Early embryo patterning in the grasshopper, Schistocerca gregaria: wingless, decapentaplegic and caudal expression

Although the molecular pathways that pattern the early embryo of Drosophila melanogaster are well understood, how these pathways differ in other types of insect embryo remains largely unknown. We have examined the expression of three markers of early patterning in the embryo of the African plague locust Schistocerca gregaria, an orthopteran insect that displays a mode of embryogenesis very different from that of Drosophila. Transcripts of the caudal gene are expressed maternally and are present in all cells that aggregate to form the early embryonic rudiment. First signs of a posterior-to-anterior gradient in the levels of caudal transcript appear in the early heart-stage embryo, shortly before gastrulation. This gradient rapidly resolves to a defined expression domain marking segment A11. The decapentaplegic (dpp) gene, which encodes a transforming growth factor beta family ligand, is first expressed in a circle of cells that delimit the margins of the embryonic primordium, where embryonic and extra-embryonic tissues abut. Patterned transcription of wingless reveals that the first segments are delineated in the Schistocerca embryo substantially earlier than previously thought, at least 14-16 hours before the onset of engrailed expression. By the late heart-stage, gnathal and thoracic segments are all defined. Thus, with respect to the molecular patterning of segments, the short germ Schistocerca embryo differs little from intermediate germ embryos. The expression of these marker genes suggests that embryonic pattern formation in the grasshopper occurs as cells move together to form the blastodisc.