The Baboon Gene Encodes Three Functionally Distinct Transforming Growth Factor β Type I Receptor Variants in Henosepilachna vigintioctopunctata

Abstract

The Transforming Growth Factor-β (TGF-β) cascade plays a critical role in insect metamorphosis and involves cell-surface receptors known as type I and II, respectively (TβRI and TβRII). In Drosophila melanogaster, the TβRI receptor, Baboon (Babo), consists of three variants (BaboA, BaboB, and BaboC), each with isoform-specific functions. However, the isoforms and functional specifications of Babo in non-Drosophilid insects have not been established. Here, we examined babo transcripts from seven coleopteran species whose genomes have been published and found that mutually exclusive alternative splicing of the third exon produces three babo isoforms, identical to the Drosophila babo gene. The same three transcript variants were accordingly recognized from the transcriptome data of a coleopteran Henosepilachna vigintioctopunctata. RNA interference (RNAi)-mediated knockdown of all three babo transcripts at the fourth-instar larval stage hindered gut modeling and arrested larval development in H. vigintioctopunctata. All the resultant larvae became arrested prepupae; they were gradually dried and darkened and, eventually, died. Depletion of HvbaboA rather than HvbaboB or HvbaboC is similar to the phenotypic alterations caused by simultaneous RNAi of all three babo isoforms. Therefore, our results established diverged roles of the three Babo isoforms and highlighted the regulatory role of BaboA during larval-pupal transition in a non-Drosophilid insect species.