To evaluate the potential for systemic RNAi effects in
insects, an experimental approach using species other than
Drosophila has been pursued. Insect systemic RNAi was
first documented in the coleopteran Tribolium castaneum
(flour beetle) by two independent studies. In the first, a
homologue of the Drosophila sensory bristle-forming gene
Tc-achaete-scute (Tc-ASH) was identified and targeted.
Injection of Tc-ASH dsRNA into larvae at a single discrete
site resulted in a ‘loss-of-bristle’ phenotype over the entire
epidermis of adult insects [20]. In the second study, a
parental RNAi effect transmissible between generations
was demonstrated by identifying and targeting developmental
genes. Injection of dsRNA specific to (i) Distalless
(leg development gene), (ii) maxillopedia (homeotic gene)
and (iii) proboscipedia (encoding a homeotic protein
required for the formation of labial and maxillary palps)
was used to produce an RNAi effect in both mother insects
(injected) and developing progeny embryos after egg hatch
[21]. Thanks to its well-documented, robust systemic RNAi
response and the recent completion of its genome sequence,
Tribolium is becoming an accepted model for the study of
systemic RNAi in insects. Intriguingly, a recent genome
comparison of C. elegans and Tribolium revealed a lack of
conservation of a systemic RNAi mechanism [22]. For
example, Tribolium lacks a C. elegans-like RdRP, so the
signalamplification observed in Triboliummust be based on
a different gene with a similar activity, or possibly even a
different mechanism. RdRP-like activity has been demonstrated
in cell-free extracts from Drosophila embryos, even
though the RdRP gene is not present in insects [23].