200 R. Asokan et al.
Therefore, there is an urgent need to look
for an effective, ecofriendly alternative for pest
management, where a new and novel approach
called RNA interference (RNAi), popularly called
Gene Silencing is poised to play a vital role. Em-
ploying this RNAi approach, it is now possible to
control insect pests by silencing some of the vital
genes that play an important role in insect-host
plant interaction, growth and development, flight,
reproduction, etc. by delivering cognate double-
stranded RNA (dsRNA) either as spray or through
transgenic plant. But RNAi approach has to be
tailor made and cannot be universal, as the mecha-
nism of RNAi is highly species specific. Hence the
basic mechanisms underlying RNAi for selected
species of insect pests that belong to three impor-
tant orders of insects has to be studied thoroughly
before venturing into field level application.
General Mechanism of RNAi
RNAi is an innate immune response that re-
tard the gene expression by degrading specific
mRNA molecules. RNAi mechanism is gener-
ally employed for maintaining genome integrity
against the invasion by transposons and viruses.
The RNAi pathway is initiated by the cellular
enzyme called Dicer (RNase III), which cleaves
the long double-stranded RNA (dsRNA) into
short fragments of 20–25 base pairs. One of the
two strands of each fragment known as guide
strand is incorporated into the RNA induced
silencing complex (RISC) and base pair with
complementary mRNA sequences and cleaves
the cognate mRNA thus arresting the target
gene expression (Fig. 1 ). This mechanism is
also known as posttranscriptional gene silencing
(PTGS) in plants. In some organisms like Cae-
norhabditis elegans and ticks the silencing sig-
nal generated in a cell is communicated to other
cells which are exposed to the dsRNA which is
known as systemic silencing. Two components
which are needed to elicit systemic silencing are
RdRP (RNA dependent RNA polymerase) and
sid 1 and 2 genes (systemic interference defec-
tive). Presence of at least one of them has been
documented in many insects except Drosophila
melanogaster (Fig. 2 ).Fig. 1 General mechanism of RNAi