Molecular cloning and characterization of a SID-1-like gene in Plutella xylostella

RNA interference (RNAi) signal can spread from the point where the double-stranded RNA (dsRNA) was initially applied to other cells or tissues. SID-related genes in Caenorhabditis elegans help in the spreading of this signal. However, the mechanisms of systemic RNAi are still not unveiled in insects. In this study, we cloned a full-length cDNA of sid-1-like gene, Pxylsid-1, from Plutella xylostella that contains 1,047 bp opening reading frame encoding a putative protein of 348 amino acids. This transcript is very much similar to the sil-1 in Bombyx mori (68.8%). The higher expression levels of Pxylsid-1 were found at the adult and fourth-instar stages compared to the second-instar stage with 21.48- and 10.36-fold increase, respectively. Its expression levels in different tissues were confirmed with the highest expression in the hemolymph, which showed 21.09-fold increase than the midgut; however it was lower in other tissues. The result of RNAi by feeding bacterially expressed dsRNA targeting Pxylace-1, which showed that the mRNA level of Pxylace-1 decreased by 34.52 and 64.04% after 36- and 72-h treatment, respectively. However, the mRNA level of Pxylsid-1 was not significantly induced when the Pxylace-1 was downregulated. Furthermore, we found that downregulation of Pxylsid-1 did not affect the RNAi effect of Pxylace-1. Hence, the Pxylsid-1 may not be involved in absorption of dsRNA from the midgut fluid. A further study is needed to uncover the function of Pxylsid-1.