Activin β Is Critical for Larval-Pupal Transition in the 28 Spotted Lady Beetle Henosepilachna vigintioctopunctata.

The activin cascade is activated when a pair of extracellular ligand (Myoglianin, Myo; Activin β, Actβ; Dawdle, Daw) binds to two pairs of transforming growth factor β (TGF) serine-threonine receptor kinases, TGF-β type I (Baboon, Babo) and II receptors. However, the roles of activin way have not well been explored in non-Drosophilid insects. In the present paper, we compared the functions of Activin β (Actβ) ligand and receptor isoform BaboB in post-embryonic development in a defoliating ladybird Henosepilachna vigintioctopunctata. RNA interference (RNAi) for Hvactβ but not Hvbabob upregulated juvenile hormone signal, reduced ecdysone pathway and impaired larval-pupal transformation. The arresting Hvactβ RNAi larvae formed pupa-specific black markings below the larval exuviae. Thus, the impairment of metamorphosis may be caused by failing to complete ecdysis behavior due to nonfunctional muscles. Consistently, larval body sizes were smaller and adult appendages were shorter in the Hvactβ RNAi larvae, in contrast to those in the Hvbabob depleted beetles. Conversely, knockdown of Hvbabob but not Hvactβ changed the pigmentation of adult elytra. Our results suggest that Actβ exerts regulative roles in JH production, ecdysteroidogenesis and organ remodeling, thus contributing to modulate the larva-pupa-adult transformation, through a BaboB independent way in H. vigintioctopunctata.