A basic helix-loop-helix/Per-Arnt-Sim transcription factor Taiman is essential for metamorphosis in Henosepilachna...

Insect Taiman (Tai) is a basic helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) transcription factor that plays a role in juvenile hormone (JH) signalling during the larval stage in various insect species, as well as in 20-hydroxyecdysone (20E) signalling during the oogenesis of Drosophila melanogaster. It forms a heterodimer with another bHLH-PAS transcription factor Methoprene-tolerant (Met) mediating JH signalling and acts as a steroid receptor coactivator of the functional 20E receptor heterodimer, ecdysone receptor (EcR) and ultraspiracle (USP). However, whether Tai plays the dual functions in other insects deserves further exploration. In this study, we determined the role of Tai in Henosepilachna vigintioctomaculata, a Coleopteran potato pest mainly distributed in northern China. HvTai mRNA levels were higher just before and right after the ecdysis within each larval instar and at early and late stages of prepupae than those in the intermediate days. RNA interference (RNAi)-aided knockdown of Tai at the second, penultimate and final larval instar stages caused 100% mortalities 9, 11 and 10 days post dsRNA administration. All Tai RNAi larvae failed to complete moulting; they were wrapped in the old larval exuviae, gradually darkened and shrivelled and ultimately died. The Tai-depleted larvae did not complete gut clearing; the larval guts contained food residues. Moreover, knockdown of Tai disrupted both JH and 20E pathway genes. Our findings demonstrated that Tai was necessary for metamorphosis in H. vigintioctomaculata. The findings also imply that H. vigintioctomaculata is an ideal model for further exploration of how Tai contributes specifically to hormone signalling.