Many genes have pleiotropic effects in vivo but have one dominant mutant phenotype that masks other informative phenotypes; thus, we decided to see if we could elicit such masked phenotypes by titrating the concentration of IPTG and, by extension, the degree of RNAi. We titrated the IPTG concentration from 1 pM to 1 mM in three log increments; for comparison, we also tested no IPTG (uninduced) and 10 mM IPTG. We analyzed four genes that could possibly be titrated to other distinct phenotypes: unc-37, which has a known allelic series with strong alleles resulting in embryonic lethality and weak alleles resulting in an uncoordinated (Unc) phenotype [10]; hlh-2, which is embryonic lethal by RNAi injection and is expressed in some neural precursors which eventually form the ventral nerve cord ([11] and M. Krause, personal communication); mei-1, which is required for meiotic spindle formation and is embryonic lethal by RNAi injection, but for which a weak mutant allele produces a high-incidence-of-males (Him) phenotype [12,13]; and rba-2, a very strong embryonic lethal gene by RNAi injection which is also involved in repression of vulval cell fates [14].
