Study On The Mechanism Of Nuclear Receptor Factor FTZ-F1 In Response To Parasitism And Pesticide Stress In Spodoptera Litura

Fushi tarazu-Factor1（FTZ-F1）,as a key nuclear receptor factor in ecdysone signaling pathway,not only participates in the transmission of ecdysone signal in insects,FTZ-F1 also regulates the synthesis of 20-hydroxyecdysone（20E）and Juvenile hormone（JH）by feedback regulation.Therefore,FTZ-F1 is an important regulator of insect development.So far,studies on insect FTZ-F1 have focused on the regulation and mechanism of insect growth and development,while there are few reports on the mechanism of insect response to parasitic wasps and pesticide stress.To investigate the functions of FTZ-F1 on S.litura after parasitism and under pesticide stress,Spodoptera litura and its dominant parasitic wasp Meteorus pulchricornis were used as research materials and our study could provide an important basis for expanding the understanding of FTZ-F1 functions.The main research results are as follows:（1）In this study,a nuclear receptor factor FTZ-F1（Slit FTZ-F1）was identified from the transcriptomes of S.litura constructed by our research group.First of all,real-time quantitative PCR（qRT-PCR）analysis of Slit FTZ-F1 expression from the beginning of the3^rdinstar S.litura larvae to the end of the 4^thinstar larvae showed that the expression level of Slit FTZ-F1 was significantly increased before and after the molting.Meanwhile,the tissue expression spectrum showed the expression of Slit FTZ-F1 in hemolymph was significantly higher than that in other tissues of S.litura.What’s more,RNA interference（RNAi）tests,illustrated that Slit FTZ-F1 silencing of larvae at the beginning and end of the3^rdinstar larve resulted in failure of molting,malformation of larvae,reduction of pupation rate,and increased risk of death.Silencing Slit FTZ-F1 also contributed to a significant decrease in the concentrations of 20E and JH in the hemolymph of S.litura larvae.The expression of key nuclear receptor genes（Slit Ec R,Slit USP,Sli E74,Slit E75,Slit E78,Slit E93,Slit Br C,Slit Hr3,Slit Hr4）in 20E signaling pathway was inhibited.Therefore,Slit FTZ-F1plays a key role in the 20E signaling pathway and development of S.litura.（2）Hemolymph hormone concentration of S.litura was measured continuously from1^stday to 8^thday after parasitism by M.pulchricornis.The results showed that the 20E and JH concentration in the hemolymph of the S.litura gradually increased from first day and reached the highest values on 5^thday（20E:7.0 ng/L,JH:129.2 ng/L）.The expression level of Slit FTZ-F1 in larvae was significantly higher than that in control group and reached the peak at the 3^rdday after parasitism,and the expression level in hemolymph was significantly higher than that in other tissues.Silencing the Slit FTZ-F1 of host larvae resulted in the failure of its normal molting,increased malformation rate and postmolting mortality rate,and significant decline of juvenile hormone and molting hormone.The expression of key nuclear receptor genes（Slit Ec R,Slit USP,Slit E74,Slit E75,Slit E78,Slit E93,Slit Br C,Slit Hr3,Slit Hr4）in 20E signaling pathway was inhibited.At the same time,it also resulted in a decreased egress rate and increased risk of cumulative death.Microinjection of 20E into the host S.litura larvae decreased the egress rate of M.pulchricornis.Therefore,Slit FTZ-F1still plays an important role in the development of host S.litura,and Slit FTZ-F1 and its regulated host 20E synthesis significantly affected the development of the progeny of M.pulchricornis.（3）In order to investigate whether Slit FTZ-F1 is involved in the tolerance process of S.litura larvae to insecticides,the expression of Slit FTZ-F1 and Glutathione-S-transferases（GSTs）in S.litura under insecticide stress was determined.Both chlorfenapyr and phoxim could induce the expression of Slit FTZ-F1 and Slit GSTs in different time.Silencing Slit FTZ-F1 can lead to up-regulated expression of Slit GSTs（Slit GST-D1,Slit GST-E1,Slit GST-E2,Slit GST-E3,Slit GST-E4,Slit GST-E5,Slit GST-E6,Slit GST-E7）.In addition,the expression of Gpyl FTZ-F1 and GpGSTs was significantly up-regulated in Glyphodes Pyloalis,an important pest of mulberry,after the treatment with phoxim and phoxim.These results suggested that FTZ-F1 might play a role in pesticide detoxification in mulberry pests besides development regulation.