Could the absence ofRNA transportmechanisms explain
why Drosophila cannotmanifest a systemicRNAi response?
Homologues of the C. elegans sid-1 gene have been identified
in insects such as Tribolium castaneum, Bombyx mori and
Apis mellifera but not in the Drosophila genome. sid-2
homologues have only been detected in nematodes closely
related to C. elegans. A sid-1 homologue has also recently
been identified in aphids [17]. However, recent evidence
suggests that dsRNA uptake into cultured Drosophila S2
cells does not involve a sid-1-based mechanism but takes
place by receptor-mediated endocytosis [18,19] because
pharmacological inhibition of endocytosis also inhibited
RNAi effects. Endocytosis of dsRNA also seems to occur in C. elegans because knockdown of components of the
endocytotic pathway by RNAi results in worms with a
‘loss-of-RNAi-function’ phenotype [18]. These results
suggest that receptor-mediated endocytosis is a widespread
mechanism for dsRNA uptake and might well occur across
different insect orders. If this is the case, herbivorous insect
pests fromdifferent orders canbe effectively targeted by oral
delivery of dsRNA. Further understanding of the complexities
of insect dsRNA-uptake mechanisms might facilitate
the targeting of specific insect pests.