RNAi screen in Tribolium reveals involvement of F-BAR proteins in myoblast fusion and visceral muscle morphogenesis in insects

In a large-scale RNAi screen in Tribolium castaneum for genes with knock-down phenotypes in the larval somatic musculature, one recurring phenotype was the appearance of larval muscle fibers that were significantly thinner than those in control animals. Several of the genes producing this knock-down phenotype corresponded to orthologs of Drosophila genes that are known to participate in myoblast fusion, particularly via their effects on actin polymerization. A new gene previously not implicated in myoblast fusion but displaying a similar thin-muscle knock-down phenotype was the Tribolium ortholog of Nostrin, which encodes an F-BAR and SH3 domain protein. Our genetic studies of Nostrin and Cip4, a gene encoding a structurally related protein, in Drosophila show that the encoded F-BAR proteins jointly contribute to efficient myoblast fusion during larval muscle development. Together with the F-Bar protein Syndapin they are also required for normal embryonic midgut morphogenesis. In addition, Cip4 is required together with Nostrin during the profound remodeling of the midgut visceral musculature during metamorphosis. We propose that these F-Bar proteins help govern proper morphogenesis particularly of the longitudinal midgut muscles during metamorphosis.