Transcriptome analysis of Spodoptera litura reveals the molecular mechanism to pyrethroids resistance

Spodoptera litura is a destructive agricultural pest and has evolved resistance to multiple insecticides, especially pyrethroids. At present, the resistance mechanism to pyrethroids remains unclear. Four field-collected populations, namely CZ, LF, NJ and JD, were identified to have high resistance to pyrethroids comparing to pyrethroid-susceptible population (GX), with resistant ratio ranging from 11.5- to 9123.5-fold. To characterize pyrethroid resistance mechanism, the transcriptomes between two pyrethroid-resistant (LF and NJ) and a pyrethroid-susceptible (GX) populations were compared by RNA-sequencing. Results showed that multiple differentially expressed genes were enriched in metabolism-related GO terms and KEGG pathways. 35 up-regulated metabolism-related unigenes were selected to verify by qRT-PCR and 15 unigenes, including 4 cytochrome P450s (P450s), 5 glutathione S-transferase (GSTs), 1 UDP-glycosyltransferase (UGT), 4 carboxylesterases (COEs) and 1 and ATP-binding cassette transporters (ABC), were all up-regulated in the four pyrethroid-resistant populations. The expression levels of CYP3 and GST3, which were annotated as CYP6A13 and GSTE1, respectively, showed positive correlation with their pyrethroid resistance levels among the four pyrethroid-resistant populations. While the expression levels of CYP5, CYP12, COE4 and ABC5 showed good correlation with their pyrethroid resistance levels in at least three populations. UGT5 had the highest expression level among the tested UGT genes in the four pyrethroid-resistant populations. RNAi mediated silencing of CYP6 increased the cumulative mortality treated by beta cypermethrin and cyhalothrin significantly, while silencing of GST3 increased the cumulative mortality treated by fenvalerate significantly. CYP3, CYP5, CYP6, CYP12, GST3, COE4, UGT5 and ABC5 play important roles in pyrethroid resistance among the four pyrethroid-resistant populations. Our work provides a valuable clue for further study of pyrethroid resistance mechanisms in S. litura.