All four genes were 100% embryonic lethal at 1 mM IPTG (Table 2). For the first three genes, decreasing the IPTG concentration to 1 μM reduced the embryonic lethality sufficiently to expose high levels of a secondary phenotype. For unc-37 and hlh-2, 100% of the surviving worms were Unc. For mei-1, a Him phenotype was seen: 8% of the surviving worms were male, which is significantly greater than the 0.5% which normally arise by non-disjunction in a wild type hermaphrodite culture. As we further decreased the IPTG concentration to 1 pM, these levels of embryonic lethality tapered off, as did the penetrance of secondary phenotypes. With no IPTG, all the worms fed these genes had wild-type progeny. For rba-2, however, embryonic lethality was observed at all concentrations of IPTG, including 56% lethality without any IPTG. Therefore, these bacteria are likely to express a low level of T7 polymerase in the absence of induction, which is sufficient to produce an RNAi phenotype for some genes. From these data, we conclude that it is possible to titrate RNAi phenotypes by feeding bacteria
