To determine which tissues are susceptible to RNAi by feeding, we fed bacteria expressing green fluorescent protein (GFP) dsRNA to hermaphrodite worms with a transgenic GFP reporter gene expressed in all somatic tissues (egl-27::gfp; Figure 2a) [15]. After being fed for 24 hours at 15°C, GFP expression was markedly reduced compared to similarly treated unfed worms (n = 28; compare Figure 2b and a). After 48 hours, with the exception of the nervous system, GFP was not detectable in somatic tissues; furthermore, neural GFP was dramatically reduced in 91% of worms (n = 22; Figure 2c). A similar level of inhibition of GFP expression was observed after 72 and 96 hours of feeding (Figure 2d,e). Although GFP fluorescence in fed worms was abolished or severely reduced, GFP was sometimes expressed at high levels in late-stage embryos derived from these worms (data not shown). This suggests that some zygotic embryonic transcripts may be difficult to silence, possibly because a continuous supply of dsRNA cannot be provided through the eggshell. In summary, RNAi by feeding efficiently silences genes in most C. elegans somatic tissues; however, the nervous system has a delayed and somewhat less robust response to RNAi compared to other tissues.
