genes in Fe R were all significantly higher than those in SS.The results suggested that long-time pressure of fenpropathrin could lead to an overexpression of vg and vgr genes in T. cinnabarinus, and the protein content of both vg and vgr increased accordingly, which could lead to enhancement of fecundity. This mechanism might be part of the reasons that explain the pests resurgence in field.